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WAR DEPARTMENT FIELD MANUAL
FTELD ARTILLERY
GUNNERY
hegraoedUNCLASSIFIEDby
DOD DIR. 5200.
AUTH06
THIS IS NOT A REVISION. This manual contains
CI, 20 December 1945 and C2, 20 February
1947 to the 1 June 1945 edition placed at the
back, following original texts, and will not be
issued to individuals possessing that edition.
UNCLASSIFIED
MHttEkTNo pe
3SEMINATION OF RESTRICTED
person is entitled solely by virtue of
his grade or position to knowledge or possession of
classified matter. Such matter is entrusted only to
those individuals whose official duties require such
knowledge or possession (See AR 380-5).
WAi DEPARTMENT - / JUNE 1945
WAR DEPARTMENT FIELD MANUAL
FM 6-40
This manual supersedes FM 6-4-0, 11 February 194-2, including Cl,
21 July 1942; C2, 7 July 1943; and C3 , 16 December 1943, and WD7C
105, 1942
FIELD ARTILLERY
GUNNERY
WAR DEPARTMENT 1 JUNE 1945
1SSEM1 NATION OF RESTRICTED MATTER—
NfTperson is entitled solely by virtue of his grade or position to know-
ledge or possession of classified matter. Such matter is entrusted only
to those individuals whose official duties require such knowledge or
possession. (See AR 380-5.)
WAR DEPARTMENT
Washington 25, D. C, 1 June 1945
FM 6-40, Field Artillery Gunnery, is published for the informa-
tion and guidance of all concerned.
[AG 300.7 (1 Mar 45)]
By order of the Secretary of War:
Official:
J. A. ULIO
Major General
The Adjutant General
G. C. MARSHALL
Chief of Staff
Distribution:
AAF (10); AGF (40); ASF (2); AAF Comds (2); Arm & Sv Bd (1);
Def Comd (1); Tech Sv (1) except 3, 9 (10); PC&S (1); PG 9 (5);
Gen & Sp Sv Sch (10) except CW Sch (50), CA Sch (50), Inf Sch
(50), Armd Sch (50), TD Sch (50), AAA Sch (50), FA Sch (1500);
USMA(50); ROTC 4, 6(1); ASF TC 3(5); RTC 2, 7, 17, 18,
44(5); 6(25); A (10); CHQ.(10); D (10); B6 (5); R6 (5); Bn6
(50). T/O & E: 2-28(5); 3-25(5); 4-155(50); 7-26(5); 17-
16(5); 17-25(5); 17-45S (5); 18-25(10); 18-35(10); 44-7(5);
44-10-1(5); 44-12(5); 44-15(20); 44-115(20); 44-200-1 (5).
For explanation of symbols, see FM 21-6.
CONTENTS
Paragraphs Page
PART ONE. GENERAL.
Chapter 1. Introduction 1-6 1
Chapter 2. Elementary ballistics 7-13 3
Chapter 3. Effects of projectiles; fuze action . . 14-20 8
Chapter 4. Dispersion 21-28 16
PART TWO. THE FIRING BATTERY.
Chapter 1. Initial laying; commands and
reports 29-36 23
Chapter 2. Fire commands and their execu-
tion „ 37-78 26
PART THREE. OBSERVED FIRES.
Paragraphs 79 to 169, inclusive, pages 44 to 150, inclusive, are super-
seded by Change 2, FM 6-40, 20 February 1947, which will be found
at the back of this manual.
Paragraphs Page
PART FOUR. SURVEY.
Chapter 1. Survey principles.
Section I. Basic principles 170-176 151
II. Echelons of survey 177 159
III. Responsibility of commanders
for survey 178 161
IV. Survey elements in the field
order 179-180 161
Chapter 2. Grid systems and plotting.
Section I. Fire control grid 181-188 163
II. Point designation grid, polar
coordinates, and polar
plotting 189-190 173
Chapter 3. Aerial photographs.
Section I. Vertical photographs 191-200 176
II. Oblique photographs 201-206 192
Chapter 4. Survey equipment and its use.
Section I. Principal instruments 207 204
II. The tape and taping 208-213 204
III. The aiming circle and battery
commander's telescope 214-223 207
IV. The transit and accessories 224-233 212
V. The altimeter 234 218
VI. The military slide rule 235 221
Chapter 5. Basic survey operations and methods.
Section I. General 236-237 222
II. Determination of distances '. . . . 238-243 222
III. Location of points 244-248 227
IV. The target area base 249-256 237
V. Determination and transmis-
sion of direction 257-259 240
VI. ' Survey cooperation with .obser-
vation battalion units 260 246
VII. Extension of control 261-262 246
Chapter 6. Survey computations 263-269 248
iv
Paragraphs Page
Chapter 7. Survey accuracy.
