Professional Papers No. 29 Engineer Field Manual 3rd Ed. 1909

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PROFESSIONAL PAPERS, No. 29 
CORPS OF ENGINEERS, U. S. ARMY 
Third (Revised) Edition 


ENGINEER FIELD MANUAL i 


PARTS I-VI 


1 RECONNAISSANCE 
Ml BRIDGES 
NM ROADS 
IV RAILROADS 
V FIELD FORTIFICATION 
Vi ANIMAL TRANSPORTATION 


PrePagip Unda THE DIRECTION OF THE 
CHIEF OF ENGINEERS, U, S. ARMY 


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PROFESSIONAL PAPERS OF THE CORPS OF ENGINEERS, U. S. ARMY 
No. 29 


ENGINEER FIELD MANUAL 


PARTS I-VI 


I. RECONNAISSANCE 

Il. BRIDGES 

Ill. ROADS 

IV, RAILROADS 

V. FIELD FORTIFICATION 

VI. ANIMAL TRANSPORTATION 


PR’PARED UNDER THB 


Direction or THE Cuter or Encineers, U. S, Army 


THIRD (REVISED) EDITION 


WASHINGTON 


GOVERNMENT PRINTING OFFICE 
1909 


WAR DEPARTMENT. 
Documznt No. 355. 


OFFICE oF THE CulnF oF ENGINEERS, 


‘War DEPARTMENT, 
OrFice oF THE OnJer oF STAFF, 
Washington, November 19, 1909. 

The Engineer Field Manual, United States Army, prepared under the direction of 
the Chief of Eugineers, U. 8, Army, is published for the information and guidance 
of all concerned; it will not be modified except by specific authority given in each 
cage. 

Any changes or suggestions that may occur to officers or others using the manual 
will be submitted to the Chief of Engineers for consideration in connection with the 
publication of future editions, 

By order of the Secretary of War: 

J. FRANKLIN BELL, 
Major General, 
Chief of Btaf’. 


War DEPARTMENT, 
OFFICE OF THE CHIEF OF ENGINEERS, 
Washington, March 12, 1907. 


The Adjutant General. 


§1r: 1. By authority of the Secretary of War, six parts of the Engineer Field 
Manual, compiled under the direction of this office by Lieut. Col. Smith S. Leach, 
Corps of Engineers and General Staff, have been published in five separate volumes, 
These parts are: Part I, Reconnaissance; Part II, Bridges; Part III, Roads; Part 
IV, Railroads, and Part VY, Field Fortification (in one volume); and Part VI, 
Animal Transportation. Each of these six parts received the approval of the Chief 
of Staff before its publication. 


2. It is now desired to publish under a single cover these six parts, revised and 
corrected, for issue to the service, when ready for distribution. 


8. In addition to the correction of such errora as have been. discovered in the 
original editions, it is proposed to add some new matter to bring the work up to date. 
The most important addition is a description of the new types of instruments adopted 
in 1906. It is also desired to add, in Part I, a brief description of the new military 
survey of Cuba; some additional topographical signs and symbols recently prescribed 
by the General Staff, and a brief account of the new system of angular measurement 
in mils adopted for position finding by the Field Artillery; to incorporate, in Part IT, 
a very useful table of dimensions of floor systems for stated loads and spans, and to 
incorporate, in Part VY, a plate and description of the Fort Riley redoubt, which pre- 
sents several excellent features of design. It is proposed to add the new matter at 
convenient places as nearly in its topical relation as possible, but under a caption 
‘taddenda, 1907.” 


4. The mechanical work involved in the preparation and publication of this 
revised edition would be, roughly, as follows: Drawing and engraving of four or 
five plates; making of a consolidated index; composition of the equivalent of about 
three or four pages of text; composition of consolidated index (about 48 pages); 
electrotyping of new plates, new pages of text, and new index; repaging of Parts II 
to VI, both inclusive, and printing and binding of 1,000 copies of the complete work, 
the cover to have a pocket, a pencil tube, and a broad flap folding over the back, 
The manuscript of a proposed introduction and list of authorities is inclosed. 


5. The matter in the six parts as now published is electrotyped; the electrotype 
plates are at the Government Printing Office. The expense of drawing and engray- 
ing the new plates, of preparing the new matter, and of making the consolidated 
index would be chargeable to the appropriation carried by the army appropriation 
act approved June 12, 1906, ‘‘ For pontoon material, tools, instruments, and supplies 
required for use in the engineer equipment of troops, including the purchase and 
preparation of engineer manuals,” of which there is an available balance sufficient 
for the purpose; the expense of composition, electrotyping, repaging existing elec- 
trotype plates, and of printing and binding to be borne by the appropriation for 
public printing and binding. The paper for the work is on hand in this office. 

6. I have the honor to recommend that 1,000 copies of the revised edition of the 
six parts of the Engineer Field Manual, as hereinbefore described, and their accom- 
panying plates be printed at the Government Printing Office and furnished for the 
use of this office on the usual requisition, the cost to be paid as stated in the preceding 
paragraph. 


7. A copy of each of the parts as published is submitted herewith. 


Very respectfully, 
A. MACKENZIE, 
Brig. Gen., Chief of Engineers, 
U. 8, Army. 


ENGINEER FIELD MANUAL. 
INTRODUCTION. 


In April, 1899, the Chief of Engineers directed the Commandant of the Engineer 
School to enter upon the preparation of an Engineer Field Manual, At the same 
time all officers of the Engineer Corps who had been in the field during the Spanish 
war were invited to contribute data and suggestions, and many of them did'so, At 
the Engineer School the work of compilation was committed to the instructor in 
civil engineering, then Capt. Henry Jervey, and under his control, and mostly by 
his own hand, a general plan of a manual was worked out, manuscript and platea 
prepared on the subjects of reconnaissance and bridges, and more or less complete 
notes on roads and railroads. 

The instructions of the Chief of Engineers required a topical division and publica- 
tion by parts, as completed. The part on reconnaissance was published in tentative 
form and distributed to officers of Engineers and other arms and to a few civil engi- 
neers, for comment and criticism. The parts on bridges and roads were sent in 
manuscript to certain Engineer officers for like criticism. Asa result, the method 
of treatment of subject-matter and the mechanical features of the book were defi- 
nitely determined and it was decided to revise the work already done to conform it 
to the modified plan and to republish Part I. 

At this stage, 1903, the pressure of work at the Engineer School made it necessary 
to place this duty in other hands and it was devolved upon the commanding officer 
of the First Battalion of Engineers, and shortly thereafter the relation of that offi- 
cer to the preparation of the manual was made personal, instead of ex-officio, and all 
subsequent work has been by the same hand. 

By July 1, 1906, six parts had been published—reconnaissance, bridges, roads, 
railroads, field fortification, and animal transportation. These parts are now col- 
lected in a single cover, with corrections of errors which crept into the first edition 
and some additions of new matter which has become available since the first publi- 
cation. The most important of these additions, made by direction of the Chief of 
Staff, is the incorporation of the signs, etc., for finished maps, published by anthor- 
ity ‘of the Secretary of War in 1904. A few minor changes which have been ap- 
proved, will be noted. 

The opportunity now first offers to make acknowledgment of sources from which 
material has been drawn and of assistance rendered by persons in the preparation 
and publication of the manual. 

As to authorities, a list is appended of works which have been \consulted and 
from which facts or suggestions have been derived. Other works have been con- 
sulted, but nothing having been taken from or suggested by them, they are not men- 
tioned, The titles in the list which appear in full-face type have been relied upon, 
more or less, as standard and as guides to topics and arrangement. Buta single 
work seems to deserve further mention, and that is the incomparable Trautwine, 
the indebtedness to which is too obvious to require mention, but too important to 
permit it to be dispensed with, Substantially no matter from any source is quoted. 
The exigency of space required everything used to be rewritten with a view to con- 
densation, In addition to the works cited, much valuable information, especially 
as to railroads and field fortifications, was obtained from the reports of military ob- 
servers with the Japanese and Russian armies and from fugitive publications as to 
the war in Manchnria. Of the latter, the Journal of the Royal Fngineers of Great 
Britain deserves special mention. 

Pergonal assistance in the preparation of text has come exclusively from brother 
officers of the Corps of Engineers, with the single exception of ‘‘ Landscape Sketch- 
ing,” paragraph 85, and plates 39 and 40, ‘‘ Reconnaissance,” which was abstracted 
from material furnished by Professor C. W. Larned of the Military Academy. In 
verifying, criticising, and correcting the work of the compiler, many officers have 
rendered assistance in greater or less degree, and none who have had opportunity to 
assist have refused. Buta few have given so much of time and labor as to make 


7 


8 ENGINEER FIELD MANUAL. 


mention by namean act of simple justice. Lieutenant Colonel Abbot, who has handled 
the manual in the office of the Chief of Engineers during the entire period of prepara- 
tion and publication, has contributed never-failing enthusiasm, encouragement, and 
counsel, which haye been.of the greatest possible assistance. Major Rees read crit- 
ically the parts on reconnaissance, bridges, and roads. Major Sibert and Lieuten- 
ants Johnston and Spalding did the same for railroads. Captain Connor read the 
same part and forwarded a paper of his own on the subject, from which some sugges- 
tions were taken. Major Gaillard read the parts on field fortification and animal 
transportation and made valuable suggestions from personal experience with pack 
trains. Captain Cheney read the part on animal transportation and made valuable 
suggestions. This part was also read by Dr. Hunter, V.8., Sixth Cavalry, and Mr. 
Daly, chief packer, upon whose approval much of its value rests, The original draw- 
ings for Parts I and 1I were made by enlisted men of the Second Battalion of Engi- 
neers, under the supervision of Major Judson, instructor of military engineering at the 
Engineer School. The names of these men, unfortunately, have not been made of 
record. These drawings were revised and those for Parts III and VI made by Ser- 
geant Pihlgrem, of the First Battalion of Engineers, assisted for a short time by 
Corporal Flugel of the same organization. The drawings for Parts IV and V and the 
-Addenda were made by Mr. 8. P. Hollingsworth, of Washington, D.C. The index- 
ing, partial and consolidated, was done by Mr. G. T. Ritchie of the Library of Con- 
gress. Mr, Pickering Dodge, chief clerk, U. 8. Engineer Office, Washington, D. C., 
contributed valuable assistance in final proof reading. 


LIST OF BOOKS CONSULTED. 


Theory and Practice of Surveying. Johnson. 

Military Topography and Sketching. Root. 
Tables and Formula. Lee. 

Higher Surveying. Gillespie. 

Roads and Railroads. Gillespie. 

Engineer's Pocketbook Trautwine. 

U. 8. Bridge Equipage and Ponton Drill. 

Military Bridges. Haupt. 

Roads and Pavements, Baker. 

Masonry Construction. Baker. 

Highway Construction. Byrne. 

Economic Railroad Location. Wellington. 

Railroad Construction, Webb. 

Notes on Track. Camp. 

Railroad Curves. Allen. 

The Railroad Spiral. Searles. 

The Roadmaster’s Assistant. Railroad Gazette. 
Locomotive Breakdowns. Emergencies, and their Remedies. Fowler. 
Text-book on Locomotives. International Correspondence Schools, 
Train Rules and Train Dispatching. Dalby. 

Block Signal Operation. Derr. 

Letters of an Old Railway Official. Hine. 

Manual of Field Engineering. Beach. 

Field Fortification. Fieheger. 

Manual of Military Engineering. Ernst. 

Attack of Fortified Places. Mercur. 

Royal Engineers Aide Memoire. 

Handbook of Modern Explosives. Tissler. 

Woolwich Text-book, Parts I and II. 

Chatham Text-book, Parts II and III. 

Text-book of Field Engineering. Phillips. 

Field Fortification. Hutchinson. - 

British Manual of Field Engineering. 1903. 

Destruction of Obstacles in Campaign. Bornecque, Tr. Burr. 
U. S. Field Service Regulations. 2 

Manual of the Quartermaster’s Department, U. S. Army. 
Horses, Saddles, and Bridies. Carter. 

Packer’s Manual. Daly. : 

Treatise on Feeding and Training of Mules. Riley. 
Military Transport. Furse. 


PART I. 


RECONNAISSANCE. 


PART I—RECONNAISSANCE. 


1, Topographical reconnaissance, as here treated, includes suitable means for 
obtaining and recording all needful information of a terrain in the shortest possible 
wae and within the limits of accuracy required for the operations of troops in the 

eld, 


Also, the interpretation of a record when made, to determine from it the favorable 
or unfavorable effect of the terrain, for the purpose of directing military operations 
with reference thereto. 


2, The information to be obtained in a topographical reconnaissance may be 
grouped under the headings of time, cover, resources, and nomenclature, The 
jmaap should permit a determination of the time which a column will require to 
pass between any two given points by showing tho distance between them and the 
condition of the road or country which must be traversed, as regards its effect on 
the rate of march; the accidents of ground which will afford cover to the army or to 
the enemy; the location, quantity, and quality of water, fuel, grass, etc., and should 
give to each feature its local name. Tho last requirement is of great importance 
and is the one most often neglected, 


3. The fundamental topographical operation is the determination of the 
direction and distance of one point from another point, 


The direction of one point from another is composed of two elements: First, the 
angle made by the line joining the two points, with a vertical plane passing through 
one of them. This angle is measured in a horizontal plane and is called the 
azimuth; second, the angle made by the line joining the two points, with a hori- 
zontal plane passing through one of them. This angle is measured in a vertical 
plane passing through both points, and for convenience will be called the gradient. 


4. Azimuths.—As an infinite number of vertical planes may pass through a 
given point, it is necessary to select one as the origin of azimuths. In topographical 
reconnaissance the plane selected is that of the magnetic meridian at the point. 
Its direction in a horizontal plane is the line of rest of a freely suspended and bal- 
anced magnetic needle, and this line is the origin of azimutha. 


From this origin azimuths are measured in degrees of arc from 0 to 360, passing 
from the north point through the east, south, and west: to north again. Azimuths 
of 0° to 90° are in the northeast or first quadrant, fig. 1; those of 90° to 180° are in 
the southeast or second quadrant; those from 180° to 270° in the southwest or third 
quadrant, and those from 270° to 360° in the northwest or fourth quadrant, 


Azimuths are bearings between stations taken in the direction of progress of thé 
reconnaissance. Bearings taken in the other direction are called back azimuths. 
If the stations are numbered in the order they are occupied, a bearing front a lower 
to a higher numbered station is an azimuth, and « bearing from a higher to a lower 
numbered station is a back azimuth, 


The method of stating azimuths described above is that commonly used in sur- 
veying when direction is maintained by carrying an azimuth, It is the simplest 
to understand and use, and permits the augle between any two lines to be read at a 
glance. 


There are other ways of expressing azimuths, adapted to special conditions or cir- 
cumstances. In astrouomical work and tables the azimuth is reckoned from the 
south, through W., N., and E., 360° to south again, Any astronomical azimuth 
differs from the corresponding survey azimuth by 180°. 


In navigation azimuths are reckoned from the mariner's compass, and are 
called bearings. The dial is divided into 32 points and each point into quarter 
points. The names of the points and their relation to survey azimuths are shown 
In fig. 1. : 

Land surveyors reckon bearings in both directions from N. andS. Their com- 
passes are graduated 90° in each direction from the N. and 8. points and a bearing 
is stated by giving the angle and direction from N, or S., whichever may be nearest, 
as N. 46° W., 8. 29° EK, 7 


Reconnaissance. 


1-3. 


i 


B Needle 
C Card 
D Pivot 
E Stop 


RECONNAISSANCE. 13 


Formerly such bearings were reckoned from the nearest cardinal point, N.,8., E., 
or W., as W. 44° N., which corresponds to N. 46° W. This method is very conven- 
ient for giving directions in orders and reports. It is shown in the middle circle of 
fig. 1. See par. 4a, p. 15. 


5, The compass is the standard instrument for the determination of azimuths 
in topographical reconnaissance. It consists of case, needle, card, pivot, and stop, 
figs. 2 and 3. 


The card may be fixed to the case or movable, attached to the needle and re- 
volving with it. The stop raises the needle from the pivot and clamps it against the 
glass cover. A good compass must have a needle sufficiently magnetized to settlo 
accurately and a pivot free from rust and roughness. If the needle becomes too 
weak, it may be remagnetized by rubbing gently from pivot to point on a permanent 
or electro magnet, each end of the needle to be rubbed on the pole which attracts 
it. In returning the needle for another stroke, carry it a foot or more from the 
magnet. The pivot may be polished with Putz pomade or similar substances on a 
soft stick. 


If possible, however, turn in the defective compass and get a good one in place 
of it. 


A needle loses part of its magnetism if kept fora long time out of the plane of 
the magnetic meridian. In storing a compass, care should be taken to place it in 
the case or on the shelf with the N. end of its needle pointing north, 


6. Dip.—tThe earth’s magnetic poles are beneath the surface, and the end of a 
symmetrical needle is drawn downward out of the horizontal plane so as to point to 
the nearest pole. This displacement from the horizontal plane is called dip, and is 
measured in degrees of arc. The dip increases generally with the latitude. Imme- 
diately over a magnetic pole the needle stands vertical, or has adip of 90°. Near 
the equator, where north and south poles exert an equal influence, the needle may 
be horizontal, or the dip 0. 


For reading azimuths the needle must be kept in a horizontal plane, which is done 
by a small movable counterweight. For considerable changes in latitude, asin pass- 
ing from the United States to the Philippine Islands, the counterweight will require 
adjustment to keep the needle horizontal, and in passing from the northern to the 
southern hemisphere, the counterweight must be changed to the opposite side of the 
pivot. 


7. There are two adopted forms of compass for topographical reconnaissance, 
one of the fixed and one of the movable card type. 


The box compass is shown in fig. 2. The card is fixed and graduated counter- 
clockwise from N. 360° to N. again. The E. and W. points, if marked, are reversed. 
The stop is operated by opening and closing the lid. The lid is hinged parallel to the 
north and south line, and when open ita upper edge forms a convenient line of sight. 
The needle when stationary can be read to the nearest degree by the eye, and to half 
a degree with a reading glass. 


Another pattern which has been issued has the lid on an E, and W, side, and the 
sighting line is a fine Line drawn across the lid. 


Some of the box compasses in use are graduated clockwise. Care must be taken 
in using these. The true azimuth is 360° minus the reading of the needle. The 
actual reading of such a compass should never be recorded; the corresponding azi- 
muth only should be set down. It will be safer to add a rough graduation in the 
proper direction. 


8. The prismatic compass is shown in fig. 3. It is of the movable-card type. 
It is read through a reflecting inverting magnifying prism, The prism revolves on 
an axis and is over the circumference of the card for reading, and against the edge of 
the case for carrying. It slides up and down in the support which attaches it to the 
case, which motion permits it'to be focussed on the scale. The focus for each obser- 
ver should be determined when the compass is resting on a level surface, and not 
thereafter varied. If, when so adjusted, the scale is out of focns when the sight is 
taken, it shows that the card is not horizontal, and the case muat be tilted until the 
scale comes into focus. The needle may be compensated for dip by a bit of sealing 
wax stuck on the underside of the card. The leaf sight folds down for carrying, and 
in so doing stops the needle, 


14 ENGINEER FIELD MANUAL. 


In the pattern illustrated, the metal cover goes on outside the leaf sight when 
folded dawn, When the compass is used, the cover is removed and placed for con- 
venience on the bottom of the case, where it fits closely. In-another pattern, the 
metal cover has a window in it opposite the prism, and is not removed when sighting. 
The leaf sight folds down outside the cover and is not protected. See par. 8a, p. 15, 


9. Compass errors.—The magnetic and true meridians generally do not coincide, 
The angle which the needle makes with the true north at any place is called the 
declination of the needle, or magnetic declination at that place. For latitudes 
of 60° and less the declination ordinarily varies between limits of 20° east and 20° 
west. For high latitudes the declination is greater and more irregular. 


There are daily and secular variations of declination at every place, but they are 
too small to have any bearing on the class of work now under consideration, and for 
purposes of topographical reconnaissance the declination at any place may be con- 
sidered constant for the period of the survey. 


A close watch must be kept for the change in de¢lination from place to place, and 
for local disturbances of the needle due to the proximity of magnetized substances, 
natural or artificial. 


Change of declination or normal direction of the needle should be checked fre- 
quently. If a change is observed, it is certain to have taken place gradually, and, 
if desired, may be distributed among the courses run, though the change will seldom 
be great enough in a single day’s work to make its distribution practicable. 


Abnormal deflections of the needle, due to local disturbances, are sudden and 
erratic and should not be distributed among all the courses, but only among those in 
which there is reason to believe the disturbance occurs. 


A simple way to detect—not measure—such disturbances is to take frequent back 
azimuths. If the position of the needle is norma] at both stations, the azimuth and 
back azimuth will differ by 180°. If there is local attraction on the course, it will 
usually be stronger or cause a greater deflection at one station than at the other, and 
the azimuth and back azimuth will not differ by 180°. 


Another way is, when taking the bearing to a station, to select a well-defined point 
beyond and on the same course. On arriving at the new station, take a bearing 
from there to the selected point ahead. If it is the same as the first bearing to that 
point, there probably is no local disturbance. If the two bearings to the same poiut 
differ, there probably is local disturbance. : 


A course in which local attraction is detected or suspected should be noted, and if, 
on closing, an azimuth correction is necessary, it should be applied to the suspected 
courses. 


10, Gradients.—There can be but one horizontal plane through a given point, 
and it may be determined by the spirit level or plumb line without serious error. 
Gradients are measured by taking the angle of the line of direction with a horizontal 
line through the point. : : 


11. Gradients are commonly called grades or slopes and are expressed in degrees, as 
1°, 2°, 334°, 614° slope, etc. 

Each angle corresponds to two slopes, one up and one down from the initial point. 
Rising grades may be recorded with a + before, or an R after the number of degrees; 
falling grades with — before, or F after. On a map, general slopes are indicated by 
an arrow pointing in the direction of the drainage, with the gradient written beside 


it, thus —£—>». Road grades are indicated by an arrowhead at top and bot- 
tom of the grade, the one at top pointing toward the road and the one at bottem 
away from it, thus : 
4° Ki 

Gradients are also expressed by the relation between the change of elevation—rise 


or fall—and the corresponding horizontal distance. This relation is stated in vari- 
Ous Ways. 


By the rise in ft. per 100 ft. hor. or the ft. rise as a percentage, as “‘ the slope is 4 in 
100, or 4 per cent.” 


RECONNAISSANCE, 15 


By the ft. rise for.1 mile of hor. distance; as ‘‘ the grade is 50 ft.,” or ‘‘a 50 ft. 
grade.”” This method and the preceding are commonly used for BR. R, track grades. 


By the number of ft. hor. corresponding to 1 ft. rise; as 3 to. 1,10 to 1. This 
method is commonly used for slopes of emhankments and excavations when less 
than:45°, 7 


By the ft. rise corresponding to 1 ft. hor.; as,lon1,60n1, This method is com- 
monly used for slopes of embankments and excavations, étc., from 45 to 75 degrees, 


By the number of inches hor. corresponding to 1 ft. rise; as, 3 ins. to the ft., 1 
inch in the ft: This method is commonly used for gradients of 70 degrees and over, 
and is called batter. 


ADDENDA, 1907. 


4a. A special method of azimuth measurement has beon adopted for use in the fire 
control of field artillery. The unit, called a mil, is the arc whose length is one one- 
thousandth of the radius, By computation this arc is 3’.437-+. This length is not 
commensurate with the length of the circle being contained in it 6,283.24 times. 
For convenience of graduation, the circle is divided into 6,400 equal parts, assumed 
to be mils, the angular value of each’ of which is 3’,375, differing from the computed 
value by nearly 2 %, which error enters into all determinations and is neglected, 


Each change of 1 mil in az. corresponds to a change in position in a direction per- 
pendicular to the line of sight of one one-thousandth of the range. This method 
“reduces all elements of fire control to functions of the range. a 


8a. The prismatic compass, model 1906, is shown in fig. 67c, p. 93, It differs from 
the types described in par. 8 in having the protective cover and leaf sight combined 
as shown in the figure, The inner glaas cover of the full size of the case protects the 
card, The middle cover is hinged and the front sight is provided by a slit in the 
cover, in the middle of which is a thin metal strip. Holes and screws are provided 
2 perme the convenient attachment of a wire or thread in case the sighting strip is 
roken, 


13a, The clinometer level, model 1908, fig. 67b, differs from the type shown in fig. 
4,in having a tangent screw, A, a reading glass, B, and in having supporting 
brackets in the angle between the top of the sight tube and the graduated arc to 
prevent the latter from being bent. 


l4a, The gravity clinometer adopted in 1906 is shown in fig. 67d. It consista 
of a circular case in which is a graduated circle controlled bya pendulum. The line 
of sight is through the peep L and a glass-covered opening at M@. The zero line is 
engraved on the glass. A mirror near the center refiects the scale back to the peep. 
Looking through the instrument the object is seen on the zero line, and at one end 
of the latter a graduation of the acale is visihle. The graduations are from zero at 
the horizontal each way to 45°, the graduations and numbers for elevation being in 
red and those for depression in black. 


A’sliding bar at H unlocks the spring-controlled stop, which, when pressed, frees 
the pendulum and graduated circle, and when released stops them again. 


To use, move the locking bar F to free the stop H; hold the instrument in the left 


hend with the forefinger on the stop; depress stop; bring lino of sight on object and 
Tead. 


16 


ENGINEER FIELD MANUAL. 


TaBLe I. 


12. Comparison of the different methods of expressing gradients: 


In this table the different methods of expressing gradients have their values given 
for the usual range and to the customary degree of accuracy of their use. 


Angl Ft. per 100ft.| Ft. to the | 1 vertical om | 1 horizontal | Batter ins. 
ngte. hor., or %. | mile, hor. or in— to— to the foot, 
Degrees. Horizontal. Vertical. 
0.44 23 229° seco e ees bee so esas 
% 87 46. 1 Tbe” \)esrees been! : 
oA 1.31 69.1 16 : 
1 1.74 92.2 67 a 
1% 2.18 115.1 46 = 
14 2. 62 138.3 38 = 
184 3. 06 161.2 33 S 
2 3.49 "184.4 29 = 
4 4.37 230.5 23 : 
3 5,24 276.7 19 . 
3% 6.12 322.9 16 = 
4 6.99 369, 2 14 = 
‘ay 7.87 415.5 13 : 
5 8.75 461.9 11.4 
6 10.61 555 9.6 es 
7 p75) EA ee 8.1 <j 
8 14. 05 71 # 
9 + 6.3 ~ 
10 6.7 = 
15 3.7 . 
20 2.7 - 
25 21 = 
30 1.7 5 
40 5 ed eee, es 
45 1 1 = 
BO cee os oe 1,2 - 
60 = 1.7 = 
65 eo 2.1 “ 
70 2 2.7 im 
75 a 3.7 
80 i 5.7 
81 = 6.3 
82 2 TAL 
83 se 8.1 
84 al 9.5 
85 | 11.4 
8534 fi 13 
86 aes, 14 
86% me 16 
87 x 19 
gilg ef 23 
88 x 29 
88 iS 33 
6, am 38 
8887 a 46 
eee en 57 
89: Z 76 
ans) = 115 
$0, eee a a ee 


229 


RECONNAISSANCE. 17 


13. The clinometer is tho instrument adopted for measuring gradients, with the 
horizontal plane indicated by a spirit level. It consists, fig. 4, of a sight-tube, 4, 
with a graduated vertical arc, B, fastened to it, and a level-tube, C, with attached 
index arm, D, revolving about a horizontal axis through the center of the vertical 
arm. The base of the sight-tube is a plane parallel to the line of sight. Under the 
center of the level-tube is an opening in the sight-tube, inside of which is a mirror 
occupying one-half the width of the sight-tube and facing the eye end at an angle 
of 45° with the line of sight. A horizontal wire extends across the middle of the 
sight-tube in front of the mirror. When the bubble is brought to the center, its 
reflected image seen from the eye end appears to be bisected by the wire. 


The central position of the bubble indicates that the level-tube is horizontal, and 
the reading of the index arm upon the arc is the angle between the axis of the level- 
tuhe and the line of sight. This reading should be 0° when these lines are parallel. 
The vertical arc is graduated each way from 0° at its middle point. The index arm 
has a double vernier whose smallest reading is 10’ of arc. Gradients of more than 
15° are difficult to measure on account of the foreshortening of the level-tube as 
reflected in the mirror, See par. 13a, p, 15. 


When the vernier is set at 0°, the instrument may be used as a hand level to locate 
points at the same elevation as the eye. The graduation on the inner edge of the 
vertical limb corresponds to the ordinary fractional method of iudicating slopes, as 
lon 2,l10n 10, etc. This scale should be read on the forward edge of the index arm, 
or in some forms on a special index mark on a shorter part of the arm. 


The level=tube is made parallel to the sight-tube by the adjusting screws 
E, fig. 4. To test and correct the adjustment, place the instrument on a smooth 
surface, the more nearly horizontal the better, and mark carefully the position of 
one side aad one end of the sight-tube. Center the bubble by moving the index 
arm, and read the vernier. Reverse the instrument, bringing the other side and 
end of the sight-tube to the marks. Center the bubble by moving the index arm, 
and read again. Note and record for each reading its direction from 0°, whether 
toward or away from the eye end of the sight-tube. Note and record also the loca- 
tion of the eye end in each position with respect to some fixed object, so that the in- 
strument can be replaced iu the first position or second position at will. 


If the first and second readings are the same, the adjustment is correct. If they 
differ, take the mean of the two and set the vernier at that reading on the side cor- 
responding to the first reading. Place the instrument in the first position and bring 
the bubbie to the center by means of the adjusting screws EZ. For a check, set the 
same reading on the side corresponding to the second reading and place the instru- 
ment in the second position. The bubble should come to the middle. 


14. The determination of gradients by the plumb line is quicker and sim- 
pler, but less precise than with the clinometer, though exact enough for ordinary 
purposes. Ifa line of sight be taken along the edge of a board and a line be drawn 
on the board perpendicular to the sighting edge, this line, when the board is held in 
a Vertical plane, will make the same angle with the plumb line that the sighting 
edge makes with the horizontal, or, in other words, will indicate the gradient, fig. 5. 


Such a construction is called a slope board and is readily improvised. The scale 
may be constructed by sweeping an arc of a circle AB, fig. 5, from the point C, at 
the intersection of the perpendicular and the sighting edge. From the perpendicular 
at D lay off each way on the arc chords equal in length to the radius CD divided by 
57.3. Itis convenient to take a radius of 5.73 ins., or 534 ins. scant, when the 
chords will be + in., or a radius of 73, ins., when the chords will be 34 in., accord- 
ingly as the scale ustdis graduated to 10ths or 8ths. 

Short radial lines drawn at the ends of the chords form a graduation in degrees, 
The scale may be drawn on the lower edge of the board by prolonging the radial 
lines as indicated in the figure. The plumb line is suspended from the point C. 


In use, the board is held so that the plumb line swings free but very close to the 
board. The sighting edge is directed to the object and when the line is steady the 
board is quickly tilted so that the line draws across the edge. The board is then 
turned toa horizontal position or nearly so, and the reading taken; or, when the 
line is steady, it may be pressed against the board with the finger and held in place 
until the reading is taken. With o straight scale and for steep grades, the latter 
method is better. See par. 14a, p. 15. 


87625—09-—2 


Reconnaissance. 4.8. 


RECONNAISSANCE. 19 


15. EBlevations.—From the slope and distance the elevation of a point above an 
assumed plane of reference may be derived. The difference of height of any two 
points is known by comparing their elevations above a common plane, called the 
plane of reference, or datum. 


The plane of reference is taken low enough so that no point of the area to be 
covered by the reconnaissance will be below it. This makes all elevations positive. 
Knowing the height of a point above this plane of reference, the elevation of any 
other point may be obtained by taking the gradient and distance to that point, deriv- 
ing from them the difference of height between the two points, and adding this dif- 
ference to the elevation of the first point if the gradient is rising, or subtracting it 
if the gradient is falling. 

The elevation for a given gradient and distance depends upon whether the distance 
is measured along the gradient or along the horizontal. Distances paced are along the 
gradient. Those measured with a chain will also usually be on the slope, thongh 
sometimes care is taken to hold the chain horizontal, in which case the table for 
horizontal distances is to be nsed. Those determined by intersections or scaled from 
@ map are along the horizontal. 


The differences of elevation corresponding to various gradients and any distances 
may be taken from the following tables. 


TaBre II. 


16. Differences of elevation for gradients of 0° to 30°, and horizontal distances, 


. 


Difference of elevation for horizontal distances of— 


Gradient in 
degrees, 
1, 2. 3. 4, 5. 6. Te 8. 8. 

% 00087 | 00174 | 00261 | 00348 | 00435 | 00522 | 00609 | 00696 | 00783 

1 00174 | 00340 | 00523 | 00698 | 00872 | 01047 | 01221 | 01396 | 01570 
1% 00262 | 00524 | 00786 | 01048 | 01310 | 01572 | 01834 | 02096 | 02358 
2 00349 | 00698 | 01047 | 01396 | 01745 | 02094 | 02443 | 02792 | 03141 
24 00436. | 00872 | 01308 | 01744 | 02180 | 02616 | 03052 | 03488 | 03924 
3 00524 | 01048 | 01572 | 02096 | 02620 | 03144 | 03668 | 04192 | 04716 
4 | 00699 | 01398 | 02097 | 02797 | 03496 | 04195 | 04894 | 05594 | 06293 
5 00875 | 01750 | 02625 | 03500 | 04375 | 05250 | 06125 | 07000 | 07875 
6 01051 | 02102 | 03163 | 04204 | 05255 | 06306 | 07367 | 08408 | 00459 
7 01228 | 02456 | 03684 | 04912 | 06140 | 07368 | 08596 | 09824 | 11052 
8 01405 | 02810 | 04216 | 05621 | 07027 | 08432 | 09837 | 11243 | 12648 
9 01584 | 03168 | 04752 | 06336 | 07920 | 09504 | 11088 | 12672 | 14256 
10 01763 | 03526 | 05289 | 07053 | 08816 | 10579 | 12343 | 14106 | 15869 
12 02125 | 04251 | 06376 | 08502 | 10628 | 12753 | 14879 | 17004 | 19130 
14 02493 | 04986 | 07479 | 09973 | 12466 | 14059 | 17453 | 19946 | 22439 
18 02867 | 05734 | 08602 | 11469 | 14337 | 17204 | 20071 | 22939 | 25806 
18 03249 | 06498 | 09747 | 12996 | 16245 | 19494 | 22743 | 25992 | 29241 
20 03639 | 07279 | 10919 | 14558 | 18198 | 21838 | 25477 | 29117 | 32757 
22 04040 | 08080 | 12120 | 16161 | 20201 | 24241 | 28282 | 32322 | 36362 
24 04462 | 08904 | 13356 | 17809 | 22261 | 26713 | 31166 | 35618 | 40070 
26 04877 | 09754 | 14631 | 19509 | 24386 | 29263 | 34141 | 39018 | 43895 
28 05317 | 10634 ) 15951 | 21268 | 26585 | 31902 | 37219 | 42536 | 47853 
30 05773 | 11547 | 17320 | 23094 | 28867 | 34641 | 40414 | 46188 | 51961 


The diff. of elevation for any gradient and any hor. distance may be obtained by 
multiplying the dist. by the tang. of the angle or gradient, Table XIV. 


20 ENGINEER FIELD MANUAL. 


Taste III. 


17. Differences of elevation for gradients of 0° to 30°, and distances measured 
on the slope. 


Difference of elevation for sloping distances of— 
Gradient in 
degrees. 
1. 2. 3. 4, 5. 6. 7. 8. 9% 

% 00087 | 00174 | 00262 | 00349 | 00436 | 00523 | 00611 | 00698 | 00785 

1 00174 | 00349 | 00523 | 00698 | 00873 | 01047 | 01222 | 01396 | 01571 
1% 00262 | 00523 | 00785 | 01047 | 01309 | 01571 | 01832 | 02094 | 02356 
2 00349 | 00698 | 01047 | 01396 | 01745 | 02094 | 02443 | 02792 | 03141 
2M 00436 | 00872 | 01308 | 01746 | 02181 | 02617 | 03053 | 03489 | 03926 
3 00523 | 01047 | 01570 } 02093 }) 02617 |) 03140 } 03663 ) 04187 | 04710 
4 00697 | 01395 | 02093 | 02790 | 03488 | 04185 | 04883 | 05580 | 06278 
5 00871 | 01743 | 02615 | 03486 | 04358 | 05229 | 06101 | 06972 | 07844 
6 01045 | 02090 } 03136 | 04181 | 05226 | 06272 | 07317 | 08362 | 09407 
7 01219 | 02437 | 03656 | 04875 | 06093 | 07312 | 08531 | 09749 | 10968 
8 01392 | 02783 | 04175 | 06567 | 06959 | 08350 | 09742 | 11134 | 12525 
9 01564 | 03129 | 04693 | 06257 | 07822 | 09386 | 10950 | 12515 | 14079 
10 01736 | 03473 | 05209 | 06946 | 08682 | 10419 | 12155 ; 13892 | 15628 
12 02079 | 04158 | 06237 | 08316 | 10395 | 12475 | 14554 | 16633 | 18712 
14 02419 | 04838 | 07258 | 09677 | 12096 | 14515 | 16934 | 19354 | 21773 
16 02756 | 05513 | 08269 | 11025 | 13782 | 16538 | 19294 | 22051 | 24807 
18 03090 | 06180 | 09270 | 12361 | 15451 | 18541 | 21631 | 24721 | 27811 
20 03420 | 06840 | 10261 | 13681 | 17101 | 20521 | 23941 | 27362 | 30782 
22 03746 | 07492 | 11238 | 14984 | 18730 | 22476 | 26222 | 29968 | 33714 
24 04067 | 08135 | 12202 | 16269 | 20337 | 24404 | 28471 | 32539 | 36606 
26 04384 | 08767 } 13151 | 17535 | 21918 | 26302 | 30686 | 35070 | 39453 
28 04695 | 09389 | 14084 | 18779 | 23473 | 28168 | 32863 | 37658 | 42252 
30 05000 | 10000 | 15000 | 20000 | 25000 | 30000 | 35000 | 40000 | 45000 


The diff. of elevation for any sloping distance and any angle or gradient may be 
found by multiplying the dist. by the sime of the angle, Table XIV. 
Kxplanation of use of Tables II and III: 


Rule.—From the line of the given gradient, take out the tabular numbers corre- 
sponding to each of the figures of the given distance, beginning at the right, and set 
them down; each one place to the left of the one above it. Retain the ciphers at 
the beginning of the last tabular numher taken out, if any. Other left-hand ciphers 
may he dropped. 

Add the tabular numbers, and point off from the left the number of places equal 
to that of the left-hand figure of the distance, counting any left-hand ciphers. The 
result is the difference of elevation, in the same unit as the distance, 


_Examples.—For the diff. of elevation corresponding to a gradient of 3° and a 
distance of 6,273 ft., on the slope— 


From Table III-— 


For 8 opp. 3° and under 3, 1570 

For,7 opp. 3° and under 7, 3663 

For'2 opp. 3° and under 2, 1047 

For 6 opp. 3° and under 6, 03140 retain leading cipher. 


Aa 6 is in 4th place, point off 4, 0328. 2900 
Diff. of elevation = 328,29 ft. 


RECONNAISSANCE. 21 


2d. What diff. of elevation for gradient of 5°, and horizontal distance of 7,180.56 yds.? 
From Table 11— 


Opp. 5° and under 6, 5250 

Opp. 5° and under 5, 4875 

Opp. 5° and under 8, 7000 

Opp. 5° and under 1, 875 

Opp. 5° and under 7, 06125, retain leading cipher. 


7 isin 4th place, point off 4, 0628, 299000 
Diff. of elevation = 628.299 yds. 


18. Barometric leveling.—The weight of the atmosphere at sea level is 14.703 
lbs. per sq. in., equal to the weight of a column of mercury 29.92 in. high, or a 
column of fresh water 34.7 ft. high. 


The aneroid barometer records the pressure of the atmosphere in inches, the 
same as a mercurial barometer, tbe reading being taken from a pointer moving on 
a circular scale. It must be carefully handled as it is sensitive to sbocks, A screw 
head will be seen through a bole in the back of the outer case by which the needle 
may be brought to any desired reading, and the instrument corrected whenever it 
can be compared with a standard. With the aneroid, corrections for instrumental 
temperature can not be made, and for this reason smal! pocket instruments are 
preferable, as carried in the pocket they are not exposed to so great changes in thia 
respect. 


The pressure of the atmosphere varies with the altitude above sea level, and 
it also varies with the moisture, temperature, and latitude, which do not depend 
upon the altitude. 


In measuring altitudes with the barometer these other causes of variation must 
be eliminated so far as possible. It is best done by simultaneous observation at both 
stations, If the stations are not far apart all disturbing conditions will be substan- 
tially the same at each and therefore eliminated, except temperature, which, with 
considerable difference of altitude, will always be less at the upper than at the lower 
station. 


If simultaneous observations can not be made, the stations should be occupied with 
as little interval of time between as possible, and better results will be obtained if the 
time of observation can be so chosen as to take advantage of calm, bright, dry 
weather. 


When the bygrometric conditions are very uniform an aneroid read at intervals 
on a day’s march over a rough country will give a fairly good idea of the profile. 


ENGINEER FIELD MANUAL. 


TaB_e IY. 


19. Table of elevations above sea level from barometer readings (United States 
Coast and Geodetic Survey), for mean hygrometric conditions and mean temperature 


of 60° F.: 
Barow-| Altitude | ,,- Barom- | Altitude | 7. Barom-| Altitude |,,. 
eter above ne eter above eae eter above ae 
reading.| sea level.| “"" * |lreading.| sea level.) “°° |/reading.| sealevel.| ~~ 
Inches. Feet. Feet Inches. Feet. Feet. Inches, Feet, Feet, 
18.0 |. 18,918 | —15.1 22.2 8, 204 | —12.2 26.4 3,483 |. —10,3 
«1 | 18,767 15.0 a] 8, 082 12.2 5 3, 380 10.3 
.2! 18,617 14.9 a4 7, 960 12,2 .6 3,277 10.2 
.3) 13,468 14,9 7) 7, 838 12.1 eT 3,175 10.2 
«4 ] 18,319 14.7 6 7,717 12.0 -8 3, 073 10.1 
-6 | 13,172 14,7 “7 7,597 12.0 9 2,972 10.1 
.6 | 13,025 14.6 .8 7,417 119 27.0 2, 871 10.1 
.T] 12,879 14.6 9 7, 858 11.9 “1 2, 770 10.0 
.8 | 12,733 14.4 23.0 7, 239 11.8 -2 2,670 10,0 
9} 12,589 14.4 ee 7,121 11.7 3 2,570 10.0 
19.0 | 12,445 14,3 ay 7, 004 11.7 4 2,470 9.9 
-L | 12,302 14.2 .3 6, 887 11.7 i) 2,371 9.9 
-2|] 12,160 14,2 4 6,770 11.6 6 2,272 9.9 
-3 | 12,018 14.1 a] 6, 654 11.6 7 2,173 9.9 
.4] 11,877 14.0 -6| ‘6,538 11.5 8 2, 075 9.8 
.5 | 11,787 13.9 27 6, 423 11.5 a) 1,977 9.8 
6] 11,598 13.9 8 6, 308 11.4 28.0 1, 880 9.7 
-T| 11,459 13.8 9 6,194 11.4 fl 1, 783 9.7 
-8] 11,321 13.7 24.0 6, 080 11.3 +2 1, 686 9.7 
+9] 1,184 13.7 v1 5, 967 11.3 +3 1, 589 9.7 
20.0 | 11,047 13.6 2 5, 864 11.3 «4 1, 493 9.6 
~  .1 4 10,911 13.5 13 5, 741 11.2 az) 1, 397 9.6 
-2| 10,776 13.4 4 5, 629 11.1 -6 1,302 9.5 
-3 | 10,642 13.4 5 5,518 11.1 “7 1,207 9.5 
-4 | 10,508 13.3 +6 5,407 11.1 8 1,112 9.5 
«6 | 10,375 13.3 -T 5, 296 11.0 .9 1,018 9.4 
-6 ) 10,242 13.2 8 5, 186 10.9 29.0 924 9.4 
-7 | 10,110 13,1 9 5,077 10.9 1 830 9.4 
8 9,979 13,1 25,0 4, 968 10.9 2 736 9.3 
9 9, 248 13.0 1 4, 859 10.8 3 643 9.3 
21.0 9,718 12.9 +2 4, 751 10.8 4 550 9.2 
al 9,589 12.9 a3 4, 643 10.8 5 458 9,2 
2 9, 460 12.8 4 4,535 10.7 6 366 9.2 
3 9, 332 12.8 6 4, 428 10.7 +7 274 9.2 
4 9, 204 12.7 6 4,321 10.6 8 182 9.1 
i) 9,077 12.6 27 4,215 10.6 9 91 9.1 
6 8,951 12,6 8 4,109 10.5 30,0 00 9.1 
7 8, 825 12.6 9 4, 004 10.5 ol — 91 9.0 
8 8,700 12,5 26.0 3, 899 10,5 2 —181 9.0 
9 8,575 12.4 1 3, 794 10.4 13 —271 9.0 
22.0 8,451 12,4 2 3, 690 10.4 4 —361 9.0 
1 8, 327 12.3 .3 3, 586 10.3 5 ~451 8.9 


RECONNAISSANCE. 28 


TaBie V. 