Section I. Standards 270-273 254
II. Operations required to attain
standards 274-275 255
Chapter 8. Procedure.
Section I. Grid sheet 276-282 258
II. Battle map . . 283-291 270
III. Photomap 292-302 277
Chapter 9. Night survey 303-304 289
PART FIVE. MAP DATA AND CORRECTIONS.
Chapter I. Determination of map data 305-316 291
Chapter 2. Determination of corrections by
registration 317-321 303
Chapter 3. Determination of corrections from
a metro message 322-325 307
Chapter 4. Application of corrections 326-330 312
PART SIX. FIRE DIRECTION; MASSING OF FIRES.
Chapter 1. Fire direction, general.
Section I. General 331-341 318
II. Attack of targets 342-354 328
Chapter 2. Fire-direction centers 355-358 337
Chapter 3. The observed fire chart 359-364 348
Chapter 4. Examples of massed fires 365-373 358
Paragraphs Page
Appendix I. Service practice 367
II. -Calibration 375
III. Dead space and visibility 379
IV. Use of field glasses 380
V. Common mistakes and their prevention or
detection 379
• VI. Methods of mil-gridding oblique photo-
graphs 386
VII. Adjustment of naval gun fire, using ground
observation methods 399
VIII. Heavy artillery 407
IX. Abbreviations; definition of terms 421
INDEX 433
This manual supersedes FM 6-40, 11 February 1942; CI, 21 July 1942;
C2, 7 July 1943; and C3, 16 December 1943, and WDTC 105, 1942.
PART ONE
GENERAL
CHAPTER 1
INTRODUCTION
1. GUNNERY. Gunnery is the practical handling of artillery fire. It
consists generally of two phases: preparation of firing data and conduct
of fire.
2. FUNDAMENTALS OF ARTILLERY FIRE.
a. The power of the artillery lies in the ability to concentrate ef-
fect from widely dispersed positions and to shift this effect from target
to target.
b. The task of the artillery is to locate the enemy and destroy him
by fire power, or to neutralize enemy action by the threat of destruction.
C. To be effective, artillery fire of suitable density must hit the tar-
get at the right time and with the appropriate projectile and fuze.
d. With good observation, effective fire can be placed on an enemy
target. The search for targets and for probable locations of targets
is most important. The observer must be skillful in the use of maps
and photographs, and familiar with the methods and tactics of the
enemy. Limited observation results in greater expenditure of ammu-
nition and reduces the effectiveness of fire. Lack of observation must
not preclude the delivery of fire.
3. SELECTION OF METHODS.
a. Gunnery methods are based on practical experience. Prescribed
methods cannot cover all possible situations.
b. To overcome the adverse conditions of the battlefield, the ar-
tilleryman must possess initiative, good judgment, and a thorough
knowledge of gunnery. He must be able to estimate situations
promptly, to select appropriate methods of preparation of firing data
and conduct of fire, and to estimate the number of rounds and quantity
For military terms not defined in this manual see TM 20-205, and for
list of Training Publications see FM 21-6.
1
of artillery which should be used. The artilleryman must continually
keep in mind the location of friendly troops so that artillery fire may
best support them.
4. ACCURACY. Field artillery doctrine demands delivery of fire by
the most accurate means which time and the tactical situation permit.
Inaccurate fire wastes ammunition and forfeits the confidence of the
supported troops in the artillery.
5. TARGETS. Artillery is of no value to the supported arms without
targets upon which to place fire. All intelligence agencies must be
exploited to the utmost to determine the location or suspected location
of targets for the artillery.
6. SCOPE. Battery gunnery is covered generally in parts one, two,
and three. Battalion (and higher) gunnery is covered generally in
parts three, four, five, and six. The text, however, should be studied
as a whole, since no part can be entirely divorced from any other.
2
CHAPTER 2
ELEMENTARY BALLISTICS
7. GENERAL. The point of impact of a projectile for a given range
is determined from the firing tables when all conditions of weather,
ammunition, and weapon are standard. However, the projectile is
acted upon inside and outside the tube by conditions nonstandard, with
resultant dispersion and a different point of impact from that desired.
An understanding of these factors and a reduction of their effects will
increase accuracy.
8. INTERIOR BALLISTICS.' Certain factors affect the projectile within
the tube:
a. Wear of the tube, especially the forcing cone, is the normal re-
sult of firing; it is much greater when higher charges are fired than
when low charges are fired. It is also increased by the firing of dirty
ammunition and by improper care of the tube. A worn tube will
permit an increase in the volume of the powder chamber by allowing
the projectile to be rammed farther forward. It will also permit uneven
seating of the projectile, which may allow gases to escape; and may
also allow improper centering of the projectile, with resulting varia-
tions in muzzle velocity and instability in flight.
b. Cleanliness of the tube must be maintained to reduce erosion,
e. Hard, uniform ramming is necessary to obtain uniform seating
of the projectile and hence more uniform muzzle velocity.
d. The rotating band must be smooth and free from burs and scars
to permit uniform seating and to prevent the escape of gases.
e. Powder must be of uniform temperature and moisture content.