20. Coefficients for temperature correction.—Argument (t+¢’) = Sum of 
temperatures at the two stations: 


5 x yl 
t4t'. Coefficient C. t4t’. Coefficient C. t+’. Coefficient C. 
° . fe} o 
0 —0. 1024 60 —0.0380 || 120 +0, 0262 
10 —0. 0916 70 —0. 0273 130 +0, 0368 
20 . —0. 0806 80 —0. 0166 140 +0, 0472 
30 —0. 0698 90 —0. 0058 150 +0. 0575 
40 —0. 0502 100 +0. 0049 160 +0. 0677 
50 —0. 0486 , 110 +0. 0156 170 +0, 0779 
60 —0. 0380 120 +0. 0262 180 +0, 0879 
Examples: 
‘ Barome- | Temper- 
Station. ter. ature, 
Inches. oF, 
59.9 
42.1 
From table of elevations.--______ Sacramento = —12.7 
Summit = 6,901.0 
. ‘ Diff. = 6,913.7 
t+ = 102° 


1. @ = +0. 0070 . 
.'. Temperature correction, 6,913.7 « 0.007 = +48.4 


H = 6,962.1 feet. 


Barome- | Temper- 


Station. ter. ature, 


Inches. oF, 
28. 075 67.3 
22, 476 38.5 


t+) = 95°,08 
ww. O = +0.0004 
. . Temperature correction, 6,060 x 0.0004 


II 


42.4 


IT = 6,062. 4 feet. 


24 ENGINEER FIELD MANUAL. 


21, Use of compasses.—A good needle requires time to settle even when the 
case is firmly supported, and the user should cultivate the knack of catching it at 
the middle of its swing, which is the desired reading. If the compass can be sup- 
ported, itis always better to dose. Then the sight can be carefully taken and the 
position of the eye changed to read the needle, Wait till the swing gets down to 
4° or 5°, which it will usually do in a few seconds. Then catch the highest and 
the lowest readings on the same swing and take their mean for the true reading. 
If the first swings are vory large, catch the needle with the stop near the middle of 
the swing and release it quickly. This will suddenly check the swings and shorten 
the time in which the reading can he taken. 


In using the box compass without a support, hold it sufficiently helow the eye so 
that the swing of the needle can be seen. Point the edge of the lid in the required 
direction, catch the needJe with the stop in the middle of a swing and hold it stopped 
until the reading is taken. Stop readings are less accurate than sight readings, as 
the needle may be displaced slightly when off the pivot. When the stop is used press 
it quickly and firmly. Always sight a fixed-card compass from the south end of the 
card and read the north end of the needle. . 


With the prismatic compass the stop is not used except to check the swings. 
Utilize a support if practicable. The prism having been adjusted for focus, as 
already explained, par. 8, adjust the case so as to bring the scale into focus, and 
when the swings become small, read the extremes and take the mean. 


Compasses for night marching are on the market, but are not very reliable. They 
have the dial rendered luminous by a paint. After exposure to the gun or strong 
daylight, they give off ligbt, at first rather strong, but rapidly diminishing in inten- 
sity. After a few hours they are not bright enough to be of much use, 


The surest preparation for night marching is a provision for illuminating the 
compass by ordinary means without allowing the light to be seen. 


22. To determine the declination of the compass: 


1st method ; from the sun,—Prick a small hole in a piece of tin or opaque paper and 
fix securely over the south edge of a table or other surface perfectly level, so that the 
sunlight coming through the hole will fall on a convenient place on the surface, fig. 9. 
The hole may be 2 ft. above the table for long days and 18 ins. for short ones. Half 
an hour before to balf an bour after noon, mark the position of the spot of sunlight 
on the borizontal surface at equal time intervals of about 10 min. Draw a curve as 
bd, fig. 9, through the points marked, and from point ¢ in the horizontal surface and 
in a vertical line with the hole a sweep an arc ef intersecting bd in two points. The 
line cg, drawn from ¢ through a point on the are midway between the iutersections, 
is the true meridian. The line 6d illustrates the method merely. Its form varies 
with the sun’s declination. 


2d method; from the sun or a star.—Observe the magnetic hearing of the sun, a 
planet, or a bright star at rising and ‘setting on the same day, or at setting on one 
day and at rising on the next. Take the difference between the sum of the rising 
and setting azimuths and 360°. One-half of this difference is the declination of the 
eompass or variation of the needle, east if the sum of the azimuths is ess than 360°; 
west, if it is greater. In using this method, the observations are better taken 
when the object is just above the true borizon, or ata gradient of zero, . This can 
usually be done if a high point is chosen for the observations. If it can not be 
done, be careful to take both observations with the object at the same gradient. 
This is most important with the sun, Under the least favorable conditions, 
an inequality of 1° in the gradients at the times of observation on the sun may intro- 
duce an error of 14° in the result. If using a star, choose one which rises nearly 
east from the point of observation, and, the inequality of a degree in gradients will 
not be material. 


The change in declination of the sun between observations can not affect the result 
more than 24°. 


Both observations need not be made at the same point, but should not be more 
than 10 miles apart in east and west, or north and south directions. 


The two foregoing methods are applicable in the northern or southern hemisphere. 


3d method; from Polaris.—The true north pole is about 1° 12’ distant from Polaris 
on a line joining that star with one in the handle of the dipper, and another in Cas- 


Reconnaissance. 9-11, 


< 
tu 
Qa. 
° 
wn 
wn 
<¢ 
(S) 


Plumb line ~ 


26 ENGINEER FIELD MANUAL. 


siopeia’a Chair, fig. 10. One of these stars can be seen whenever Polaris ia visible. 
The polar distance of Polaris is decreasing at the rate of 19” a year. It also varies 
during the year by as much as 1’, The latter variation may be neglected, and the 
former also for a series of years. = 


Imagine Polaris to be the center of a clock dial, with the line joining 12 and 6 o’clock 
vertical und with the position of one of the lines described considered as the hour 
hand of the clock. ‘The distance in azimuth of Polaris from the true north may be 
taken from the following table: 


TABLE VI. 


23. Table showing the azimuths of Polaris in different positions with respect 
to the pole. Epoch 1911; polar distance 70’, Latitude 0° to 18° north. This table 
may be used until 1930. 


Clock reading of—| Azj- || Clock reading of— Clock reading of — F 

muth Azimuth Azi- 

of of muth 

& U 2 o |Polar- Cy aa Polaris, é aes -s poe 
Cass. Maj. is, Cass. Maj. Cass, Maj. fe 
, fe} , io} Pi 
XII:30 VI:30 18 |) ITIT:30 X:30 49 j| VIII II 358 69 
I VII 35 v XI 35 IX III 358 50 
1:30 | VII:30 49 V:30 XI;80 18 x IIlI 358 59 
Il VIII 6L VI:30 | XITI:30 | 369 42 X:30 | 1TI1:30 | 359 11 
III Ix 70 || VII I 359 26 XI v 359 25 
IIII° x 61 | VII:30 1:30 | 359 11 ||; X1:30 V:30 | 369 42 


For higher latitudes add to the small azimuths or subtract from the large ones, as 
follows: 


Lat, 19°—30°, 2,. Lat. 519°—-53°, f,. 
Lat. 319°—37°, vy. Lat. 56°87, yo. 
Lat. 38°—42°, 3. Lat. 58°—599, 48, 
Lat. 43°—46°, 4, Lat. 60°—61°, fy. 


Lat. 47°—60°, 


It is well to keep track of the position of Polaris by noting it frequently and taking 
the corresponding clock time. Then if on a cloudy night a glimpse of Polaris is had, 
the observation may be taken even though the other staracan not be seen, 


24. For practical details of the observation, the following may serve as a 
guide: Select aclear space of level ground not too near buildings or any object 
which might cause local disturbance of the needle. Drive a picket, leaving its top 
smooth and level, about 18 ins. above the ground. Six feet north of the picket sus- 
pend a plumb line from a point high enough so that Polaris, seen from the top of 
the picket, will be near the top of the line, fig. 11. Tbe line should be hard and 
smooth, about jj; in. diam. The weight at tbe bottom of the line should hang in a 
vessel of water or in a hole dug in the ground to lessen its vibration. Drive a sec- 
ond picket in range with the first one and the plumb line, a short distance north of 
the latter. Makea peep sight by punching a hole about 7, in. diam. in a piece of 
paper and hold it on tbe top of the first picket; adjust it so that the star is behind 
the plumb line when looking through the peep. Note the position of one of the 
stars on the imaginary clock face at the moment the observation is taken. Mark 
the position of the peep on the top of the first picket, and lay a straightedge or 
stretch a line from that point touching the plumb line to the second picket. Place 
the north-and-south edge of the compass box against the line or straightedge, and 
Mab oe needle. Hind the azimuth of the star at the time of observation from 

able VI. 


RECONNAISSANCE. 27 


If the az. of Polaris rant VI) and the reading of the needle are both less 
or both greater than 180°, their diff. is the declination; east if the needle read- 
ing is less, west if it is greater, If one of these quantities is less and the other 
greater than 180°, add 360° to the lesser and take the diff. which is the declination; 
east if after the addition is made the needle reading is less, west if it is greater 
than the tabulated az. 


This method will give results true to within 34°. 


25. Distances passed over are ordinarily measured by the stride of a man or a horse, 
or by the revolutions of a wheel. Distances not passed over are determined by 
intersection, or are estimated. 


Pacing on foot,—The length of a man’s pace at a natural walk is about 30 ins., 
varying somewhat above and below. Each sketcher must determine his own length 
of pace by walking several times over a known distance. An unnatural stride 
should never be taken. Knowing the length of a pace or step, the measurement of 
a distance is only a matter of counting steps. The counting may be done mentally, 
and with practice becomes 4 subconscious operation, leaving the attention free to 
take note of surrounding objects and conditions. The greatest danger is of dropping 
one hundred paces. It is better to keep a tally of the hundreds. See par. 25a, below. 


On level ground, carefui pacing will give distances correct to 3% or less. The 
normal length of pace decreases on slopes. The decrease varies with the slope and 
with the direction, whether ascending or descending. The following table gives the 
length of pace on slopes of 5° to 30°, corresponding to a normal pace on a level of 
30.4 ing, 


ABLE VIL. 
Slopes. oe | 5° | 10° | 15°°} 20° | 95° | 30° 
Length of step ascending____________ 30.4 | 27.6 | 24.4 | 22.1 | 19.7 | 17.8 | 15.0 
Length of step descending_--_-----__ 30.4 | 29.2 | 28.3 | 27.6 | 26.4 | 23.6 | 19.7 


For the same person, the length of step usually decreases with fatigue. Sketchers 
should test their pace when fresh and when tired, and if there is an appreciable 
difference, use one length:for the morning and the other length for the afternoon 
work. 


26. A distance ou a slope measured by foot pacing may be reduced to the correct 
horizonta] distance for plotting on the map by the following table, which takes ac- 
count of the decrease in length of pace, Table VII, and also of the reduction to the 
horizontal, Table XII. This table can be used only when the length of pace has 
been determined on level ground, which should usually be done. When a consid- 
erable stretch of road is found with fairly uniform slopes, a special average rating 
may be made over a distance involving a fairly representative range of slopes and 
this average rating may be used without reduction. 


ADDENDA, 1907. 


25a. A pace tally is issued for use when desired. It is the size and shape of an 
ordinary watch. ° 

28a. The most convenient timer for mounted pacing is the type known as the 
football watch. It has a stop and start arrangement independent of the fly back 
and gives a cumulative record of the times in motion. 


ENGINEER FIELD MANUAL. 


28 


Tague VIII. 


9°39P G'GGS | 8°ELO | GIR | BLL | ZGGS | 9°9BL | L’EGD | S°E*B | S°8OL | F098 | 9°SIB 
our #°98G | 9°ShS | 8°SGE | F'SFO | P'OLP | 4°669 | HPSS | O'ZEL | 9°6%9 | B'FOL | SSL 
8°6SE 9°02 b LLP Lets O'LLe | S°U9 1 °S8h GOr9 | 6°0G¢ 3699 8 C89 
F808 S'FLZ | 2°60b | 9°b6Z Bagg | h'AG | SGLh | OOPS | BLE | O°SLG | BPS 
O°LSS O'6LL | O'LbE | G"CtS OPES | OLE | GORE | G°LGF | SEE | O'SLE | 0'SSE 
9°06 Stl | S'3L | F'9GL 6°SS | O'OFE | GLLS | O'99E | S'FIE | BBE | 9°L9E 
BFS FLOL | 9°F02 | E°LEL P'SLL | 29GB | CLOG | GPL] | L°9ES | BOBS | BLL] 
8°20T 9°LLO | b°9EL | 3°860 O° LIL | S°PLL | O'REL | OBL | H°LSL | SIBL | 8 ‘OSL 
L°L60 0°890 | 9°GZL | €°60 LULL | E'99t | L°1SL @'ORL | 91ST | 8°LLAT 
£°360 P'b9O | 8°SSL | F°880 S°SOL | €°LoL | L°FSL LULL | LOLL | L’OoL 
¥°180 6°090 | G°SIL | 9°E80 O'OOL | 9°8FL | B°LIL | 9°SSL | 8°SEL | S°SOL | LEST 
&°280 $°L90 | L°6OL } 9°8L0 T'F6O | 8°GEL | GOIL | P'OPL | G°SSL | O'ESL | O°FFL 
L'L20 L°SS0 | €°GOL | L°€L0 6880 | LUSL | O'FOL | GLEL | L°SLL | F'SbL | 99ST 
0610 1060 | ¢°s60 | L°890 £°280 | F°2SL | O'L60 | Lat |; BOLL | BSL | 9°93 
8°990 ¢'9PO | L°880 | 8°E90 F°9LO | O9'SIL | L060 | O'EIL | SOL | SPSL | S°LIL 
L"190 O"ePO | S*ISO | G’8c0 9°0L0 | 6°FOL | 2°80 | 8°GOL | F'FEO {| L°FLL | F'SOL 
g*990 F680 | 0'°SLO | O'FSO L°$90 | 1°960 | 2°9L0 | O°LOL | 9°980 | O°SOL | #°660 
¥1S0 8'Se0 | G°S90 | L’GtO | 8'080 | 8’scO | F°L8O0 | $690 | S°L6O | L°8L0 | ¥"s60 | ‘060 
“TAOT ‘dy “TMOG ‘da “uMog “da "uMog “dq “amog ‘dn “umog ‘d 
oot 08% 00% oS oO of ee 


—yo sadojs 20y ‘|RzUOzL0Y UO seoed Jo aquinag juepeaInby 


poinsveta saved jo sequin 


:Surpuaosep pue Fur>ueose ‘sadojs yuasepip uo peoed soouejsip jo JBIMOZOY 0} BOTJONPsY *17 


RECONNAISSANCE. 29 


Table VIII gives directly the horizontal equivalents of the distances usually 
occurring in foot pacing. If desired, other distances may be obtained by combina- 
tions, i‘ 


From 1 to 9, take ‘the first figure, left-hand cipher included, of 100 to 900 for the 
whole number and the second figure for the tenths. 


From 10 to 90, take the first two figures, left-hand ciphers included, of 100 to 900 
for the whole number andthe third figure for tenths, 


For 290 take 100 + 190; for 440 take 140 + 300, etc. 
Example: For the horizontal equivalent of 738 paces on a 5° rising slope, 
700 + 30 + § = 632.8 + 27.1 + 7.2 = 667.1. 


28. Pacing mounted,—The average walk of a horse ig a mile in 16 mins., or 384 
miles per hour, making 120 steps, covering 110 yds. per min., the step being 0.916 of 
a yd., or 33 ins. 


The average trot is a mile iu 8 mins., or 714 miles an hour, making 180 steps, cov- 
ering 220 yds. per min., the length of step being 1.22 yds. or 44 ins, 


It will generally be found more convenient in pacing, both on foot and 
mounted, to count the steps of one foot only, and multiply the number counted by 
the stride of one foot, which is twice the length of step given above. In this case, 
the number counted is doubled for use with the tables and scales given herein, 


Timlng.—Counting the steps of a horse diverts the attention more than is desir- 
able, and it is better to determine distances in mounted reconnaissance from the times 
occupied by the horse in passing over them. The rating is done by ascertaining the 
time required to pass over a known distance. Time and step ratings should be taken 
together by counting and timing at once. Ratings should be taken before the 
reconnaissance, if possible, but for short stretches of hasty work, the averages given 
above may be used without serious error. See par. 28a, p. 27. 


Horses travel better in pairs, and two men should be sent out together, one to do 
the sketching and the other to give his entire attention to taking the time and keep- 
ing his horse at a regular gait. It is better to rate the pairs together. If it has not 
been done, take the rate of the timer’s horse. 

When a sketclier is traveling with a party and must keep their gait, an occasional 
count of his horse’s steps for a minute or two will give a special scale for use in 
plotting. , 


29, The speed of a horse over road grades, even in moderately hilly countries, ia 
not affected by the slope sufficiently to make an allowance necessary. Distances up 
and down grades measured. by timing in mounted reconnaissance will require no 
correction except that to the horizontal, Table XII, which may be applied if the 
slopes exceed 5° or 6°, This statement does mot apply to distances measured by 
mounted pacing or counting the steps of a horse. 


30. The walk is the normal gait for reconnaissance.—If greater speed is 
necessary, the timer may go on while the sketcher is taking angles and plotting; 
the latter taking the trot or the gallop and overtaking the timer just before he 
reaches the next station, This method should be used only when the required dis- 
tance can not be covered at a walk. 


If circumstances require short distances to be covered at a trot or gallop, the times 
may be reduced to walking time by multiplying by 2 for the trot and 3 for the 


gallop. 


81, The odometer is an instrument for recording the number of revolutions of a 
wheel, The adopted form is in a leather case, 44g ins, in diameter by 244 ins. 
thick, figs. 6,7,and 8. It ia attached by straps to the front wheel of ‘a wagon, fig. 6. 
To read, the case is opened, the registering train, fig. 7, withdrawn, and the 
number of revolutions read from the scale. Multiply the diameter of the wheel by 
3.1416 for the circumference; multiply the circumference by the number of revo- 
lutions for the distance traveled by the wagon. 

The bearings of the odometer must be kept free from grit and may be oiled with 


fine oil used sparingly; gummy oils or grease must not be used. If good oil ia not te 
be had, rub the bearings with a soft lead pencil. 


30 ENGINEER FIELD MANUAL. 


Odometer readings are valuable as a rough check on a day’s march. They are not. 
accurate, but are free from large errors. ‘Two instruments on the same wagon will 
not always agree. On heavy roads, mud or sand, there is a slip, sometimes positive 
and sometimes negative. 


Tase IX. 


32. Number of revolutions per mile, of odometers attached to wheels 36 ins. 
to 48 ins, diam.: 
Diam. of wbeel: Revolutions. 


36 inches__. 
37 inches 


48 inches____._------__-___~~----~------------- -------. 


Sizes of wheels of some military wagons: Ambulance, 3614 ins.; ponton (light) 
tool and chess, 425¢ ins.; escort, 4434 ins.; ponton (heavy) 45 ins.; army six, 4714 ins, 


33. Estimation of distances is a knack which may be cultivated by practice to 
a degree of accuracy far beyond that which is at first attainable, and quite sufficient 
for the location of many objects off the traverse line. Short distances are more closely 
estimated than longer ones; those on a level, than those up or down hill. When 
the intermediate ground can be seen, the estimation will be closer than when it can 
not. 


A rough estimate of distance may be made from the velocity of sound, as by know- 
ing the time that elapses between seeing and hearing the discharge of a gun, or the 
fall of an ax. Note the time in seconds and multiply by £00 for the distance in yds. 


Distances across water are usually underestimated. The distance of the visible 


horizon on water in miles is 1.225 VE; ZH being the height of the observer above the 
water surface in feet. : 


A cartridge or other small heavy object fastened to a string 10 ins. long and al- 
lowed to swing through a small angle or arc will beat half seconds approximately. 


34, The location of a point by intersection is done by taking azimuths to it 
from two known points. As each of these azimuths when plotted must pass through 
the unknown point, it must be at their intersection. 


An observer at an unknown point may locate himself from two visible known 
points by taking an azimuth to each. From the known points plot the correspond- 
ing back azimuths and they will intersect at the point of observation. This process 
is called resection. It is subject to errors of local attraction. (Par. 9.) 


The accuracy of a location by intersection is affected by the relation of the azimuths 
and of the distances. The greatest accuracy results when the azimuths differ by 90° 
or 270° and the distances are equal; in which case the two azimuths and the base 
form a right-angled triangle. A difference of azimuths of less than 30° or more than 
330° should be avoided. 


Errors in length of the base, or distance between the known points, affect the 
distances in the same proportion. If the base is 5 or 10% in error, both the dis- 
tances will be in error in the same direction by the same percentage. 


Distances are most easily determined from intersections by plotting the points and 
scaling. The distances are horizontal. If gradients are taken at the same points as 
the azimuths or at one of them, the elevation of the unknown point may be deter- 
mined after the distance has been scaled, 


RECONNAISSANCE. 31 


35, Tape-chains are adapted for the accurate measurement of considerable dis- 
tances. The tape-chain isa steel tape detachable from the reel on which it is car- 
ried, and with a handle at each end. It is graduated in feet, the last foot to tenths 
and the last tenth to hundredths. ‘ 


Metallic tapes are of linen with wires woven in longitudinally. They are grad- 
uated in the same way as tape-chaing, and also in feet, incbes, and eighths. Metallic 
tapes are used for the exact measurement of short distances, as dimensions of build- 
ings, lengths of bridges, etc. They stretch slightly, but not enough to introduce 
appreciable error. 

In using tapes note carefully whether the small divisions are inches or tenths 
of feet. See that the first graduation is the proper distance from the end, and 
if the tape has been spliced note whether the graduations on either side of the splice 
are the right distance apart. 


Rules are used for measuring sbort distances and dimensions and are usually 
graduated in feet, inches, and sixteenths, fig. 45. 

Rules approximately correct may be improvised in several ways. If a rule or rod 
graduated to feet be grasped in both hands, palms down, with the outside edges of 
the hands at consecutive foot marks and tbe thumbs extended toward each other 
along the rule, the tips of the thumbs will meet or pass, and by carefully noting 
their relative positions a foot may be approximately reproduced at any timo by 
grasping a stick in the hands, placing the thumbs in the proper position, and mark- 
ing the outside of the hands. A length may be measured in feet by passing along it 
hand over hand, placing first the edges of the hands together and then the thumbs 
as described. 

Every military topographer should know the length of his shoc, his exact height, 
and the length of his forefinger. A copper cent is 34 in. in diameter. 

It is impracticable to adopt and adhere to one system of graduation. 
The decimal system is most convenient for computation. For field measurements 
an observer will make fewer mistakes with the system he is familiar with and should 
be allowed to use it. 


, The following table will convert units of one system into those of the other: 


Tass X. 


36. Table for conversion of inches and sixteenths into decimals of a foot 
and the reverse. "The quantities in the table are thousandths of a foot. The deci- 
mal point is omitted. 


wa olafalalsfala|alalalafa]e als |a 


000 | 005 | 010] 016 | 021 | 026 | 031 | 036 | 042 | 047 | 052 | 057 | 062 | 068 | 073 | 078 
083 | 089 | 004 | 099 | 104 | 109 | 115 | 120 | 125 | 130 | 135 | 141 | 146 | 151) 156 | 162 
167 | 172 | 177 | 182} 188 | 193 | 198 | 203 | 208 | 214 | 219 | 224 | 229 | 234 | 240 | 245 
250 | 255 | 260 | 265 | 271 | 276 | 281 | 286 | 292 | 297 | 302 | 307 | 313 | 318 | 323 | 328 
333 | 339 | 344 | 349 | 354 | 369 | 365 | 370 | 375 | 380 | 385 | 391 | 396 | 401 | 406 | 412 
422 | 427 | 432 | 438 | 443 | 448 | 453 | 458 | 464 | 469 | 474 | 479 | 484 | 490 | 495 
500 | 505. | 510 | 516 | 521 | 526 | 531 | 536 | 542 | 547 | 552 | 557 | 562 | 668 | 5731578 
583 | 589 | 504 | 599 | 604 | 609 | 615 | 620 | 625 | 630 | 635 | 641 | 646 | 651 | 656 | 662 
667 | 672 | 677 | 682 | 688 | 693 | 698 | 703 | 708 | '714 | 719 | 724 | 728 | 734) 740 | 745 
750 |:755 |'760| 766! 771 | 776 | 781 | 787 | 792 | 797 | 802 | 807 | 813 | 818 | 823 | 828 
833 | 839 je 849 | 864 | 869 | 865 | 870 | 875 | 880 | 885 | 891 | 896 | 901 | 906 | 912 


HP OODARTAWNHO 
» 
= 
5 


ee 


917 | 922 | 927 | 932 | 938 | 943 | 948 | 953 | 958 | 964 | 969 | 974 | 979 | 984 | 990 | 995 


382 ENGINEER FIELD MANUAL. 


TaBLe XI. 


37. 16ths of an inch in decimals of an incn: 


vs | fe | os | to] te | | oe | oe | He | te | ie | 48 | a8 | te te 


063 | 125.| 188 | 250 | 313 | 375 438 | 500 | 563 | 625 | 688 | 760 | 813 | 875 938 


38. Reduction to the horizontal.—Distances measured along a slope may re- 
quire a correction before plotting them on a map, as all map distances are, or are 
supposed to be, measured in a horizontal plane. Such corrections, when made, are 
called reduction to the horizontal. The following table givos horizontal dis- 
tances corresponding to sloping distances for gradients up to 30°. This table is to 
be used in the same way as Tables II and III. The correction for slopes of 6° and 
less is too small to be plotted on the customary scales and is nsually neglected. In 
flat or ordinary rolling country, the correction will rarely be necessary. 


TasLeE XII. 


39. Horizontal distances for gradients of 0° to 30° corresponding to distances 
on the slope: 


3 3 Horizontal distances for sloping distances of— 

we 

ae : 

Sa] hb 2. 3. 4. 5. 6. 7. 8. | 9 
1] 09998 | 19997 | 29995 | 39994 ; 49992 | 59991 ; 69989 | 79988 | 89986 
2] 09994 | 19988 | 29982 | 39976 | 49969 | 69963 | 69957 | 79951 | 89945 
3 | 09986 | 19972 | 29959 | 39945 | 49931 | 59918 | 69904 | 79890 | 89877 
4 | 09976 | 19951 | 29927 | 39902 | 49878] 69854 | 69829 | 79805 | 89781 
5 | 09962 | 19924 | 29886 | 39848 | 49810 | 59772 | 69733 | 79695 | 89657 
6 | 09945 | 19890 | 29836 | 39781 | 49726 | 659671 | 69616 | 79562 | 89507 
7] 09925 | 19851; 29776 | 39702.) 49627} 59553 | 69478 | 79404 | 89329 
8 | 09903 | 19805 | 29708 } 39611 |, 49513 | 59416 | 69319 | 79221 | 89124 
9} 09877 | 19764 | 29631 | 39507 | 49384 | 59261 | 69138 | 79015 | 88892 


10 | 09848 | 19696 | 29544 | 39392 | 49240 | 59088 | 68936 | 78785 ; 88633 
12 | 09781 | 19563 | 29344 | 30126! 48907 | 58689 | 68470 | 78252 | 88033 
14 | 09703 |] 19406 | 29108 | 38812; 48515 | 58218 | 67921 | 77624 | 87326 
16 | 09613 | 19225 | 28838 | 38450) 48062 | 57676 | 67288 | 76901 | 86513 
18 | 09510 | 19021 | 28532 | 38042 | 47653 | 67063 | 66574 | 76084 | 85595 
20; 09397 | 18794 | 28191 | 37688 | 46985 | 56381 | 65778 | 75175 | 84572 
22 | 09272 | 18544 | 27815 | 37087 | 46359 | 55631 | 64903 | 74175 | 83446 
24 | 09135 | 18271 | 27406 | 36542 | 45677 | 654813 | 63948 | 73084 | 82219 
25 | 09063 | 18126 | 27189.| 36252 | 45315 | 54378 | 63441 | 72505 | 81568 
26 | 08988 | 17976 | 26964; 35952} 44940 | 53928 | 62915 | 71903 | 80891 
27 | 08910; 17820 | 26730 | 35640 | 44550 | 53460 | 62370 | 71280 | 80190 
28 | 08829 | 17659 | 26488 | 35318 | 44147 | 62977 | 61806 | 70636 | 79465 
29 | 08746 | 17492 | 26238 | 34985 | 48731 | 52477 | 61223 | 69969 | 78716 
30 | 08660 | 17320 | 25981 | 34641 | 43301 | 51961 | 60622 | 69282 | 77942 


The hor. dist. corresponding to any sloping dist., and any angle or gradient may 
be found by multiplying the sloping distance by the cosine of the angle, Table XIV. 


40. The protractor is an angular scale of equal parts used for plotting azimuths. 
That adopted for reconnaissance is the rectangular form, figs. 12 and 13. It is 


RECONNAISSANCE, 33 


graduated on one face, which will be called the A face, fig. 12, from 0° to 180°, and 
on the other, or B face, fig. 18, from 180° to 360°. The graduation is clockwise on 
both faces. It has a scale of inches and tenths along one edge. The protractor may 
be used ag ruler, acale, triangle, and parallel ruler. 


To plot a given azimuth from a given point, draw a meridian through 
the point. If the azimuth is less than 18°, lay the protractor down A face up with 
the center at the point and the edge on the meridian, 0° to the north. Make a 
pencil dot on the paper at the proper graduation on the edge of the protractor. 
Move the protractor so that one of its edges passes through the two points and draw 
a line, which will be the desired azimuth. 


If the azimuth is more than 180°, lay the protractor down B face up, 360° to the 
north, and proceed ag before. The moving of the protractor after setting off the 
angle and before drawing the line may be avoided by adding a counter=clockwise 
graduation to the protractor. The sum of the two graduations at any point will be 
180°, Place the center of the protractor and the given azimuth, read on the 
counter-clockwise graduation, on a meridian, and slide the protractor up or down, 
keeping the two points on the meridian until one of the long edges passes through 
the given point, when the azimuth may be drawn along that edge. 

A semicircular protractor is shown in fig. 14. It is usually double graduated, in 
opposite directions from 0° to 180°. With this form an azimuth may be laid off and 
the line drawn along the diameter without moving the protractor. Lay the pro- 
tractor down with the center on a meridian. If the azimuth is less than 180°, place 
ita number of degrees on the counter=clockwise scale on the meridian north of the 
center, fig. 15, If it is greater than 180°, subtract its number of degrees from 360 
and place the difference on the clockwise scale over the N. end of the meridian, 
fig. 16. In either case slide the protractor up or down, keeping the center and the 
graduation on the meridian until the diam. passes through the point, when the 
az. may be drawn along the diameter of the protractor. Fig. 17 shows a triangle 
graduated for use as a protractor. 


41. Improvised protractors.—If a rule is at hand, a protractor may be made 
as described for slope board in par. 14 by extending the 1° graduations around a half 
or whole circle. If without compasses, measure off the radius on a piece of 
paper, stick a pin through one extremity for a center and a fine pencil point through 
the other extremity and sweep the circle. 


If without a rule, fold a piece of paper carefully through the middle. The 
folded edge should be straight. Place the ends of the folded edge together and fold 
again. The two edges now make an angle of 90°, Fold again through the middle 
and the angle will be 45°. Now fold in three parts and the angle is 15°. Spread 
the paper out flat and the creases will represent radii of 15° intervals. These may 
be divided into three equal parts by the eye, and the protractor will then read to 5°. 


The hour graduations of a watch are 30° apart, and the minutes 6°. 


42, The scale of a map is the ratio between dimensions on the map and the 
corresponding dimensions on the ground. If the lengths on map and ground were 
expressed in the same unit, the scale ratio would always be expressed by the number 
of ground units corresponding to tho map unit. If 1 in. (map) corresponds to 
120,000 ins. (ground), the ratio, or scale, is plainly 1+ 120,000, or as usually de- 
scribed, 1 to 120,000. This fraction is called the representative fraction, and 
designated R. F. But ground distances are so much greater than map distances that 
they are ordinarily expressed in a larger unit, which makes the scale ratio less 
apparent. If 1 in. (map) equals 10,000 ft. (ground), the scale is still 1 to 120,000 
because 10,000 ft. equal 120,000 ins. The map unit is aliost always inches. 
Hence, a good rule for obtaining the scale ratio is to reduce the given number of 
ground units to inches, which will indicate the ratio. 


Another method of stating scales, much employed in military map making, ig to 
take ratios which will give 4, 1, 2, 3, 6,12, or 15 ins. on the map to 1 mile on the 
ground, and call the scales 4, 1, 2, 3, 6, 12, or 16 ins. to the mile, Such scales can 
be put into terms which express the ratio by dividing 63,360, the number of ins. in 
1 mile, by the number of ins. given in the scale. Thus, 1 in. to1 mile equals 1 + 
63,360; 2 ins. to 1 mile equals 1 + 31,680; 3 ins. to 1 mile equals 1 -+- 21,120, ete. 


87625—09-———3 


Reconnaissance. 12-17. 


RECONNAISSANCE. 35 


The scale ratio is true for all units. If a scale ratio is 1 + 9,600, 1 in. (map) 
= 9,600 ins. (ground); 1 ft. (map) = 9,600 ft. (ground); 1 meter (map) = = 9,600 
meters (ground), etc. 


‘When the scale of a map is changed, as by reduction or enlargement, the ‘R, F. 
changes too, and hence the ratio should not be given on maps which are to be repro- 
duced. A finear scale should be drawn on eyery map. This will be enlarged 
or reduced with the map and will always be true. Such a scale is also very con- 
venient for taking distances from the map. It consists of a straight line divided into 
equal parts which are numbered with reference to the relation between distances on 
the ground and distances on the map. The nuipbers relate to distances on the 
ground and the graduations, or lengths sect off on the line, relate to distances on the 
map. A distance on the map equal to that from the zero of the scale to any gradua- 
tion corresponds to the distance on the ground represented by the number of that 
graduation. Scales are designated by the unit of their parts, as scales of miles, 
scales of feet, scales of meters, etc. « 


A scale might be constructed by drawing a scale of inches on the map and placing 
opposite the divisions the numbers expressing the equivalent ground distances. It 
is customary, however, because more convenient, to take the ‘numbers at intervals 
of 10, 100, or 1,000, or multiples of them, and make the divisions of the line corre- 
spond, A scale should be divided into a convenient number of equal parts called 
primary divisions. The zero should be between the first and second primary 
divisions, counting from the left. The primary divisions are numbered from the 
zero to the right. The primary division on the left of the zero is subdivided into 
smaller parts, called secondary divisions, and these are numbered from the zero 
to the left. The secondary are usually } or 7g of the primary divisions. 


To take off any distance from such a jeale, put one leg of the dividers on the 
primary division next below the distance sought, and the other leg on the secondary 
division corresponding to the remaining figures. 


Figs. 18 and 19 give scales for the usual range of topographic maps, which may be 
taken off on the edge of a strip of paper and transferred to amap. Jig. 20 gives 
scales for plotting distances measured by pacing on foot, and fig. 21 for those by 
pacing mounted. 


Scales may be constructed on strips of paper, wood, celluloid, or metal instead of 
on the map, and are then culled plotting scales. The scales given in figs. 18-21 
are plotting scales. A distance may be taken between dividers from any map and 
read by applying the dividers to the proper one of these scales. 


These scales are not engraved and can not be relied upon within 14. They are 


sufficiently exact for reconnaissance and, in fact, for most topographical (lrawing and 
scaling. 


Reconnaissance. 18. 


POR. F.= 7h =8:33 to 1’ 633.6 to 1 mile. 


10987654321 0 10 feet. 
SS ee ee 


9 


2 RF. =p 10 to = 528’to 1 mile. 


1098765438210 10 feet. 
eGSet =o — Od —— a =} 


3° R. Fe <5=41.66 to 1126.7 to 1 mile. 


50 25 0 50 feet. 
— = ——Sj 


ay = 50' to 1=105'6 to 1 mile. 


50 feet. 


R, F.= ahp=352 to 1” —= 15” to 1 mile. 


0 E 200 yds. 
SSS 


R. Fx sqgp=440" to 1’ = 12” to 1 mile. 


100 200 yds. 
= 


7® R. F.= yh = 833.3 to 16/34 to 1 mile. 


100 0 100 200 300 400 500 yds. 
HERR =F ———— ; ——} 


E 


8* R. = aij 880 to 1” = 6" to 1 mile. 


100 0 200 300 400 500 yds. 
CR AEN I a OE a | 


36 


Reconnaissance, 19, 


OR. F=zghgg - 1666.7 to 13717 to 1 mile. 


100 0 500 1000 yds. 
a 


10°R. F=ayhgg = 1760 to 1'—= 3:00 to 1 mile. 


1000 500 1000 yds. 
{ERE eer 


1 R. Fsqigg = 4400’ to 1’= 112 to 1 mile. 


1000 Q 1000 2000 yds. 


X 


TOR. F= sr = 5280' to 11:00 to 1 mile. 


1000 0 1000 2000 yds. 
ECR CE EO 


13° R. F=ag¢799-10560' to 10150 to 1 mile. 


1000 0 1000 ~=2000 3000 4000 5000 yds. 
[oR S00 a OO | 


14 R. F= ee =52800' to 1.10 miles to T° 
109876543210 10 miles. 
| 


15°R. F= =132000' to 125 miles to 1: 


1 
1584000 


10 30 40 miles, 


20, 


5 


Reconnaissance. 


OOFE 


SEW f Ol 
saoed jo ayeos 


00% 
id 

OT 
oT 


SIN L OLE 
saoed jo ajeog 


0081. 


o 


000T 


fe) 2 wz 


= s 8 


aed jo ajeos 


alIW 1 1% 


saced jo a/29g 


HW 1 O29 


38 


{ 


2 


Issance, 


Reconna 


SW L Olt. 


“UTUE GT FT SLOTS 9 FSO & 


aIW Lore 


= 

3, 
Q 
= 
a 
© 


shear 


06 


—-6E-—— 


8 


Min. to 1m 


g F & 6 
OW 1 OL% 


T O%T 


AN 11g 


40 ENGINEER FIELD MANUAL, 


43. A series of points connected by azimuths and distances is called a traverse, 
and the operation of determining the azimuths and‘distances is called traversing. 
The latter term is usually extended to include all azimuths, distances and eleva- 
tions taken while running such a line, 


A traverse line with elevations along it may also becalled a profile, and when the 
traverse is run for the express purpose of taking the elevations, the operation is 
called profiling; and the line on the ground and the plot of it on paper, are called 
profiles. 


Distances in topography are so much greater than elevations that both can not 
conveniently be represented on the same scale. It ig usual to take a scale for eleva- 
tiong called the vertical scale, much larger than the scale of distances, or hori= 
zontal scale. The ratio of the two scales is called the distortion or exaggera- 
tion, Ten or 20 ft. to the in. isa common scale for elevations. If the horizontal 
scale is 8 ins. to the mile, the resulting distortions are 176 and 88 times. Both scales 
should always be written below every profile. 


Angles on a distorted profile are also distorted, and gradients can not be plotted or 
read with an ordinary protractor. ‘ 


Angles can be plotted or read on a profile by any of the other methods of express- 
ing gradients, par. 11 and Table I, The horizontal distance is plotted to the hori- 
zontal scale and the corresponding vertical distance to the vertical scale. A special 
protractor may be made for any given distortion and used to plot and read angles 
directly on a profile having that distortion. ‘To make such a protractor, lay offa 
distance of 100 to the horizontal scale. At one end of it erect a perpendioular, and 
lay off on this, from the intersection, distances corresponding to 1°, 2°, 3°, etc., 
Table I, col. 2. These distances must be laid off 10 the vertical scale.: Draw lines 
through the points on the perpendicular and the other end of the horizontal line. 
These nee represent the angles on the profile corresponding to the slopes on the 
ground, 


44, Fieldwork.—Measurements and additional notes may be recorded and affer- 
wards plotted on a map, or may be plotted on a map as taken, or the two operations 
may be combined, as circumstances demand. A written report also will often be 
reqnired, 


45, A road sketch consists of a map of the road with a narrow belt of country on 
either side. If roads, parallel and intersecting, are not too far apart, the road 
sketches may be combined into a fairly good map of the entire area, ‘ 


The road itself will, if practicable, be traversed with the degree of precision 
already indicated as required for topographical reconnaissance. If the country is 
open, so that long sights are possible, a trained observer will get better work by the 
use of the prismatic compass and clinometer. For shorter courses, when the object 
is of sufficient importance to use a chain for distances, the prismatic compass and 
clinometer should also be used and the readings taken with the greatest care. 


Usually, however, the box compass will be used for azimuths and the slope board - 
for gradients, or else the sketching case, to be described later, par. 54, 


Side features will, if important, be located by intersection; otherwise by estima- 
tion. A convenient method is to estimate the distance of an object when it bears at 
right angles to the course, and plot it.from that point. In such case the azimuth 
will be denoted by R or L. Thus, house 800 R would mean a house at a distance of 
300 units to the right, on a line at right angles to the course through the point 
where the observation was taken. 


46. Traversing with compass and notebook.—Rule a column 34 of an inch 
wide down the center of each left-hand page of the notebook. Select for the starting 
point some object or point which can be identified by description. Standing at this 
point, sight with the compass toward some object—tree, stump, telegraph pole, or 
stone—that will serve as the second station of the traverse line. Note the reading 
of the compass and record it in the center column of the notebook, at the bottom of 
the first left-hand page, making also the symbol for@1. Observe and record also 
the azimuths of any other objects which are to be located from @ 1. All the ob- 
servations taken at this station are written in order in the central column from the 
bottom upward and are bracketed together with the station symbol. The name of 


RECONNAISSANCE. 41 


each object is written on the same horizontal line with its azimuth, on the right side 
of the page if on the right of the traverse, and on the left side of the page if on the 
left.of the traverse, If elevations are to be obtained, observe the gradients from 


@ 1 to ibe several objects and place each in the notebook next to the corresponding 
azimuth. 


Proceed toward © 2, counting paces. Halt when necessary to sketch and measure 
offsets to objects on either side of the course, to take bearings of intersecting roada, 
paths, streams, etc. When a halt is made, a mark is scored on the ground, the dis- 
tance in paces from the last © recorded in the central column and the desired notes 
made. Distances along the main line, azimuths, and gradient angles only are 
recorded in the central column. Al) descriptive matter relative to side objects is 
placed outside of that column on the side corresponding to that where the objects 
lie. Return to the scored mark and resume the pacing, beginning with the number 
recorded at the halt, so that the total count of paces at any point shall be the number 
-taken since leaving the last ©. 


The center column of the page is taken to represent the line actually paced and to 
be without width, so that offsets in the side sketches are shown measured from the 
sides of the column and not from its center. 


* On reaching the second ©, record its distance from ©1; draw a horizontal line 
across the page; write © 2 in the center column above the line, and continue as 
before to ©3. 


It is well at ©2 to take a back azimuth on @©1. This should differ from the 
azimuth of © 2 from @1 by exactly 180°. A marked discrepancy indicates error in 
observation or the effect of local attraction on the needle, and should be investigated 
before proceeding. If a back azimnth is taken, it should be the first observation 
made and recorded. 


When opportunity offers, take bearings on distant bends of the road, spires, towers, 
hilltops, tall trees, etc., and enter the angles in the center column with the name of 
each object written beside its bearing. Endeavor to get bearings of the same dis- 
tant object from several stations or from two stations at some distance apart. These, 
when plotted, should intersect at a common point if the observed bearings are cor- 
rect and the compass has not suffered local disturbance. It is not to be expected in 
work of this grade that an exact intersection of more than two bearings can be 
obtained except by accident. 


When a sketcher at any point of the traverse finds himself in prolongation of a 
line that defines or bounds a feature of the country, such as a fence, the edge of 
a wood, a reach of shore line of river or lake, a gully, canyon, or.ridge, a face of a 
building, or a stretch of road or railroad, its bearing should be taken. The same 
rule should be observed when important features come into range with each other 
from a point on the traverse, A valuable check on the relative positions of such 
features is thus obtained. 


If a traverse line is interrupted by any obstacle that interferes with the measure- 
ment of distance, its width should be estimated and the pacing resnmed on the other 
side; or, for greater exactness, make an offset, perpendicular to the traverse line if 
possible, long enough to clear the obstacle, continue the traverse parallel to the 
original course and return to the latter after passing the obstacle by a second offset 
parallel and equal to the first and in the opposite direction; or, locate points on the 
farther side by intersections. 