Variations within lots and especially between lots will cause different
rates of burning and variable muzzle velocity.
f. High charges will cause coppering in the tube, which will de-
crease the muzzle velocity of the first few rounds fired with a lower
charge. The muzzle velocity returns to normal after several rounds
have been fired at the lower charge.
g. Uniformity in density of loading must be obtained. A variation
in the volume of the powder chamber or in the position of the charge
3
in the chamber changes the speed of 'burning, with resultant variation
in velocity.
h. Variations in the weights of projectiles will cause variations in
muzzle velocity.
i. Slight variations from standard in manufacture of the tube and
variations in the adjustment of the recoil mechanism will cause minor
differences in range.
9. EXTERIOR BALLISTICS. After the projectile has left the tube and
before it reaches the point of impact, it is affected by many factors.
a. To keep an elongated projectile from tumbling during flight,
it is given a rotating motion around its axis by the rifling of the tube.
The action of air resistance, rotation, and gravity causes the projectile
to deviate from the plane of fire, and this deviation is termed drift.
b. Weight of projectile. For the same muzzle velocity, a heavier
projectile tends to travel farther than a lighter projectile of the same
size and shape.
e. An increase in air density causes greater resistance and decreased
range.
d. A variation in air temperature causes a variation in range.
e. Wind blows the projectile from the normal trajectory. A head
wind decreases the range; a wind from the right blows the projectile
to the left; the effect of an oblique wind is divided into components
parallel and perpendicular to the direction of fire.
f. Muzzle velocity greater than normal will result in greater range.
g. When tilted to reach a target above or below the horizontal, the
trajectory is altered by gravity (see par. 13).
h. The rotation of the earth affects the projectile in range and de-
flection, depending on the direction of fire.
i. The exterior surface of the projectile must be smooth. A rough
surface on the projectile or fuze will increase air resistance, decreasing
range and causing an error in deflection.
j. A heavy overcast increases air density and impedes the flight of
the projectile, decreasing range. The effect of the impact of moisture
particles against the projectile decreases its velocity.
10. CORRECTIONS. Since data in firing tables are based upon stand-
ard conditions which rarely exist, corrections for nonstandard con-
ditions must be made. Factors in the firing tables will correct for
the following known conditions: drift, variation of powder tempera-
ture, weight of projectile, air density, air temperature, differences in
4
muzzle velocity obtained from calibration and ammunition data cards,
wind, and nonrigidity of the trajectory (complementary angle of site
factors); and rotation of the earth tables for direction and range.
Using personnel may correct or provide for the following factors: undue
wear of the tube, by selection of the proper charge; cleanliness of the
tube; uniform ramming; care of the rotating band; uniform powder
temperature and powder lots; uniform placing of powder charges;
segregation of projectiles by weight and exterior surface; and measuring
the shooting strength of the gun by calibration. In spite of the appli-
cation of these corrections and extreme care in the service of the piece,
there are many factors which cannot be measured accurately. These
will cause dispersion or variation in the point of impact from round
to round under the same firing conditions. (Chap. 4, part one.)
11. FORM OF THE TRAJECTORY.
a. In a vacuum. If there were no air to offer resistance to the pro-
jectile, the form of the trajectory would be determined entirely by the
elevation, the muzzle velocity, and gravity (figs. 1 and 2). The form
would be a symmetrical curve (approximately a parabola); the angle
of fall would equal the angle of elevation, and the maximum ordinate
would be at a point half way between the origin and the level point.
Figure T. Variable elevation, constant muzzle velocity.
b. In the air. Resistance of the air retards the projectile from the
instant it leaves the piece. This makes the trajectory a more complex
curve than that in a vacuum; the angle of fall is greater than the angle
of elevation, the maximum ordinate is closer to the level point than
5
Figure 2. Variable muzzle velocity, constant quadrant elevation.
to the*origin, and the range is reduced. Air resistance is approximately
proportional to the square of the velocity, and varies with the shape
of the projectile. Retardation (the effect of air resistance on a pro-
jectile) depends upon the ratio of air resistance to mass of projectile.
In general, retardation is less for large projectiles than for smaller ones
of the same shape, because air resistance varies as the square of the
caliber while mass varies as the cube.
12. ELEMENTS OF THE TRAJECTORY (fig. 3).
a. The origin is the center of the muzzle of the piece.
b. The level point is the point on the descending branch of the
trajectory at the same altitude as the origin.