47. The unit of measure should be clearly stated in the notes. Ordinarily 
distances along the course are in paces, while estimated offsets may be in paces, feet, 
yards, or fractions of a mile, according to their distances, and also according to the 
unit in which the sketcher finds he cau make the closest estimate. 


On the usual recounaissance scales, the dimensions of buildings, widths of roads, 
bridges, etc., can not be plotted to scale. They are shown exaggerated, and the 
true dimensions, if important, must be given in figures. 


48. The best method of plotting is to plot the traverse lines and the check 
bearings first. Then any error discovered by means of the latter, or by closure on 
the initial or other known point, can be more readily corrected. When the traverse 
line has been’ adjusted, the details on either side are plotted in and do not have to be 
changed. 


42 ENGINEER FIELD MANUAL. 


The outfit desirable for the method of traversing with compass and notebook is 
the following: Notebook or sheets of paper ruled as described, prismatic or pocket, 
compass, pencil of medium hardness, rubber eraser, pocket knife, 25 ft. tape, a piece 
of twine 100 ft. long. The absolute necessities are the paper, the compass, pocket 
knife or pencil sharpener, and rubber-tipped pencil. The tape measure is to be used 
for making small measurements of distance or dimensions. The cord is useful for 
measuring depths of water, heights of structures, etc. It should be graduated to 
yards by knots. 


49. The topographic field notebook is designed to facilitate the foregoing 
method of traversing. In addition to the central column, it has columns on either 
side in which to record the offset distances, each of which is put down on the proper 
side of the central cblumn, avoiding the necessity of using the letters R and L, and 
eliminating the liability of mistakes in confusion of the direction. 


The opposite right-hand page is ruled in 1 in, squares, and has a full-circle pro- 
tractor graduated to degrees printed on it. This page facilitates a hasty plot of the 
traverse with respect to which many details can be sketched in more clearly and cer- 
tainly than they could be recorded in writing. At the bottom of the page are scales 
of tenths and eighths of inches. The alternate pairs of pages are plain ruled for 
notesand memoranda. Figs. 22 and 23 show the arrangement and illustrate the use 
of the book described, 


50. Traversing with compass and drawing board.—The observations are 
taken as in traversing with a notebook and compass, but the traverse line and such 
offsets a3 come within the limita of the sketch are plotted at once; that is, the map is 
drawn as the observer proceeds over the ground. A great advantage of this 
method is that any large error in measurement ia likely to be detected by the eye, 
as the map is compared with the ground, and errors can be corrected on the spot. 
The plotting scale of equal parts should be prepared beforehand to suit the scale of 
theimap. If thia scale can be pasted or drawn on the edge of the protractor opposite 

- the angular graduation, it is a convenience. 


The sides of the sheet of paper should be lettered N, EH, 8, and W to correspond 
with the points of the compass. If the ‘paper is ruled or water-lined, the lines are 
taken parallel to the magnetic meridian. 


Having observed the azimuth at © 1, draw through the point designating that 
station a line having the observed azimuth, Azimuth lines are erased finally asa 
rule, and hence should be lightly drawn and with a fairly hard pencil, Prolong this 
line in the direction of © 2 far enough to surely reach that @. If other azimuths 
are taken at © 1, plot them also, and note on each the object to which it bears. If 
the distance to the object is estimated, it may be laid off on the azimuth and the posi- 
tion of the object plotted at once. 


Proceeding toward © 2 totake any desired side shot, haltabreast of the object, plot 
the distance from © 1 on the course, estimate the distance to the object, and plot it 
in at that distance opposite the point plotted on the course and on the proper side, 


Arrived at © 2, lay off the entire distance from © 1, and plot and mark @2, Erase 
the azimuth line beyond © 2; take and plot any other desired azimuths. If any of 
them are to points previously sighted to, make the intersections and plot and mark 

. the points, In plotting azimuths to side objects, itis better to draw only a short part 
of the line near the object to avoid confusion of lines on the sketch and especially 
near the stution. 


61, The following outfit is desirable for traversing by this method: A thin, 
smooth board 12 x 15 ins. to which the paper is attached by thumbtacks or 
rubber bands, prismatic or pocket compass, clinometer or slope board, a rectangular 
protractor, a plotting scale, lead pencil, No. 3 or 4, rubber eraser, 25 ft, tape, 100 ft. 
of twine, watch, pocket knife, canvas cover for board and paper, notebook. A. field 
glass is also very useful. Good work can be done with a less elaborate outfit, or with 
improvised arrangements for some of those mentioned. The drawing board may be 
utilized as a slope board. . 


52. A road sketch will be long and narrow, and two or more stretches should 
be got on a board if possible. In this way a board of the size indicated will hold a 
fair day's work. When a section runsoff the paper mark it with a letter, as A, and 
make a note, Continued at B, Mark the beginning of the next section B and write 
Continued from A. : 


22. 


Reconnaissance. 


g & lofsed : 
LeemarksLefe. Var pase oe Fremarhs Let ght. 


Crossed ae Toad. | 


fuming 


Crowe ae Cr 


on 72_\700. 2380) Farm £Z____]| 
| Crele_. S800 


ALL H 


Cr 28. Crossed ary Cr. 
Tossed wagonrroad| 
Crt_2 rossed dry Cr__| 


A.f2.Pr. 700. 


a —— OT 
Jae ROOM Par rr LL. 


rossed is: veil 
PULI2 IAG — 


Creek 30 wide\|4¥40 oo 
Woods along Cr 


. /}-—————_|—_ 300 | 
Polling Prairi @ 
Pasture | {2250 


aslere- 


Lari lt. 770_\| 800 
farm £1. 


ASO Crossed wagon road. 
= Larm fF. 
(5 Le. 

3 {__ 


orn ¢ Wheat 


Creek, ary ai eae G28 


efewagon road. 
Cue le. | 
Larm lt. 29. 0. Loeve Go wih eee 


aPrrr2 


\Lefthehena O30 AM. A fei lowing wagon read. 
Sepe. 42h, 1900. 


42 distances in Fals. 


Feginning 


23. 


Reconnaissance. 


Ves 


@Q 
qa 
“Be 


ae 


RECONNAISSANCE, 45 


Wherever else a road runs off the map, make a marginal note * To + 
miles,” giving the name and distance of nearest settlement or conspicuous topograph- 
ical feature. If the road crosses one parallel to the main route, write also “ To 
crossing, miles.’ 


53. Traversing with oriented drawing board.—A drawing is said to be ori- 
ented when so placed that its true meridian is parallel to the true meridian on the 
ground. When using magnetic azimuths, making the magnetic meridians—map and 
ground—parallel, may be accepted asa proper orientation. Whena map is oriented, 
with any given point vertically over the corresponding point on the ground, a ruler 
held on the point or station on the map, and pointed in the direction of any object 
gives the azimuth of that object on the map. No angular measurements need be 
made. A compass is not necessary, hut it is very convenient as it affords the quickest 
means of orienting the map. 


To run a traverse by this method, assume on the map the initial point and 
the magnetic meridian, selecting them so that the general direction of the traverse 
will coincide with the longest dimension of the paper. Place the board over the first 
station; lay the compass on it with the north-and-south line parallel to the assumed 
meridian, and turn the board until the needle reads north. The board is then ori- 
ented, and inust be in this position whenever a sight is taken, It should also bo level, 
as nearly as can be determined by the eye. 


Place a ruler on the station point of the map and sight it in the direction of any 
object which it is desired'to plot. Draw a line along the edge of the ruler and on it 
lay off to the adopted scale the distance of the object if known or assumed. When 
all the desired azimuths have been taken from the station, sight the ruler to the 
second station and draw its azimuth, and then proceed to that station, pacing the 
distance. Arrived at the forward station, plot the paced distance, orient the hoard 
over the station, and proceed as before. If any of the objects taken at the first sta- 
tion can be seen from the second, new azimuths may be taken to them which will 
locate them by intersection, fig. 24. If no compass is at band, orient the hoard 
arbitrarily at the first station, and at the second station orient it by placing the rnler 
on the line between the two, and sighting back to the station Just left. Fig. 25 shows 
the relative positions of board and ground at four successive stations. 


54. Traversing with sketching case.—The sketching case is a compact device 
for traversing by the oriented-map method. The simplest form issued to the serv- 
ice, usnally called the cavalry sketching case, is shown in fig. 26: The compass is 
set into the board, and a movable index is provided which can be revolved to place. 
it parallel to the assumed meridian on the map. When the needle is brought paral- 
lel to the wire the board is oriented. The needle may he parallel to the index wires, 
but end for end, or 180° out of its true position, in which case the sketvher is turned 
completely around. Sach a mistake is so great and so obvious that it needs no pre- 
veutive, but a sketcher may note at the outset whether the N or S end of the needle is 
toward the stud which moves the wires and keep it in this position. 


The ruler A is pivoted to a slide, moving in a slot in the radial arm B, pivoted in 
turn to the board near the compass. The screw C clamps the ruler and slide with 
respect to the arm, and the screw D clamps the arm on the hoard. The combination 
permits the ruler to be set on any point of the board and on any direction through 
that point and clamped thero. 


To facilitate road sketching rollers KK are provided, on which 30 or 40 ins. of 
paper may be placed. The paper should roll on and off the undersides of the rollers. 
If the traverse runs off either edge of the paper draw a meridian through the last 
station, roll back the paper until that station is off the board, plot the same station in 
anew assumed position with a new meridian through it, and continyo the sketch. 
When the paper is cut and one position of the station placed over the other with the 
two meridians coinciding, the two parts of the sketch are in their true relative 
positions. 


By clamping the rnler with the stud F engaged in the notch G, loosening the 
screw D and holding the board in a vertical plane, the case may be nsed as a slope 
board. The tops of the roller screws HH form a sighting line, and the angle is read 
from the left edge of the arm, on the scale across the bottom of the board. See par. 
S40, p. 92, 


Reconnaissance, 


{Position at91 


\ ~ 
\ 

\ 

\ 

\ 

\ 

} 


see 
oe see 


ee are atO4 Fig. 25. 


Position at\Og 


Traversing by plane table and Resection 


‘ 


46 


Reconnaissance. 26-27. 


2 TO 1 MILE 20 FT, Vale 
r £° 37 4915' 


Fig. 27, Bower Sketching Case 


48 ENGINEER FIELD MANUAL. 


55, Another form of sketching case is shown in fig. 27. The radial arm of the 
cavalry case is replaced by two sliding motions at right angles to each other, which 
permit the compass to be placed over the pivot end of the ruler and bring it directly 
under the eye when aligning the ruler. Several minor details are worked out to 
promote convenience and accuracy of use. These advantages are secured at some 
sacrifice of simplicity and compactness, and this form of case will not stand as much 
rough usage as the cavalry case. 


66. Improvised instruments.—By the oriented-map method a very good sketch 
may be made with improvised instruments. Any smooth surface on which lines will 
show will answer for the board and paper, The edge of a book, an envelope, ora 
piece of paper. carefully folded makes the ruler. A narrow strip of paper folded 
double several times makes a scale of equal parts, See par. 56a, p. 69. 


57. A road reconnaissance should procure data‘on the following subjects: 


The road.—Gradients, especially the steepest; width of roadway; if paved, width, 
kind, and condition of paving; width and depth of side ditches, and whether wet or 
dry; if not paved, character of soil, sand, clay, or gravel; kind of fences and width 
between them. ‘The sketch should also show where the road is in embankment or 
cntting; where wagons can not double or pass, and where foot troops can not march 
along the side between the wagon track and the fences, 


Bridges.-—Material of piers and abutments; type and material of superstructure, 
as girder, truss, arch, suspension, wood, steel, stone, etc.; width of roadway, and 
clear headroom; safe load (see Bridges). Of bridges over the road, clear width and 
height; over streams, the nearest bridges above and below and whatever information 
can be obtained about them. 


The country.—Character of cultivation or natural vegetation; areas and density 
of timber, underbrush, vines, especially poisonous ones; marshes and fords, kinds of 
fences, nature of soil; general configuration of surface, especially high hills, long 
ridges or valleys, bluffs or slopes too steep to scale, and practicable rontes to their 
crests. : 


Streams crossed.—Name, width, depth, and surface velocity in swiftest current; 
velocity noted as sluggish, moderate, quick, or swift; elevation of high-water marks 
in relation to the road; which bank is the higher at crossing and above and below, 
and how much; accessibility of water for stock; fords at or near crossing; length, 
depth, and steepness of approaches; levees or embankments, height, and thickness 
on top; if navigahle, to what distance above and below and for what class of veasele— 
steamers, flatboats, rowboats. 


Towns and villages passed through.—Name, location on map, and population. 
Names of streets to be traversed. Material, as stone, brick, frame, log; size, 1, 2, 3 
stories, and distribution, close or scattered, of the houses in those streets; gradients 
of intersecting streets; location of railway depots, post, telegraph, and telephone 
offices; of drinking fountains and watering troughs; of elevators, storehouses, or 
other accumulations of food or forage; of blacksmith, wagon, and machine shops. 


When ordered to make a complete examination of a town or village, note 
besides the foregoing, location and size of principal buildings, balls, court and 
school houses, churches, banks, jails, and their ownership; sources, maximum 
quantity und distribution of water supply; sanitary conditions and disposal of wastes; 
location of railroads, depots, freight houses, sidings, etc.; for ali roads entering from 
the surrounding country the same information as scheduled above for streets; loca- 
tion and extent of open spaces, and of large substantial buildings standing apart; 
location and extent of high ground within range, especially that from which streets 
can be enfiladed. 


Railroads crossed.—Name, gauge, single or double track, sidings and loading 
platforms at point of crossing; crossing at grade, over or under; distance and name 
of nearest station each way; direction aud distance of nearest roundhouse, shops, etc. 


58. River reconnaissance.—Designate the banks as right or left, the right bank 
being that on the right hand when looking down the stream. If, when standing on 
the bank facing across the stream, the current flows from left to right, the observer 
is on the right bank; if from right to left, he is on the left bank, 7 


RECONNAISSANCE, 49 


Tf the stream is navigated, pilots and residents will know distances by channel 
between landings with sufficient accuracy for the purposes of a field reconnaissance. 
In making a traverse along the banks of the river, it may be desirable to cross from 

,one side to the other to save distance or avoid obstacles. When a crossing is to be 
made, at two or three stations from the point of crossing select a point on the other 
side and take an azimuth toit. From the last station take another azimuth to the 
selected point, locating it by intersection. If the conditions prevent an intersection, 
take an azimuth from the laat station to the point on the opposite bank and estimate 
the distance. 


The valley.—General configuration, heights of limiting ranges, and positions of 
passes or roads crossing them; commanding’ ground from which a stretch of the 
channel of considerable length can be enfiladed by artillery; forest growth on or 
near banks; soil and cultivation of the valley; roads parallel to river, and means of 
access to them from the river. 


The stream,—lIts width, depth, and velocity; navigability, as for steamboats, 
flatboats, rowboats, rafts, and head of navigation for each; nature of obstructions to 
navigation and possibility of removing or avoiding them; season of high and low 
water; average rise and fall; rapidity of rise and fall and causes; amount of drift; 
character of banks and relative command, Quality of water; amount and kind of 
sediment borne; usual period and thickness of ice, 


Tributaries and canals.—Width, depth, navigability, and means of crossing. 
Nature and purpose of canals; dimensions and lifts of locks; time for lockages; 
means of destroying locks and effect of destruction; floating plant found, 


Bridges and fords,—As in road report. Also for bridges note position of the 
channel and navigable width between piers; height of arches and lower cbords 
above the water at different stages; dimensions and operation of draw spans. Note 
the exact position of fords and the marks on both banks by which they may be found; 
length, width, and nature of bottom; velocity of current; position of deep holes; 
aids to crossing. Fords should not be more than 4 ft. 4 ins. for cavalry, 344 ft. for 
infantry, and 2 ft. 4ins. for guns and ammunition. Note nature of approaches to 
bridges and fords; width of roadway, slopes, soil, effect of weather and traffic. Note 
especially the defensibility of bridges and fords, 


Ferries, boats, and other means of crossing.—Position of ferries; approaches 
and practicability for horses and loaded wagons; sizes, number, and kinds of boats; 
method of propulsion; sites for military bridges or ferries; character of site for con- 
struction, use, and defense; proximity of islands and tributary streams; approaches 
and slope of hanks; width of river and maximum surface velocity of current; mate- 
rials for the construction or repair of boats, bridges, or ferries. 


Inundations,—Places suitable for inundations by damming or obstructing a nar- 
row bridge span, or by cutting a levee or dike. Note raised roads on ground liable 
to natural or artificial inundations and the safest route to follow by known land- 
marks when the road is overflowed. An extensiye inundation 2 ft. deep on level 
ground is a serious obstacle unless the roads are very sound and marked by trees, 
posts, etc. Even when so marked a dip in the roadbed of 3 or 4 ft. may render the 
road impassable. A railroad bed is soon washed out even by a slight overflow. 


59. Reconnaissance of a railroad.—Theline, Local name; terminal points and 
distances between stations and other points; gauge; single or double track; condition 
of roadbed, ties, and rails; drainage and liability to overflows or washouts; facilities 
for repair; condition of right of way for marching troops along the line. 


Tunnels and bridges.—Number and location; dimensions; strength of bridges; 
means of destroying and repairing; of blocking traffic. : 

Rolling stock.—Number and nature of engines and cars availgble; capacity for 
transporting troops between given points; facilities for constructing armored traina, 
as spare rails, old boilers, etc.; location and capacity of shops and store yards, 

Stations.—Name and location; facilities for entraining and detraining troops 
with wagons and horses; platforms on through line and sidings; ramps; side tracks, 
number and capacity; turntables; water tanka; fuel supply; storage facilities; der- 
ricks or cranes; cross-overs for teams and pedestrians. Facilities at hand for hos- 
pitals, camps, depots; for feeding men, heating coffee, watering horses during 
temporary halts, 


87625—09——4 


50 ENGINEER FIELD MANUAL. 


Other communications.—Telegraph lines; number and location of stations; 
number of wires; connections; parallel highways, roads, rivers, or canals; means of 
access from same to railroad; junctions and crossings of other lines; relative eleva- 
tion; facilities for laying temporary switches and sidings at stations or between 
crossing lines. 


Defensibility.—Heights commanding line of road; defense of stations; defense of 
road and telegraph lines against raiding parties; structures exposed to demolition; 
defense and attack of same; defiles and river crossings. 


60. Reconnaissance of a wood or forest.—Note all roads and paths, and all 
hills, ravines, and streams within the wood or skirting the edges; kinds of trees, 
density and growth; underbrush, prevalence of poisonous shrubs and vines; marshy 
or large open spaces; practicability of forming new roads by cutting; creation of 
obstacles by felling trees; if there are no roads traverse the shortest practicable path 
between the point of entrance and point of exit, and mark boulders or blaze trees, 
set stakes, or otherwise indicate this path, and also give compass bearings of the 
route to be followed. Note the exterior forms of the woods, whether parts of the 
edge flank other parts; connection with neighboring pieces of wood by scattered 
trees or clearings; undulations of the ground that would give cover to attacking 
force or to defenders. 


61. Reconnaissance of mountains.—Note the number and positions of passes 
through the mountains, of roads and trails leading to these passes, their condition, 
practicability and means of repair; steepness of slopes on the sides of roads; means 
of constructing additional roads; water courses, their direction, nature, and time of 
floods; means of crossing. Note ravines and open glades on mountain sides, lookout 
points, and good signal stations; note time and duration of snowdrifts on roads or 
passes; depth of drifta and possibility of removing them or of traveling on the surface 
of the snow. Note extent and nature of forest growth. 


62. Reconnaissance for a camp or winter quarters—Site.—Location, eleva- 
tion, and area; sanitary features, such as drainage, dryness, and general character 
of top soil; proximity of swampy ground or stagnant ponds. 


Communications.—Sufficiency of existing roads and paths, maximum grades, 
probable condition under heavy traffic and in bad weather, location and kind of 
materials available for improvement or repair, railroad or water communication and 
terminal facilities of same. 


Waiter and fuel.—Location, kind, and quantity of fuel at hand; quality and 
quantity of water; faeilities for filling water carts, for watering animals and for 
wasbing and bathing; nature of supply, as wells, springs, running streams, and its 
reliability. 

Shelter and conveniences,—Proximity of trees, brush, wood, hay, and straw 
for huts and bedding; of markets; of towns and villages. 


Defensibility.—Location of outposts and guards; location and character of 
defensive positions in or near the camp; force required to hold positions which may 
command the camp. 


63, Reconnaissance of a position.—This problem usually includes the selection 
of the position, and is therefore tactical as well as topographical. Certain relations 
and conditions must be observed in the selection, and the extent and degree in which 
they are found must be clearly shown on the map or in the report. 


The length of the position, or its development along the firing line, should be 
proportional to the force available for its occupation. Exact rules can not be given, 
but 5,000 infantry per mile or 3 men per yard is the usual estimate. . 


The flanks n@ust be secure. Impassable natural features, a river, mountain, 
or stream form the best flank. Lacking these, a wood, a deep ravine, a cliff, ora 
high hill will serve. Even with these features absent a flank may be strengthened 
by the construction of a strong earthwork, but the general rule obtains that natural 
weakness of the flanks must be made up by a greater number of men, or by the sub- 


stitution of cavalry for infantry in case the ground favors the movements of mounted 
troops. 


If the flanks are naturally strong the line should be withdrawn to make the 
entire position reentrant; if the flanks are naturally weak the connecting line 
should be held straight or advanced so as to make the position straight or salient, 


RECONNAISSANCE. 51 


The depth of the position, or its extent in rear of the firing line, should afford 
natural cover for supports, reserves, and trains, which may require a total depth of 
800 to 2,400 yds.; but a short position may be relatively shallower than a long one. 
Three or four parallel ridges, 300 to 600 yds. apart, with the intervening grouud 
practicable, form an excellent position. If the first ridge is somewhat higher than 
the rest,so much the better. Whatever cover there may be for the component parts 
of the force, whether natural or artificial, fences, ditches, trees, etc., should be shown 
oF aes aoa If digging is necessary, its amount and the character of the soil should 

e stated. : 


Strong points in front of the line, which may be occupied as outposts, should 
be shown. 


Communication should be free in every direction, concealed so far as possible 
from the enemy’s view. ) 


Artillery positions are required when that arm is represented in the occupying 
force, as will usually be the case. They should permit the guns to sweep all ground 
in front of the position over which the enemy can advance, to the limit of effective 
range, Every point in front of the position and within range which commands any 
part of it, is an element of weakness. 


Ranges at which the enemy can be seen and reached by artillery fire; the points 
beyond rifle range covered by such fire and its relative command of adverse artillery 
positions should be shown or described. 


If possible, similar information should be obtained of the ground likely to be occu- 
pied by the euemy in forming for attack, or in taking up counter position. 


64. A position occupied by an enemy must be reconnoitered from a distance, 
and few details can actually be seen. Valuable inferences may be drawn by remem- 
bering that the enemy has probably chosen his position in accordance with the prin- 
ciples above given. 


Hepecial attention should be given to the flanks and the feasibility of turning one 
of them. 


65. A position sketch will usually be on a scale of 6 ins, or 12 ins, to the mile. 
It will be.found most convenient and expeditious to make it by the compass and 
drawing-board method, par. 50, or the method with oriented board alone, par. 53. 
The traverse will include the fewest points from which the entire area can be seen, 
often only two, and all other features will be located by intersections from these 
points. Elevations may be taken by slope board or clinometer, the height of the 
first point occupied being arbitrarily assumed if not known. 


If two points can be found which overlook the area in front of them and which 
are also visible from each other, the compass may be dispensed with except for 
a meridian. Measure the distance between the two points. Assume the position of 
one of the points and of the line joining them, so as to bring the desired area on the 
paper. From the first point lay off on the line the distance between the two points 
to the adopted scale and plot the second point. The line joining the two is called 
the base, and will be near one edge of the board, if all the area to be mapped is on 
one side of the line, or toward the middle if it is on both sides. 


Place the board over the first point; lay the ruler along the base and turn the 
board until the ruler points to the second point. Keep the board in this position 
and point the ruler successively to the objects to be located, drawing the lines as 
explained in par. 53, Gradients are written along the corresponding azimuths. 
One gradient should be taken to each point determined. 


Proceed to the second point. Lay the ruler along the base and point it to the first 
point. Point the ruler to the objects to be located, marking where it crosses the line 
to the same object drawn from the first point. 


66. Contouring isa method of exhibiting relief of ground by means of lines so 
drawn on a map as to indicate points of equal elevation. The lines so drawn ona 
map and the corresponding lines on the ground are called conteurs. The word con- 

‘touring is applied to the field work directed especially to obtaining data for drawing 
contours, 


52 ENGINEER FIELD MANUAL. 


The difference of elevation of points in adjacent contours is called the contour 
interval, and is usuaily constant for all the contours on the same map. The hori- 
zontal distance between contours, measured in a radial direction with reference to 
the curvature of the contours will be referred to as contour distance, 


The theory of contouring is that no inadmissible error will be made by supposing 
the slope of the ground from a point in one contour to the corresponding point in the 
next, or along the contour distance, to bea straight line. The less the contour inter- 
val, the Jess error will be made. If in fig. 28 the curved line 4B represents theactual 
surface of the ground, and points 1, 3, 5, the elevation of successive contours, the 
broken line 1, 3, 5, will represent the assumed ground surface, and its departure from 
the line AB is the error introduced. If now the points 2, 4, and 6 are also determined, 
or the contour intervals be reduced one-half, the assumed slope is 1, 2, 3, 4, 5, 6, which 
differs less from the Jine AB than the line 1, 3; 5, and hence introduces less error. 
With points determined at very short intervals the error is practically eliminated. 


If contour distances decrease with elevation, or the contours become closer as 
they go higher, the slope is concave, and points between contours are lower than 
the straight line joining corresponding contour points. If the contours become closer 
as the ground falls, the ground is convex, or lies ahove the straight line joining cor- 
responding contour points. <A point of inflection, or change from convex to con- 
cave, is at the point where the contour distance is less or greater than those on either 
side of it. Equal contour distances correspond to uniform slope. 


67. One contour does not necessarily join all the points of the same elevation on 
the map but only those which have a continuous series of points of the same eleva- 
tion joining them. It may require several contours to take in all the points of a given 
elevation on the map. Parts of the same contour will appear as separate when the 
ground over which they could be connected is not‘on the map. The selection of the 
points to connect in one contour is the difficult part of the process and can not be 
done correctly without thorough knowledge of the principles of the method and a 
good idea of the general shape of the ground to be contoured. In military recon- 
naissance only enough elevations can usually be taken in the field to guide one who 
has seen and studied the ground in drawing the contours. No one who has not seen 
and studied the ground should be expected or permitted to draw contours from such 
data. Erroneous information may be worse than none at all. 


68. For equal contour intervals the map contours are closer together as the slope is 
steeper. It follows that for steep slopes the map contours will approach each other 
very closely, and for a vertical wall or cliff they will coincide. 


Ground contours can not cross, but map contours may cross in the very unusual 
case of a cave or a bluif overhanging by an amount which can be shown on the bori- 
zontal scale. This is so rare that it is usual to say that map contours can not cross. 


Every contour must close upon itself in a loop or else must extend unbroken from 
one point on the margin on the map to some other point on the margin. An excep- 
tion is made in the case of large streams, the contour on each bank beiug carried 
upstream until it cuts the water surface when it is dropped. The two ends must be 
directly opposite, fig. 29. In asmall stream or dry bed, the contour crosses at the 
point where the elevation of the bed is that of the contour, fig. 30. 


Maximum ridge and minimum valley contours go in pairs. A single lower 
contour can not He between two higher ones, or a single higher between two lower. 
When two adjacent contours have the same elevation, the ground between 
them will be still lower if they are valley, or still higher if ridge contours. 


69, Contours are designated by their heights above a datum plane. The height is 
expressed in feet, except when the metric scale is used, when contour intervals are 
in meters. 


The elevation of each contour should be shown in figures at points close enough 
together to allow the eye to run from one to the other with ease, It is best to break 
the contours and write the numbers between the ends. If written alongside, the 
numbers should always be on the higher side of the contour, figs. 31 and 32. 


_70. Straight contours are very rare. They may be determined by locating any 
two points, or by locating one point and observing the azimuth of the line, 


54 ENGINEER FIELD MANUAL. 


Simple curved contours are more frequent than straight ones, but are not often 
found of any considerable length. They may be determined by fixing 3 points; or 
by 2 points with the radius estimated; or by 1 point with the center assumed, 


The typical contour is a wayy line, alternately salient and reentrant, and may 
be determined with the precision needful for hasty reconnaissance by fixing the 
extreme points of the convex and concave portions. 


71. Looking at contours from the higher side, the salient parts, or those concave 
to the observer, correspond to the ridges, and the reentrant parts, or those convex 
to the observer, to the valleys. The valleys are also lines of drainage. Hence, haif 
of the points necessary to determine a wavy contour will lie on drainage lines, as 
indicated by rivers, creeks, brooks, and rivulets, and by ravines, or other depressions 
dry at most seasons. 


The slope of a drainage line grows less in the direction of flow. Tributaries, or 
branches, are usually steeper than the main stream at their junction, and also 
increase in slope toward their sources. Generally, in a limited area, the sources will 
be at nearly the same elevation. To apply this principle in increasing the amount 
of topographical relief that may legitimately be drawn from a given number of 
known elevations, let fig. 33 represent the drainage lines of an area taken froma 
civil map. Suppose the ground to have been studied and elevations to have been 
determined at 2 points, A and B. How much topography can be drawn? 


The 110 ft. contour will be above the 105 ft. and by a distance somewhat less than 
the length AB, because the slope becomes steeper and the contour distance less in 
going upstream. The succeeding contours at 10 ft. intervals will cross the tributary 
at gradually decreasing distances, as indicated, and for the same reason. The source 
is found to be about 130 ft. Take the other sources to be also 130 ft., and draw the 
contour at that level, remembering that it is concave where it crosses the streams, 
and that the part between the streams is convex and advanced, Lay off the contour 
points on the other stream lines, keeping in mind the law of slopes, and draw the 
other contours, following the same rule as for the first, 


72. If enough elevations were taken on stream lines the concave parts of the 
contours would be fairly well determined, but the convex points would still be in 
part uncertain. It is known that they are convex and salient, but not how much. 
This information is supplied by elevations taken along the ridges, crests, or divides 
which lie between adjacent drainage lines. The typical profile of a crest is a reversed 
curve, flat and couvex between the sources of streams, flat and concave near the 
junctions of streams, and steepest in the middle, with the inflection at the steepest 
point. The form of crests is not so regular as that of valleys, and less use can be 
made of it. It should be kept in miud as a basis of comparison, so that actual forms 
can be more readily remembered. 


73. The field work of contouring an area which has a sufficient relief to exhibit 
drainage lines clearly may begin by traversing these lines, with gradients taken by 
clinometer or slope board. It is most convenient to begin where collected drainage 
leaves the area to be mapped, and follow each valley to its source. 


If the valley is open and the flanks of the ridges on each side can be seen, time 
may be saved by taking level sights from some of the contour points on the drainage 
line to points on the ridges as far advanced as possible, usually where the line of 
aight is tangent to the hill. This gives two points, a a, fig. 33, near the apex of the 
salient from which the contour may be drawn often as well as by a point at the apex. 
If this can be generally done, it may not be necessary to run out the ridges, Notes 
should be made of the apparent shape of the contours near the drainage line, whether 
sharp or blunt, or whether the valley is narrow or wide. The general shape of the 
sky line of the ridge or its projection against higher ground should be noted when- 
ever a lateral view of it cau be had. 

If hill points can not be taken from the valley traverse, the ridge lines must be 
run out. Tbey must be connected in plan (distance and azimuth) and in elevation 
with the drainage lines. When drainage and ridge lines are plotted on the map, the 
contour points, if not actually observed, may be interpolated and the contours drawn. 


The symmetry of adjacent contonrs is obvious from the inspection of any contoured 
map, and this relation may be utilized where one contour has been well determined, 


-34, 


33 


Reconnaissance. 


ff 


f 


| 
ie YA as j 


ye 


53 


V 


5 


tA 


56 © ENGINEER FIELD MANUAL. 


to draw the one on either side of it from a very few pojnts, often but one. If the 
contours are wavy, they will generally be a little farther apart at the concave and 
convex points than at the reversion points between them. If the contours are not 
wavy, they are generally parallel, 


74. If the relief of the ground is so slight that the drainage and ridge lines are 
uncertain, the field work of contouring is best done by taking elevations at points 
arbitrarily selected Such points will usually be in straight lines running in the 
general direction of the steepest slope. The points are plotted’on the map, the cor- 
responding elevations written near them, and the contours are interpolated as indi- 
cated in fig. 34, assuming that the surface of the ground between observed points 
is astraight line. The closer the points are together, the less error is involved in 
this assumption. . 


If the country is comparatively flat and unbroken, profiles may be run along roads 
and paths, and contours sketched in on each side so far as they cah be seen: Then 
by going over the intervening ground and observing its shape, the portions drawn 
can be joined with the eye with sufficient accuracy. 


In towns and villages profiles along intersecting streeta and the study of the inter- 
vening space furnish data for approximate contours. 


75. Slope equivalents.—Actual distances between contours on a map depend on 
the contour interval, the scale of the map, and the gradient. For any given map 
the contour interval and scale are constant and the distances between contours 
depend on tbe slope alone. On any map with contours at equal intervals each 
gradient has its corresponding contour distance, which is called its equivalent. A 
line subdivided to show the equivalents of various gradients on any map is called 
a scale of slope equivalents for that map, or simply the scale of slopes, and by 
applying such a scale to the distance between two successive contours the slope of 
the ground between them may be read off. 


For different maps slope equivalents vary with the ratio between the contour inter- 
val and the scale. A scale of slope equivalents may be constructed for a given ratio 
and will be true for all maps having that ratio, no matter how much the scales may 
vary. The ratio may be taken as the fraction of an inch on the scale of the map cor- 
responding to the contour interval. If the scale of the map is 500 ft. to the inch 
and the contour interval 1 ft., the ratio is $5 or 0.002, which is the fraction of an 
inch corresponding to 1 ft. on a scale of 500 ft. totheinch. If the scale is 1,000, 5,000, 
10,000, or 60,000 ft. to the inch, and the corresponding contour interval is 2, 10, 20, or 
100 ft., the ratio in each case is 3, and the contour interval corresponds to 0.002 in, 
on the scale of the map and a scale of slope equivalents corresponding to the ratio 
applies. 


Fig. 35 contains scales of slope equivalents for ratios of 535 t0 goy5, Which will cover 
the usual range. : 


Lo get any desired scale of slope equivalents from the figure, divide the number of 
feet in the contour interval by the number of feet per inch of the scale, or divide the 
number of inches in the contour interval by the denominator of the R. F. The result 
is the ratio. Place the straight edge of a piece of paper horizontally on the diagram 
and passing through the corresponding point on the ratio scale on the left of the 
figure, and prick off the scale. 


Slope equivalents afford a convenient and rapid method of determining contour 
points on any line of a map the gradient of which is known, 


Any fraction of the equivalent for any slope corresponds to the same fraction of 
the contour interval. If an end of the line is on a contour, the slope equivalent may 
be stepped off along the line aud each point so determined will be # contour point. 
If the end of the line is between contours, measure off on the line the part of the 
slope equivalent corresponding to the rise or fall to the next contour point. From 
this step off the slope equivalent as before. If a fractional distance remains at the 
end of the line, find what part of the slope equivalent it is, and add to or subtract 
from the last contour elevation the corresponding part of the contour interval for the 
elevation of the end of the line. To illustrate: If in fig. 36 elevation at @ of the line 
ab ig 103, gradient + 2°, the contour interval 10 ft. and the slope equivalent cd, then 
the rise to the next contour is 110 —-103=7 ft. or 7; of the contourinterval. Seven- 
tenths of cd=ae, and hence e is the Position of the 110 ft, contour point. Lay off 


35-37, 


Reconnaissance. 


SOl}el JO BjBIS 


58 ENGINEER FIELD MANUAL, 


ef, fg, and gh = cd and locate the 120 ft., 130 ft., and 140 ft. contour points. The 
remaining distance hb = 14 of ed, hence the rise beyond hk = 2 ft. and the elevation 
of b = 142.5. 


76. In the absence of contours relief muy be indicated by hachures, which are 
short parallel or slightly divergent lines running in the direction of the steepest 
slope. Hachures should be used only to indicate areas which present slopes steep 
enough to offer cover or become obstacles, The use of hachnres is illustrated in fig, 37. 


77, The reconnaissance with a moving column will require tho simultaneous 
work of a number of sketchers and must be so organized that each sketcher shal! do 
his full sbare in the time allowed; that the sketches and reporta ghall be turned in 
about the same hour, and that the assigned ground shall be thoroughly covered with- 
out unnecessary duplication. . 


A good sketcher on foot can take about 10 miles of road in a day, or can keep up 
with a slowly advancing eolumn. A good sketcher mounted can cover 15 milesa 
day steadily, or in an emergency 20 or 25, and can keep up with infantry on a forced 
march or with cavalry marching at ordinary rate. 


The reconnaissance for a column should include besides the road traveled the nearest 
parallel road on each side and all connecting roads between them. Each mile trav- 
ersed by the column on the main road will thus involve 214 to 5 miles of sketching. 


If a reconnaissance ig to be made when a force is not in motion, the area to be 
covered will usually be so large and the time allowed so short as to make it necessary 
to combine the work of a number of sketchers. 


78, If any map is available, the area to be reconnoitered should be outlined 
on it and snbdivided into as many parts as there are sketchers, tho parts to be mado 
equal, not in size necessarily, but in amount of work aud time reqnired, the important 
point being that all the parts shall be finished at the same hour. 


Each of these parts is assigned to a sketcher, with full instructions as to the amount 
and class of work to be done, the scale to be used—which should be the same for all 
—and the place and hour at which the sketch must be turned in. If practicable, 
each sketcher should be given a tracing or copy of enough of the map to show the 
boundaries of his own task and the adjacent features of those next to his. 


If there is mo map, the area may be indicated by landmarks, but it will be usually 
necessary, and always desirable, to go over the ground and point out his task to each 
sketcher. When boundaries are definite, there need be very little overlapping. The 
amount of reduplication must increase as boundaries become more vague. 


79. The area to be mapped may be divided np in any convenient way, but it is beat 
to use roads, fences, streams, or other well-defined lines as much as possible. Lack- 
ing these, compass courses passing through well-defined points will answer. 


In a road sketch one man should be assigned to the main road or that on which 
the column is marching. Others will be assigned to such parallel and intersecting 
roads as it may be necessary to map. So far as practicable, side parties should leave 
the main road by an intersecting road, traverse a short stretch of parallel road, and 
return to the main road by another cross road, 


80. Compilation.—The sketches when turned in are consolidated, usually by 
pasting them in their proper relative positions on a large sheet of paper, or else by 
pasting them togetherat their edges eo that corresponding features will join. If one 
of them does not exactly fit, as will often happen, the adjustmeut is best made by cut- 
ting the sketch into two or more pieces and moving them with respect to each other 
go as to absorb the discrepancy. Thus, if a piece of road is half an inch too short, 
cut it at three or four places on lines perpendicular to the road and separate the 
pieces by a sixth or eighth of an inch. If too long, overlap the pieces instead of 
separating them. If a road or other feature is out of azimuth, make a cut through 
one of its ends and swing it into place. These operations may be combined. The 
adjustment is rapid and sufficiently exact. If a sketch is too much out to he adjusted 
by this process, it will usually be of little value and time will be saved by leaving it 
out of the compilation and filling in the gap freehand, using the sketch as a guide. 


Fig. 38 illustrates this method of adjustment. 


38. 


Reconnaissance. 


60 ENGINEER FIELD MANUAL. 


81. Reproduction.—As many copies of the map will be made as circumstances 
may require. The first step is to divide the map into sections of convenient and 
usually equal size, and make a tracing of each. The size of the sections will usually 
be determined by the method of reproduction to be used and the size of the apparatus 
at hand. Time will be saved if there are not more sections than there are men avail- 
able to trace, supposing that all the tracers are of approximately the same speed. If 
one of them can work two or three times as fast aa the average, two or more sections 
should be reserved for him, the idea being that the work will be done in the shortest . 
time if so arranged that al! finish at once. 


With fairly expert sketchers, it will be possible to have each ink his work before 
turning itin. A useful expedient in case of great haste is to make the sketches them- 
selves transparent by oiling and fasten them together for use instead of a tracing. 


82. The tracing made, further processes depend upon the time available and 
whether the work can be done in daylight or must be done at night. 


Of processes requiring sunlight, the most reliable, simplest, and quickest is the 
blueprint process. 

The prepared paper may be purchased in rolls of 10 or 50 yds. It should be put 
up in tin foil and each 6 or 8 rolls should be in a sealed tin case; it will then keep in 
good condition fora long time. If necessary to sensitize the paper in the field the 
following solutions must be prepared: 


Stock solution A {Wane of iron and ammonia. 


. Red pruasiate of potas 
Stock solution B as 


For use mix 4 parts of A with 3 parts of B. 


Unprepared paper may be purchased in 50-yard rolls. To sensitize the paper a 
sheet of the desired size is cut from the roll and placed on a fiat surface; the mixed 
solution is applied with a sponge to the upper surface in a smooth, even coat, care 
being taken not to wet through to the back of the paper. The sheet ia hung up in 
a dark room until dry, when it is ready for use. Only enough paper for a day’s use 
is sensitized at one time, for it does not keep well. 


The exposure takes from four to eight minutes in bright sunlight, varying with 
the intensity of the light and the transparency of the tracing Under other condi- 
tions than sunlight a much longer exposure is required; sometimes an hour or more,’ 
Care must be taken that the paper is not taken from the frame beforc it has been 
sufliciently exposed. When the margin protruding from under the tracing has a 
greenish-bronze color, open one part of the back of the frame and observe the print. 
The lines should stand out sharp and distinct on a gray hackground. Take the print 
from the frame and place it in a tray containing water sufficient to fully cover the 
print. Rinse it until the lines stand out in clear white, then hang up to dry. It is 
to be remembered that the fresher the paper is the slower it will print and the quicker 
it will wash out;the older the paper is the quicker it will print but the slower it 
will wash. : 


Additions and alterations may be made to blueprints with a 10% solution of oxalate 
of potash used as an ink. If it shows a tendency to run, add a very little mucilage. 
Common soda may be used, but the lines have a yellowish cast instead of the pure 
white which the potash gives, Additions and alterations of a drawing are conven- 
iently made by inking the lines of a blueprint with waterproof liquid india ink and” 
removing all the blue color by the potash or soda solutions. The black lines then 
remain on a white ground. They take well in photographing, and by treating the 
paper with oil, it becomes transparent enough for contact printing, being used in 
place of a tracing and in the same way. 

Brown prints.—Next in point of simplicity for daylight use is the brown-priut 
process, It is in many respects the most satisfactory of the copying processes. The 
paper is purchased prepared. 

After exposure<for about two minutes in bright sunlight, the margin protruding 
from under the tracing turns from its original light yellow to a reddish-brown color. 
The print is then taken from the frame, immersed in water, and thoroughiy rinsed 


RECONNAISSANCE. 61 


on both sides, when the lines come out in perfect white on a sepia-brown ground. It 
is then immersed in a fixing bath made from the salt which accompanies each roll 
of the paper (2 ounces of fixing salt to 1 gallon of water); this makes the print per- 
manent and also darkens the sepia-brown color, tbe lines remaining white. After 
fixing, the print must be thoroughly washed for twenty to thirty minutes and then 
hung up to dry. 

Tbe brown color being impervious to light makes this paper very valuable for 
negatives which.may be used to produce positive copies, either with the: blue or 
brown print papers, yielding an exact reproduction of the original in either blue 
or brown lines on a white background, In making the positive prints from the 
brown-paper negatives the time of exposure is somewhat longer, since the brown- 
process paper is not as transparent as tracing cloth or tracing paper. Jven very fine 
lines of the original are reproduced with surprising distinctness, due to the fact that 
in both manipulations the original is in direct contact with the sensitive side of the 
paper, so that no light can enter sideways under the lines. 


By making several negatives and printing from them simultancously, the rate of 
reproduction may be largely increased. 


83. For printing by artificial light bromide papers are used. A contact print 
from the tracing has clear white lines on a very dark brown ground, The contrast 
is clear and agreeable, Alterations may be made with a sharp red pencil, which 
makes a legible line, or by scratching through the emulsion, which makes a white 
line. A print can be obtained quickly from the light of three candles at 121 ing, 
distance. 


To develope bromide prints make a atock solution of hydrochinon, 150 gr.; 
sodium sulphite, 360 gr.; water, 12 oz. 


Yor use, to 1 oz. of stock solution add 1 dr. rodinal and 8 oz. water; or, make stock 
solution of metol, 150 gr.; sodium sulpbite crystals, 214 oz.; sodium carbonate crys- 
tals, 344 02.; bromide potash, 8 gr.; water, 20 oz. For" use, add 1 oz, stock solution 
to 4 02. water. 


Acetic acid is used to clear bromide prints after development and to stop the actiou 
of the developer, 16.02. water to 1 dr. acetic acid. 


For fixing bromide prints use hyposulphite of soda, 1 0z.; water,6 oz. A little 
alum added to the fixing bath in hot weather hardens the film, 


A bromide print may be made transparent by oil and used for contact printing by 
artificial light. It will be better, though not essential, to secure a paper for nega- 
tives thinner than that usually supplied for prints, 


The cycle of operations for quick reproduction by the bromide process is as 
foliows: 


From a tracing or transparent drawing make, say, 3 to 5 negatives. Make them 
transparent and start printing from allof them. If the sketchers are in by 5.30 p. m. 
the negatives can be ready for printing by 7p. m., and after that priuta can be 
turned out at, the rate of 15 per hour from each negative. It should not be difficult 
to have all that are needed for the next day done by 9 p. m 


84. Transfer processes,—With the hectograph the drawing is made in a spe- 
cial ‘ink and pressed face down on the surface of a gelatin compound in a metal pan. 
When the paper is pulled off the drawing appears reversed on the gelatin surface. 
A piece of blank paper pressed on the surface and then withdrawn shows the draw- 
ing direct in purplish lines. Fifty to 100 impressions may be taken, Each print is 
covered with a thin flim of the compound and is sticky, curly, aud very stubborn. 
The process is at best only a makeshift, but it is the easiest of all to improvise, and 
the simplest to operate. For quick work several pans should be provided, as each 
Toust be washed after use and should not be used again until well dried. 


The hectograph compound is made of— 
Glue or gelatin __. 
Glycerin ___ 


Kaolin, 50 parts, or some fine inert light-colored powder, may be added with 
advantage. The ingredients require prolonged mixing at 200° F., which is best 
obtained in a salt-water bath, 2 oz, salt to 1 pt. water. 


62 ENGINEER FIELD MANUAL. 


The ink is made of— . Parts, 
Nigrosine black _ 2 1 
Glycerin —.---- 

Water ____ 


Writing or drawing is' done with a fresh, clean steel } pen. The surface of the 
compound is moistened lightly with a brush or sponge and allowed to nearly dry, 
when the copy is laid smoothly on face down and rubbed to a ‘good contact through- 
out, eliminating allair bubbles. The paper is allowed to remain two or three minutes 
and then removed by starting one corner and pulling parallel to the surface. The 
sheets for impressiong are put on and removed in the same way, except that they 
are left on but a few seconds. 


With the black autocopyist the drawing is made in a special ink and transferred 
to a parchment sheet held in a special frame. This process is free from some of the 
objections to the hectograph, but it is more difficult to work. The copies are in 
printer’s ink, are permanent, and very satisfactory, 


85, Landscape sketching,—Free-hand sketching can not take the place of topog- 
raphy, but it is a valuable adjunct and should be practiced by every soldier who has 
any aptitude for pictorial drawing. 


Asketch differs from a photograph only in that it shows in sharp outline a limited 
number of the larger and characteristic features easily seen and understood, while 
the photograph shows all details, many of them so minute that they are lost in a mass 
of confused forms, with the form lines, other than the sky line, relatively incon- 
spicuous. All the lines of a perfect sketch exist in a photograph, but close scrutiny 
is often necessary to find them. If sought out and traced, however, a perfect sketch 
results, Tracing from photograpbs is excellent practice. 


The outfit for field sketching should be as simple as possible. A sketchbook with 
a canvas cover, carried in a water-tight case, together with a few lead pencils. B, F, 
and H, and pieces of softand hard rubber are the essentials for satisfactory work. 
For active field work the book should be no wider than can be carried in the pocket 
of a service blouse, and relatively long, say 5 by 9 ins. 


The point of view should as a rule be high enough to give a comprehensive 
grasp of all that is important—a rock, a knoll, a hill, a peak—depending upon the 
conditions. Face toward the middle of the field of view which is determined upon. 
Hold the board or sketchbook vertically before the eye and move it backward or for- 
ward until the sheet just fills the field. Lower the board until the sky line of the 
hills can be seen above its top edge, and with a penci] mark on that edge the points 
corresponding to the principal salients and reentranta of the hillforms. If desirable 
the board can be moved sideways far enough to enable the principal heights and 
depregsions to be marked on tbe vertical edge. By interseeting references the loca- 
tions can then be easily established on the sheet. From these points the forms can 
be sketched in with much greater accuracy. 


Proceed next to draw the hills in outline, but faintly, with attention, to the larger 
curves or humps at first. Go over them again with more care, bringing out the 
small irregularities. If any part of the horizon is visible, draw in lightly, and then 
complete the general mass of hills by drawing the water or base lines. Seek now for 
the surface character of the hills by tracing the ravine lines. The knobs and foothills 
are brought out by tracing the tree meanders that show form. Al] changes in form or 
breaks in the ground produce corresponding breake in the foliage of the tree masses, 
which show in the distance as irregular lines. If the more iroportant of these are 
sought and drawn, the general character of the hill will result. Add now the fore- 
ground crest, and the skeleton of the sketch is complete. 


The road and railroad meanders should follow as a rule, and the fences of the 
fields. Cultivated land is rendered by parallel irregularly hroken lines. Houses, 
fortifications, trenches, etc., will be drawn more or less in detail according to dis- 
tance and importance. Enemy’ 8 lines or trenches even at a great distance should be 
strongly marked by simple black lines. Tbe indication of forests and trees is the 
most difficult feature for students. The indications given in the accompanying 
sketches will show the treatment in outline work, 


Figs. 39 and 40 show a variety of forms sufficient for most localities. 


39. 


Reconnaissance. 


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yy 
4 
ae igs 


aes Si 


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2 ae dg 


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ts 


= CAY \ 
SEG, 


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3040) Uy moewuens ead) 


40. 


Reconnaissance. 


ef CRETE O= ee 
Progese scons ‘ 


SSS gy 19ugu29 


ee ——_ ——, — pee : 


=e le Gere oo <7 


£0 hisag BuypunT N 


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Cee $e 


RECONNAISSANCE, 65 


86. Hydrography.—Depth of water and character of bottom are determined by 
sounding with a pole or with a lead and Jine. The sounding pole may be impro- 
vised, or of permanent form. A convenient one is 10 ft. long, octagonal in section, 
tapering slightly from middle to ends, divided into feet which are painted alternately 
white, and black or red. There should be an iron shoe at the bottom, heavy enough 
to make the rod stand erect when free in deep water. Such a rod is convenient to 
use in water 9 ft. or less in depth. : 


If a sounding lead is not furnished, any compact weight may beused. The sound= 
ing line should be of braided hemp or cotton, % to % in. in diam., and tagged with 
cloth or leather. Tbe tagging will depend on the depth to be measured, and degree 
of precision required. Cloth of different colors may be used for different units, and 
leather tags may be distinguished by cutting notches or punching holes in them. 
The line should he thoroughly wet, stretched, and allowed todry. It should then 
be wet again and tagged while wet. The zero of the graduation is at the bottom of 
the lead or weight. A lead and line are best connected by a rawhide thong passing 
through an eye in the lead and an eye made in the end of the line. 


Soundings are usually referred to a plane parallel to the water surface, horizontal 
except in flowing streams. The plane usuuily selected is the water surface itself if 
stationary, or one of its positions if variable, so that soundings will indicate approxi- 
mately the actual depths of water. The elevation of the water surface in the position 
selected is called the datum level. If the surface elevation varies, a gauge rod 
must be set near the water’s edge, and read often enough to plot a continuous curve 
of water level, The time of beginning aud ending a particular group of soundings 
is noted. The mean elevation of the water surface during that interval is taken 
from the curve, and the soundings are corrected by the difference between the actual 
level and the datum level. If the correction to be applied is less than half a foot, it 
is usually neglected. ‘ 


The material of the bottom, as rock, gravel, sand, or mud, can usually be told 
from the feeling of the rod. or lead when it strikes. A specimen of the bottom can 
be brought up by smearing the énd of the lead with tallow. 


A correct sounding is obtained only when the line or rod is plumb and straight 
and its length correct, or its error known and applied. Except for blunders in read- 
ing the line, only one source of error operates to make the soundings too small, and 
that is a line which has stretched since it was tagged or is too long. All other 
sources of error make the soundings too large, and heuce they are apt to be so, and 
actual depths slightly less than those recorded will usually be found. 


To get a plumb sounding from a boat moving through the water, the lead is 
thrown out or the, pole inclined in the direction of motion far enough to allow it to 
teach bottom by the time the boat is directly over the spot where it strikes. Sound- 
ings taken with a line from a moving boat will always be too large. 


The most accurate soundings with !ead and line in running water are taken from 
® boat floating with the current, with line allowed to hang and move with the water. 
It is raised only a foot or so between soundings, just enough to clear the bottom, 


87. Location of soundings.—The simplest method is by two simultaneous azi- 
muths from known points on shore. If the soundings are taken on a line passing 
through one of the points, all azimuths from that point will be constant, and one 
measurement will suffice. This line is plainly marked by range flags and the boat’s 
crew instructed to keep the flagsin range. Only one instrument und observer are 
required. This is the usual method for streams and is best for all work where the 
soundings can be taken in straight lines. Locations may he made from the boat 
by two observers taking simultaneous compuss bearings to two known points on 
shore—see resection—or by two simultaneous sextant angles. The latter is less con- 
venient, as a special protractor is required for rapid plotting. 


88. The following notation or its equivalent should be made on a map or chart 
containing soundings: ‘‘ Soundings are in feet (or meters) and are referred to the 
stage of water at (location of gauge) at——o’clock, on the —— day of The 
elevation of this datum level is ft. (or meters).’’ If the reference plane is in- 
clined, add: ‘‘and its inclination is ina direction.’ The first blank is 
filled with the rate of fall expressed in any recognized way, and the second with a 
compass bearing. 


87625—09—— 5 


Reconnaissance. & 41. 


RECONNAISSANCE. 67 


89. Map reading is essentially the reverse of map making, In the latter process 
ground is measured and studied with a view of forming a mental picture of how a 
map of it will look. In the former—map reading—a map is measured and studied 
for the purpose of forming a mental picture of how the ground itself looks, All 
rules and principles heretofore stated as to relations between ground and map are 
to be used in studying the relations of map to ground. 


The following suggestions will aid the beginner: 


Note the meridian on the map and associate it in the mind with the local merid- 
ian. This may be done by turning the map so that the meridian will point to the 
north, using the compass as a guide if necessary. If there is no meridian on the 
map, look for indications of direction in local names, or for some road, stream, ridge, 
or other feature the general direction of which is known. 


Note the scale of the map. Estimate certain distances, as the total width or 
total length or distance between prominent points and test these estimates by scal- 
ing. If there is no scale, look for some indications of distance. It may possibly be 
found in local names, as Three Mile Creek, Two Mile House, etc.; roads uniformly 
spaced, as the U. S. land surveys; city blocks, which are usually about 100 yds, on 
the shorter side; railroad stations or sidings, the distance of which may be taken 
from time tables. If the map has parallels of latitude a good scale may be drawn 
by assuming 69 miles to each degree, or 1.15 miles to each minute. If the ground 
is accessible, take two convenient points shown on the map and measure the dis- 
tance between them. , 


If the map is contoured, note the contour intervals and the scale of slope 
equivalents. If the contours are not numbered, decide which are the high and 
which the low ones. Closed contours are much more likely to be elevations than 
depressions, especially if several are concentric, A single closed contour may be 
uncertain, Look for indications of marsh or water inside of it. If the contour 
interval is not given, it will be difficult to get any clue to it unless isolated elevations . 
appear on the map. If the ground is accessible the contour interval may be deter- 
mined by actual measurement of a gradient. , 


Note all topographical and cultural signs, and associate them in mind with their 
advantages or disadvantages for military operations, 


90. A problem frequently arising in map reading is that of determining what 

oints are visible from a given point. A point is visible when tho gradient to 
it, if rising, is greater, and if falling, is smaller than the gradient to any intermedi- 
ate point, ! 


For this comparison gradients are conveniently represented by the quotient of 
distance in ft. divided by the difference of elevation in ft. The point will be visible 
when this quotient is smaller, if rising, and larger if falling, than the quotient for 
the intermediate point. Thus, to determine whether the bridge near the French- 
man’s, fig. 41, is visible from Atchison Hill or is concealed by intermediate ground, 
assume the highest point of Atchison Hill to be in the center of the 1,040 contour 
and to have an elevation of 1,050. The distance from this point to the bridge is 5,610 
ft., fall 250 ft., quotient 22.4, The line of sight from this point to the bridge crosses 
the 960 ft. contour on the flank of Sentinel Hill at 3,060 ft. distance, fall 90 ft., quo- 
tient 34, hence bridge is not visible from Atchison Hill, since the gradient is 
falling, and the nearer point has the larger quotient. 


Working from the bridge the quotient for the whole distance is 22.4, as before, 
put the gradient is rising. The distance from the bridge to the high point is 2,550 
ft., rising; difference of elevation 160 ft., quotient 16, hence, as before, the top of 
Atchison Hill is not visible from the bridge, since the gradient is rising, and the 
nearer point has the smaller quotient. 


If one gradient is rising and the other falling, no computation is necessary. A 
point of rising gradient will hide a farther point of falling gradient, but will not be 
hidden by a nearer one. See par. 90a, p. 78. 


o 
91. Drawing.—The essential requirements of a good topographical drawing are 
accuracy and clearness. By accuracy is meant a faithful exhibit of moccsurements and 
observations made in the field, or of data taken from other maps. Clearness involves 
absence of confusion or crowding, and neatnessin execution. Beauty and pictorial 
effect are obtainable by skilled draftsmen only, and while always desirable, are 


68 ENG-NEER FIELD MANUAL. 


rarely necessary. Persons who are not skilled draftsmen should not attempt picto- 
rial effect, as it will detract from accuracy and clearness without substituting anything 
of equal value. A 


Avoid unnecessary haste in plotting and drawing. If possible, take time to 
check carefully all azimuths and distances plotted and be sure they are exact. , Thera 
should be no approximation on the drawing board, Although an observer may have 
simply guessed a distance to be’ 550 yds. in the absence of other information, the 
plotter should be careful to lay it down at exactly 550 yds. 


Start with clean paper and keep it as clean aa possible In the office, wipe off the 
instruments before using, especially rulers, scales, and triangles. Dust the drawing 
carefully before beginning work. Dust again wben stopping and cover with a cloth 
or paper. If necessary, dust the drawing and wash the hands occasionally while at 
work. 


Make all ink lines firm and very black. A drawing to be made in ink is 
usually drawn first in pencil, and in sucb cases a very hard pencil (4H or 6B) is best. 
If the pencil drawing is to be traced, a softer and blacker pencil should he used, but 
must be kept well pointed. 


Tndia ink in stick form gives the best results, but the time required for proper grind 
ing precludes its extensive use in military field work. The prepared india inke in 
liquid form are ready for use and are satisfactory. They must be kept well corked 
when not actually filling a pen. If the ink gets thick in the bottle so that it will 
not run freely from a fresh-filled pen, add a little water. 


The ruling, or right-line pen, figs. 42 and 43, is best for making lines of -uni- 
form thickness. The points must be kept clean,and when worn must be ground on 
a very fine stone to the form shown and to exactly equal length. The points may 
be closed and the ends shaped together, which will make them identical. Then open 
the points aud grind each on the outside to a proper edge. Right-line pens are set 
to make lines of different thicknesses by the screw D, but the range for any one pen 
is limited,and different sizes of pens are made. A very fine line can not be mado 
with a coarse pen, and it is difficult to make a very broad line with a fine one, The 
points should never touch. If a line made with the points slightly separated is too 
coarse, take a smaller pen. These pens are graded by the length over all. Five 
inches is a medium and useful size. 


Right-line pens may be filled by dipping an ordinary pen in the ink and inserting 
it between the points. A strip of paper closely folded may be used in the same way. 
In the bottles of prepared ink the cork carries a small quill for filling. Take only 
as much ink as can be used in two or three minutes. As soon as the flow becomes 
the least sluggish, the pen should be emptied and refilled. To empty or clean the 
pen, pass a piece of paper (the corner of a blotter ia excellent) between the points. 


The adjusting screw should not be disturbed while working on lines of the 
same thickness. When changing from one thickness to another, open the pen and 
clean more thoroughly. To reset fora given tbickness, draw a short length on a 
scrap of paper and lay it alongside of a line of the desired thickness, previously 
drawn. The difference will be seen, the pen can be changed and another trial made, 
and so on until the lines are matched. 


For ruled lines the ruler or curve is laid in the proper position and the pen drawn 
along the edge, lightly pressing against it. The pen should be held with the plane 
of its points perpendicular to the plane of the paper and in the direction of motion. 
The handle sbould be slightly inclined in the same direction. For free-hand lines, 
as contours, hold the pen in the same way and move the hand so as to cause the 
points to follow the line. 


In ruling with a writing pen, choose one of a size which will make a line of the 
required thickness without pressing on the paper. Dip the point only in the ink. 
If the ruler has a beveled edge place it with the top projecting, A curve or a ruler 
not beveled should be raised slightly from the paper. The pen'should not be inked 
above the point which touches the ruler. It is held as described for the ruling pen. 
Parallel tines close together may be drawn with one setting of the ruler by inclining 
the pon slightly, : 


RECONNAISSANCE. 69 


‘Writing pens are best for stream lines. When it can be done, vary the size ofthe 
pen to suit the thickness of Jine. When using a writing pen free-hand do as much 
of the work as possible by drawing the pen toward the body in about the direction 
of the down stroke in writing, 


For lettering, sigas, and all free-hand work with the writing pen, keep tbe pen 
clean and freshly inked and the ink free from dust and of ‘proper consistency to flow 
freely without dripping from the pen in blots. 


In using a circular pen, fig. 43, set the legs of the compasses so that they will span 
the right distance and the pen point will be vertical. The lead of a pencil point 
should be sharpened to the shape of the ruling-pen points with the flat side toward 

- the pivot leg of the compasses. When using compasses with pen or pencil, incline 
them slightly in the direction of motion and rotate the head between the thumb and 
forefinger. Very slight pressure only should he necessary beyond the weight of the 
instrument. 


Fig. 46 represents the most convenient instrument for measuring the length of 
curved or broken lineson a map. The smal] wheel is run over the line and its length 
in the unit of the instrument is read from the dial. This length is converted into 
actual length by the scale of the map. 


92, Papers.—Manila paper of cream or buff tint, usually called detail paper, is 
suitable for sketches and drawings which are to he traced or used in the field. Only 
the. better grade stands erasing and that imperfectly. This paper comes in rolls 36, 
42, and 54ing. wide, It may be ordered hy the pound or yard. 


White drawing paper may be had in rolls or sheets mounted on muslin or 
unmounted. Whatman’s cold-pressed fine-grain is most generally useful. It comes 
in sheets of names and sizes as follows: Royal, 19 x 24 ins.; Imperial, 22 x 30 ins.,; 

“Double Elephant, 27 x 40 ins.; Antiquarian, 31 x 53 ins. Roll papers are 27 to 63 
ins. wide. 


Sheet papers unmounted and kept flat are best for field topographical use. 


93. If a blot drops on the drawing take a piece of blotting paper, tear a corner or 
edge to expose a fresh surface, and hold it in the blot without touching the drawing 
until the surplus ink is absorbed. Then press a dry blotter firmly on the spot and 
let it dry thoroughly before attempting to erase. A piece of newspaper may be used 
instead of blotting paper, but should be slightly moistened to basten the absorption. 
For a large blot several pieces may be required. 


94.’ Erasers for ink are of steel or rubber. A steel eraser or penknife must be 
very sharp to give good results. An eraser of gritty rubber is most generally used. 
It is best to use an erasing shield of thin metal or celluloid, fig. 44, which exposes 
the area to be erased through one of the openings and protects the rest. 


95. Tracing linen is usually dull back, haviug one side glazed and the other dull. 
Erasing can be done on the glazed side only. The glazed side is used for ink and 
the dull side for pencil work. The glazed side requires preparation hefore use to 
remove excess Of paraffin, which prevents ink from running well and clogs the pen. 
Rubhing hard with fresh blotting paper is the simplest method. 


Tracing paper is alike on hoth sides. It will not erase. Most varieties are lesa 
transparent than tracing cloth. . 

In tracing it is helpful to use a dull-pointed instrument in the left hand—a stylus 
or top of a penholder—to press the linen against the drawing at the point where the 
pen is resting. 


ADDENDUM, 1907. 


56a. A handy device, easily improvised, for attaching a compass toa book or other 
object to orient it for use as a plane table, bas been proposed by Lieut. E. K. Massee, 
Seventh Infantry. It is shown in fig. 602c. The material used should be non- 
magnetic. 


AT 


42 


issance, 


Reconna 


ttt de 


RECONNAISSANCE. val 


96. Conventional signs,—The symbols or signs used to represent topographical 
features are designed to be rapidly made and readily understood, and to resemble or 
suggest the actual features they represent. Multiplicity of signs is not desirable, 
and a verbal designation or description of the features is often more intelligible and 
more quickly recorded. For instance, it is better to write the names of the growing 
crops of a district, as tobacco, corn, or cane, than to cover the entire area with a 
symbol. Another metbod of expediting mapping is to surround an area with 
@ narrow border of the proper sign and leave the middle blank. 


The commonly used signa are given in figs.48 and 49. See also par, 96a, p. 128. 


97. Titles, notes, etc.—Every finished drawing should have a descriptive title, 
consisting of— 


(1) The designation of the organization under whose auspices it is made, as Engi= 
neer Department: Bureau of Insular Affairs, War Department; Division 
of the Philippines; ist Division, 2d Corps. 


(2) Its kind, as map, sketch, plot, plan, profile, section, or elevation. If 
more than one kind of drawing appears on the sheet, each should be mentioned in 
the title, as Plan and sections of battery; Plan, section, and elevations of 
guardhouse, etc. 


(3) Ita subject, if it relates to a particular object, feature, or purpose. 
(4) Its locality.. This and the preceding may be interchanged in position. 


(5) Ita sources, as Compiled from, etc.; Reduced from, etc.; From a sur- 
vey, etc, 


(6) Its authorship. If the work has been done by one person, acting nnder the in- 
structions of another, both should be named, as under the direction of Colonel John 
Doe, General Staff, by Captain William Roe, 1st U. S. Infantry. . 


(7) Its date. 
(8) Its linear scale; its contour interval; its scale of slope eqnivalents. 


Titles should be adapted in size and boldness to the size and importance of the 
sheet. They should be divided into lines, following mainly the divisions just stated. 
The middle letter of each line should fall on a line drawn vertically through the 
middle of the space allotted to the title. Lines should be alternately long and short, 
and if the long lines are symmetrically disposed, the effect is better. 


To prepare a title, write down the matter under the various heads, with proper 
connecting words, and divide it up into lines. Then block out the title, observing 
the division of lines decided upon, and make such alterations as seem desirable. 
Finally, ietter the title on the map. The following is an example: 


Division of the Philippines, | Sketch map { ofa tract of laud northeast of | Zam- 
boanga, | Island of Mindanao, | showing the proposed location of a | cantonment 
of U. 8. troops, | From a reconnaissance by | Capt. A... B_..., | Chief Engr., 
Department of Mindanao, | Jan. 15,1904. | Scale | Contour interval, 20 ft. 


Notes.—Besides the title, such information as wili belp to a proper understanding 
of the meaning and value of the map should be given in the form of notes. These 
usually relate to methods used in the aurvey, datum points, etc. 


Fig. 50 shows the title corresponding to the above example, with notes, 


Meridian.—The magnetic meridian should be shown, and the true meridian also 
if the declination is known, The true meridian may be a line, of 3 ins. or upward 
in length, with a star at its north and the feather of an arrow at ita south end. The 
magnetic meridian may be an arrow crossing the former at the middle point and 
making with it an angle equivalent to the declination. 


Border,—The drawing should be inclosed in a rectangle, preferably with its sides 
N.and§. and E. and W,. The border consists of two parallel lines, the inner one 
medium fine, the outer one medium heavy, with a space between them equal to the 
width of the outer. For geographical maps a double border is used, with space 
between sufficient to contain the numbers of meridians and parallels. 


Reconnaissance, 48, 


Soil and Cultivation. 


3 uu 
abe alee ale 
TELNET: 
pI ATIOES HALAL 
ea e 


Fil a 
Rice swamps 
ditch and dikes. 


Sand Mud and Salt marsh. Fresh marsh Cypress 
and gravel, Tidal Flats. pond, swamp, 


Enclosures, Communications. 
Wire Fence te 


~ Babed Publica, = ST TOTO 


—— 9 —- 0 9 + 0 


Smooth Telegraph 


Rail fence, 


R.R, single track. 


Wooden tence, RR, double track, 


DOD OTANI 00 CASEI G00 
Stone fence. 


Hedge. 


Reconnaissance. 


Military Signs. 


Infantry 
Incolumn Goooooo 


In line —SS 


Cavalry 
In column NG wy 


In line —— | 


Artillery ehgpaba & 


Sentry } Vedette é 


Headquarters S ¢ 


Battle | 


Palisades 


entanglement 


Redoubt 


ee A 


AAAAAA 
Camp AAAAAA 


Trenches eee 


Gun 
battery 


Mortar 
battery 


Abattis =P MPP UP 
Chevaux de Ececesessinsesestieseiea 


frise 


Miscellaneous. 


Dry run 


te 7 


"Arg y 
ot unl ec 
tame Gully 


Mine or 
Quarry 


OR 
{o] Weil 
\A 


Wind Mill 


=m Church 


See 
oteto)|+ t+) Cemetery 
ototol/totet 
+ to+ Jlote+9} 


B.S. 
ML Blacksmith Shop 


gp Wagon Shop 


Grist Mill 


For additignal symbols see figs. 72 to 84, ° 


Reconnaissance 4 50. 


DIVISION ortHe PHILIPPINES. | 


SKETCH MAP 
OraTRACTor LAND Nortueast or 


ZAMBOANGA, 
ISLANDorMINDANAO. 


Vhowing the proposed location ofa 


CANTONMENT or U.S. TROOPS. ° 


FROM A RECONNAISSANCE BY 


CaPTAIN A_..B_ 


Carer EncRLe PT. oF MINDANAO. 
Jan. 15, 1904... 


SCALE: 


500 


Contour Lnterval 20’ 


r eer ae a2 


Nore ~ Elevations are above mean hwal 
QM whart in Zamboanga. 


RECONNAISSANCE, 75 


Lettering.—Names and figures relating to points on the map should be made 
parallel to one side. Names and figures relating to extended features or large areas 
are disposed along the feature or across the area in straight or curved lines. 


Ornamental lettering should be avoided. A plain unshaded letter is best. All 
needful variety of effect and prominence may be obtained by the size, spacing, 
weight, and inclination of such letters and the larger initials for important words, 
Fig. 51 shows the style of letter described, upright and inclined—usually called 
italic—with normal, condensed, and extended spacing. Fig. 51 A is a scale for spac- 
ing letters and determining the length of a given line, This scale gives equal apace 
to all letters, which is not strictly correct, but is simple and does well enough for 
present purposes. It is the method necessarily employed in typewriters and the 
eye is accustomed to it. 


For ordinary or normal lettering the height of letters is the width of the letter 
space in the second line below that adopted for the widths of the letters, fig. 51 A. 


For condensed lettering take for the height the space in the third or fourth line 
below; and for extended letters make the height equal to the width or take it from 
the first line below. 


A very good effect may be obtained by the exclusive use of capitals, The small 
letters require one-half the space of capitals in the same liue. They are not so easy 
to make well as the capitals, but can be made more rapidly and look better on the 
face of the map. A very good general rule is to use inclined letters for all names 
and oe on the face of the map which relate to water and upright letters for those 
which do not. 


98. Enlargement and reduction.—The simplest method is by squares. Divide 
the origina! into squares of 2 ins. or less by lines drawn parallel to the borders, fig. 
52. Divide the paper on which the copy is to be made into squares with sides cor- 
responding to the same distance on the scale of the copy that the side of a square on 
the original itself does to the scale of the original, fig. 53. If a plotting scale of the 
original be placed on the side of a square on the original and the plotting scale of 
the copy on the side of a square of the copy, the readings should be the same. The 
square on the copy will be larger if the drawing is to be enlarged and smaller it it is 
to be reduced. The ratio between the sides of the squares on the original and the 
copy is the ratio of reduction or enlargement. This ratio must not be confused with 
the ratio of areas of the two maps, which is different and not important, 


_ Select a square of the original and reproduce ita contents in the corresponding 
square of the copy; or take a feature of the original, as a road or stream, and trace 
its course through several squares. 


Usually the position of a point in a square or on one of the sides can be estimated 
with sufficient accuracy. Important points may be located by measurement of dis- 
tances from the nearest sides of the squares, using the scalé of the map and the 
scale of the copy respectively. 


Instead of drawing the squares on the original, they may be drawn on tracing 
linen or paper laid over it, or fine threads may be stretched to form the squares. 
Every drawing board should havea scale of inches on each edge marked with fine 
saw-cuts or with small tacks to facilitate the drawing of squares, 


99, To measure an irregular area,—Lay over the area a piece of cross-section 
tracing paper, fig. 54. Count the full squares inside the area and to.tbeir number 
add the sum of the estimated fractional ones. In the figure the fractional squares 
to be added areshaded. Multiply the equivalent number of full squares in the area 
by the area of one square to the scale of the figure. If the scale is 500 ft. to the 
inch=250,000 sq. ft. to the sq. in., and the squares ;', of an inch on one side, then 
the area of one square is ;4, of a sq. in., and its value to the scale of 500 ft. tol 
in. =2,500 sq. ins, 17.36 sq. ft. Thenumber of squares counted, multiplied by 17.36, 
is the number of square feet in the area, 


If the scale is distorted, the area per sq. in. of the drawing is found by multiplying 
the scales together. Thus, in a profile plotted to a hor. scale of 500 ft. tol in. and 
a vert. scale of -10 ft. to L in., the area of a sq. in. of the drawing is 500 x 10 = 5,000 
sq. ins, On sucha profile a square of 7, iu. on aside, or 73, in. area, corresponds 
to 50 aq. ins. - 


Reconnaissance 51-51 A, 


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Reconnaissance. 


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%8 ENGINEER FIELD MANUAL. 


100. Verniers.—-A vernier is an auxiliary scale by means of which the principal 
scale can be read more closely than can be shown by actual subdivision. Sy 


Consider AB, fig. 55, a8 part of a scale of equal parts. Construct the auxiliary 
scale or vernier CD, the total length of which is equal to 9 of the smallest divisions 
of the principal scale, but divided into 10 equal parts instead of 9, which makes each 
division of the vernier 4 the length of the division of the scale. Ne 


When the zero division of the vernier, indicated by an arrow, is coincident with a 
division, ‘as 31, of the scale, the reading is 31 and it is obvious that the first division 
of the vernier is to the left of 32 in the scale by 4 of the distance between 31 and 
32. Similarly, the second, third, etc., division of the vernier is 2, 3, etc., tenths to 
the left of the 33, 34, etc., division of the scale. To make any division of the verhier, 
as 2d, 3d, 5th, or 8th, coincide with the division of the scale next ahead of it, the 
vernier must be moved to the right 2, 3,5, or 8 tenths of the length of one division 
of the scale, and the arrow will then be opposite a point on the scale 2, 3, 5, or§ 
tenths of the distance from 31 to 32; or at 31.2, 31.3, 31.5, or 31.8, The quan: 
tity obtained by dividing the value of one division of the scale by the number of 
divisions of the vernier is called the least count of the vernier, But one inter- 
mediate vernier division can coincide with a scale division at ‘the same time and the 
number of the coincident vernier division, counting from the arrowhead, is the 
number of times the least connt must be added to the last scale division passed by 
the arrow to get the true reading. 


To read any vernier, note the value of the last scale division passed by the zero of 
the vernier and to it add the least count multiplied by the number of the coincident 
vernier division. 


A vernier constructed as described is always read ahead of the zero, or in the di- 
rection in which the scale graduations increase, and is called a direct vernier. 
Verniers may also be constructed by dividing the length of a certain number of divi- 
sions of the scale, as 11, into eqnal parts one less in number, as 10. The principles 
of operation and method of reading are the same, except that the coincident line is! 
to be found behind the zero of the vernier, or in the direction in which scale gradu- 
ations decrease. This form is called retrograde. It is but little used. 


If the scale is graduated in both directions, as ia often the case, the vernier is 
donbled, the zero in the middle and each side forming a direct vernier for the grad- 
uations increasing in the same direction. This form is called double direct, fig. 56. 
The most compact form is that shown in fig. 57, called the falded vernier, in which, 
the graduations are numbered from the middle to one end and continue from the 
other end to the middle. This is read as a direct vernier in either direction. If the 
coincident line is akead of the middle or in the direction of increasing graduation, 
take its number from the middle as zero. If it is behind the middle, or in the direc- 
tion of decreasing graduation, take its number from the nearest end, counting the end 
liue as numbered on the vernier. 


Verniers are also constructed on cylindrical surfaces, fig. 58, and on conical sur- 
faces, fig. 59. The principles and method of reading are the same for all. : 


ADDENDUM, 1907. 


90a. A quick rough test as to whether an intermediate point obscures the view 
between two other points may be made by setting up at three points on the map 
pencils or other suitable objects having the corresponding elevatious marked on 
them on a convenient assumed scale. Sight, or stretch a thread, along the pencils. 
If the middle mark is above the line joining the other two, each of the two extreme 
points isinvisible from the other. If the middle point is below the line, each ex- 
treme point is visible from the other. 


ADDENDUM, 1909. 


91la. The statement in paragraph 91 of former editions that vinegar may be used 
to thin prepared india ink is erroneous. The effect of vinegar is to precipitate the 
coloring matter. Water may be used with satisfactory results, or spirits of ammoni@ 
if quickness of drying is important. Glycerin is recommended, but its use is likely 
to delay drying. 


Reconnaissance. 55-59, 


80 ENGINEER FIELD MANUAL. 


10l. The engineer's transit.—This instrument is shown, and the names of its 
parts indicated in fig. 60. To use the transit, set up the tripod, the legs extending 
far enough to give a stable base and so as to make the top surface of tue head hori- 
zontal or nearly so. On level ground the legs will be equally extended. On inclined 
ground, the leg on the lower side will be straighter aud the others more inclined. 
Remove the cap from the tripod and screw on the instrument in its place. Hang 
the plumb line on the hook depending through the tripod head, and adjust its length 
to bring the point of the plumb bob as close as possible to the setting point. Unclamp 
the vernier and turn the transit so that one of the plate levels is parallel to one pair 
of leveling screws. The other plate level will be parallel to the other pair. Bring 
the bubbles of the levels to the center in succession by means of the leveling screws. 
Always turn one of a pair down as the opposite one is turned up and avoid 
more pressure of the screws against the plate than is necessary for a firm bearing. 
If a screw turns hard at any time it is either sprung or has been set up too tight. 
In turning a pair of leveling screws always move the thumbs toward each other or 
away from each other. The bubble will follow the motion of the left thanmb. 


With the level bubbjes in the centers of their tubes, the plate will be level if the 
bubbles are in adjustment. Turn the transit slowly in azimuth and watch the bub- 
bles. If they remain in the centers, the plate is level and the levels are also 
correct. If either bubble leaves the center, the amount of its motion indicates tha 
amount by which it is out of adjustment. If the amount is small it may be neg- 
lected; if large, the adjustment should be made as hereafter described. For short 
lines the level error may be neglected if the entire bubble remains in sight during 
tbe entire revolution. Adjust the leveling screws in this case so that the travel of 
the bubble will be equal on Loth sides of the center. 


Having leveled the plate, draw out the eyepiece until the cross hairs are clearly 
defined. The instrument is now ready for use or adjustment. Adjustments should 
be invariably made in the order in which they are described, 


ist adjustment.—To make the axes of the plate levels perpendicular to the axis 
of the instrument and therefore parallel to the plate: . 


Having set up and leveled, clamp the limb and revolve the plate 180°. If either 
bubble recedes from the middle of its tube, bring it back by raising the lower, or 
depressing the higher end, one-half by the main leveling screws, and one-half by the 
small screws which fasten the level to the plate. Again revolve the plate 180° and 
if the bubble still recedes from the middle, correct the error as before and repeat 
the operation until the bubble does remain in the middle in both positions of the 
plate, When the adjustment is complete, both bubbles will remain in the center 
with the plate in any position. 


2d adjustment.—To place the intersection of the crogs wires in the straight lind 
through the optical center of the object glass and perpendicular to the horizontal 
axis of the telescope: 


The first adjustment completed, direct the telescope to some small, well-defined, 
and distant object. With the screw which moves the object-glass slide adjust the 
latter so that the distant object is as distinct as possible. Both cross wires and 
object should now he clearly seen. Note whether the image appears to move with 
reference to the wires when the eye is moved from side to side across the opening 
of the eyepiece. Such displacement is called parallax, and indicates that the image 
is not exactly in the plane of the cross wires. Move the object glass by its thumb- 
screw until the parallax ceases. This must be done every time the transit 
is used to read an angle, as well as when adjusting it, 


Unclamp the plate and lay the intersection of the wires upon the middle of 8 pin 
200 or 300 ft. distant; clamp the plate; plunge tbe telescope, that is, revolve it about 
its horizontal axis, and havea pin driven at the same distance from the transit so 
that its middle shall be seen exactly at the intersection of the cross wires. Revolve 
the plate 180°; clamp and lay exactly upon the middle of the firet pin. Again plunge 
the telescope and look at the second pin. If the intersection again strikes the pin 
the adjustment is correct, but if the pin appears to one side of the intersection, bring 
it back one-quarter of the way by the side reticle screws, turning one in as the other 
is turned out. If the instrument is erecting (most transits are) loosen the reticle 
screw on the side toward which the wire should move in the field and tigbten the 
other one. If inverting, turn the other way. Repeat the process until the pins are 
cut exactly in the middle without reference to position of transit or telescope. The 
adjustment will then be correct. 


Reconnaissance 60-—60a, 


A-Tripod. 
B- « head. 

c { Plate clamp. 
Vernier « 
a tang.screw, 
or 


Vernier « 
E> Limb clamp. 
F— tang. screw. 
G-— Main leveling 

Screws. 
H-H-Verniers 
|-!-Plate levels. 
K- Vert. limb. 
L- «  «vernier. c~<E 
M— « «tang. screw. @ 
N- « « clamp. B 
O-— Attached level. 
P— Telescope. 
Q— Eye piece 
R-R-Reticle screws. Aj 


Fig. 60a 


87625—09-——6 81 


82 ENGINEER FIELD MANUAL. 


3d adjustment.—To make the horizontal axis of the telescope perpendicular to 
the vertical axis of the instrument: 


The instrument leveled, lay the telescope on a point at the top of a nearly vertical 
line, such as the corner of a building or a steady plumb line. Clamp the plate 
and depress the telescope until the horizontal wire is near the lower end of the ver- 
tical line, and note the position of the intersection of the wires with respect to the 
selected vertical, whether to right or left of it, and how much. Revolve the plate 
180°; plunge the telescope, and again bring the intersection of the cross wires on the 
top point of the selected vertical; again depress the telescope and bring the horizontal 
wire to the bottom of the vertical. If the intersection of the wires is again on the 
same side and at: the same distance from the selected vertical the adjustment is cor- 
rect. If itis not ao, raise or lower the movable support by the proper adjusting 
acrews so as to correct half the difference, and repeat the operation. If the instru- 
ment is erecting, raising the support will move the intersection away from it; or low- 
ering the support will move the intersection toward it. If inverting, the reverse. 


4th adjustment.—To make the vertical wire perpendicular to the horizontal 
axis: 

Level carefully and lay the top of the wire on a definite point. Elevate the tele- 
scope slowly and note whether the point remains on the wire. If not, loosen two 
adjacent reticle screws and tap the head of one very gently until the point will travel 
on the wire from end toend. Then tighten the screws. If gently tapping on a 
screw head does not move the wire, tap on the opposite side of the opposite screw. 


For a transit without vertical limb or attached level, known in the trade as a plain 
transit, the adjustments are now complete, If the transit has an attached level, 
its axis is made parallel to the line of sight by the— : 


5th adjustment.—Set up midway between two stakes, which have their tops at 
about the same elevation, and with the bubble of the attached level tin the center, 
read a rod on each stake. The difference in the readings is the true difference in 
level of the tops of the stakes. Move the instrument toward one of the stakes, and 
set it up so that the eyepiece is about over the center of the stake. Place the rod on 
the stake near the eyepiece, and set the target in the middle of the field as seen 
through the object glass. Set up the rod on the far stake with a target set at the read- 
ing just taken through the object glass, plus or minus the difference of level between 
stakes—plus if lower, minnsif higher. Bisect the target with the horizontal cross 
wire, The line of sight must now be horizontal, and keeping the vertical motion 
clamped so as to retain the pointing, adjust the bubble of the attached level to the 
center by means of the small screws at the movable end of its tube. Botb line of 
sight and axis of bubble are now horizontal and therefore parallel. 


Note that the position of the horizontal wire in the field is a matter of convenience 
tainly. Itis best to have it near the middle of the fleld and it can be placed there 
by inspection with alt needful precision. 


6th adjustment.—If the transit has a vertical limb in addition to the attached 
level, the line of sight and axis of the attached Jevel made parallel to each other by 
.the preceding adjustment should also be so adjusted that the vertical scale will read 
zero, when they are horizontal. If the vernier of the vertical limb is adjustable, 
bring the bubble of the attached level to the center and then adjust the vernier to 
read zero. If the vernier is fixed, the reading, when the attached level is hori- 
zontal, may be taken as an index erfor and applied to all readings, or the line of 
sight may be- adjusted to the vernier. To do this, establish a horizontal line from 
the center of the level to the target, as explained in the preceding adjustment. Set 
the vertical limb so that the vernier reads zero, and bring the intersection of the 
wires on to the target by the top and bottom reticle screws. Then keeping the in- 
tersection on the target, bring the bubble of the attached level to the center by its 
adjusting screws. The line of sight and the axis of the attached level are now 
parallel, and are horizontal when the vertical limb is at zero, which completes the 
adjustment. ‘ 


102. Use of the transit.—To measure a horizontal angle, set up over the vertex 
of the angle to be-measured, and direct the telescope algng one of the sides of the 
angle. Clamp limb and ‘plate—if the latter is set at zero it is more convenient—and 
with the tangent screw of the limb bring the intersection of the cross hairs ona 


RECONNAISSANCE. 83 


definite point of the line. Read each of the two verniers and record, calling one 
vernier A and one B, Unclamp the plate—not the limb—and direct the telescope 
along the other line. Clamp and bring the cross hairs to a definite point with the 
vernier tangent screw. Readand record as before. Take the differences of the two 
readings A and B, respectively. If these differences are the same, it is the value of 
the angle. If not, take the mean of the differences as the value. For greater 
accuracy, the method of repetition isused. After the first measurement is made, 
unclamp the limb—not the plate—and resight on the first point by means of the 
limb tangent screw, and proceed as before. The reading of the vernier is now twice 
the angle. Continue the repetitions until the desired number are made. The last 
reading divided by the number of measurements is the value of theangle. To guard 
against errors, it is well to read and record after each measurement. 


To measuré a vertical angle.—Point the instrument; clamp the horizontal 
motions and make the readings on the vertical limb. For greater accuracy when 
there is a complete vertical circle, revolve the instrument through 180°, plunge the 
telescope, and take new readings. If the results differ, use the mean. 


To run out a straight Hne.—Set up accurately over the initial point. Point 
the telescope in the required direction, and establish a second point. These two 
determine the line which is to be run out. Set up over the forward, or second point; 
lay the telescope on the initial point; clamp limb and plate; plunge telescope and 
set a point forward. If the adjustments are good, this third point will be in line 
with the first and second and the line may be prolonged by repeating the steps taken 
at the second point. 


Tf the adjustments are not good, set a third point as before. Then unclamp the 
Jimb and turn 180° in azimuth and lay on the initial point. Clamp and plunge 
again and set another third point beside the first one. Take the middle point be- 
tween the two for the true third point. This method eliminates errors of adjust- 
ment, except those of the plate levels. These are so easily observed and corrected 
that they should never exist when close work is required. 


103. Traversing.—The transit must be set at each station with the 0-180° line 
of the azimuth circle parallel to its position at preceding stations. Thys is called 
carrying an azimuth. The direction chosen for the 0-180° line is usually the 
true ¥. and §., or as near it as data at hand will permit. 


Having observed the second station from the first, proceed to the second, set up, 
and set one of the verniers at its reading from the first to the second station, plus 
180°, or at the back azimuth, Point at the first st
…[truncated]