C. The base of the trajectory is the straight line joining the origin
and the level point.
d. The plane of fire is the vertical plane containing the axis of the
bore when the piece is fired.
e. The line of fire is the trace of the trajectory on a horizontal plane.
f. The line of site of a point is the straight line connecting the origin
with that point.
q. The plane of site is the plane containing the line of site and a
horizontal line perpendicular to it.
h. The line of elevation is the axis of the bore prolonged when the
piece is laid.
i. The angle of fall is the angle between the base of the trajectory
and the tangent to the trajectory at the level point.
j. The angle of impact is the angle between the tangent to the tra-
jectory at the point of impact and the plane tangent to the surface Of
the ground at that point.
k. The slope of fall is the tangent of the angle of fall and is ex-
pressed as 1 on 10 (or so much).
I. Other elements of the trajectory are indicated in figure 3.
6
Figure 3. Elements of the trajectory.
13. RIGIDITY OF THE TRAJECTORY. The theory of the rigidity '
of the trajectory is the assumption that the trajectory may be tilted up
or. down through small vertical angles about the origin without ma-
terially affecting the shape of the trajectory. This assumption is
utilized to obtain the quadrant elevation for a target above or below
the piece by adding algebraically the angle of site of the target to the
elevation necessary for range only. When large elevations are used
with large angles of site, errors may be introduced by utilizing this
assumption; therefore, in carefully prepared fire, elevation corrections
should be determined from the complementary angle of site tables given
in firing tables (accurate to angle of site of 25 mils or difference in
altitude of 1000 feet, whichever is less).
CHAPTER 3
EFFECTS OF PROJECTILES; FUZE ACTION
14. PENETRATION IN SOIL After a projectile strikes the ground and
before it detonates, its path depends upon:
Angle of impact;
Shape, weight, velocity, and rotation of projectile;
Condition of surface of ground;
Composition and compactness of soil.
The resulting action is unpredictable except that certain general
statements apply. These are:
a. Other conditions being equal, the amount of penetration varies
with weight and striking velocity.
b. Increased compactness of the soil reduces the amount of pene-
tration.
c. When the angle of impact is small (see par. 12j), the projectile
tends to ricochet. When the angle of impact is moderately large, the
projectile first penetrates and then tends to rise. If penetration is very
great, the burst may produce a camouflet, that is, a hole underground,
\
\
\
\
Figure 4. Effect of burst, deep penetration, steep slope
of fall (cross section).
the surface of the ground remaining unbroken (see. fig. 4). If penetra-
tion is moderately great, a crater is produced. Whether a camouflet
or a crater is produced depends upon depth of burst, character of soil,
and force of the detonation. When the angle of impact is large, the
projectile continues downward until it stops or detonation occurs.
d. The rotation of the projectile, resistance of the soil, and inequal-
ities of resistance may cause a projectile to turn from a straight path.
The amount and direction of the deviation are unpredictable.
8
15. EFFECT ON CONCRETE. Observed effects on reinforced con-
crete of excellent quality are shown in the following table.
Thickness of concrete
perforated by single
round (Face normal to
angle of fall) (feef)
Number of rounds, falling in circle
of given diameter*, necessary to
perforate various thicknesses of
concrete at given ranges
Weapon; Projectile
(maximum charge)
Range
600
Range
1000
Range
2000
Range
1000
Range
2000
Range
3000
Remarks
Thickness
(feet)
Thickness
(feet)
ThickneBS
(feel)
3
6
7
3
5
7
3
5
7
57-mro AT gun; AP
2.7
2.6
2.1
No
t a
vai
lab
te
Best to fire at ports
and embrasures.
Other employment
is an expedient.
75-mm gun M3
(medium tank) ;
HE M48, fuze M78
2.1
1.9
1.7
5
14
27
8
21
41
13
36
72
3-inch gun (TD);
HE M42B1. fuze
M78
3.2
3.0
2.5
1-2
5
10
3
8
16
5
14
27
Effective weapon at
short ranges.
90-mm gun; HE
M71, fuze M78 or
APC M82
4.0
3.8
3.4
1
3
7
1
5
10
3
8
14
Effective alternate
weapon.
105-mm how M2-
Al; HE Ml, fuze
M78
2.3
2.1
1.8
5
14
27
8
20
40
10
27
53
Effective against
light concrete at
short ranges.
4.5-inch gun Ml;
HE M65, fuze M78
4.7
4.6
4.1
1
1-2
5
1
3
8
1
4
10
155-mm how Ml;
HE M107, fuze
M78
4.0
3.9
3.6
1
8
7
1
4
9
1
6
11
155-mm gun M19-
17 (carriage M12);
HE M101, fuze
M78
5.5
5.2
4.9
1
1-2
3
1
2
4
1
2
6
Good weapon for
perforating concrete.
155-mm gun Ml;
ii ij iya lull 1 Ui£C
M78
6.8
6.6
6.1
1
1
2
1
1
2
1
1
8
Best results. Self-
propelled weapon is
desirable.
155-mm gun Ml;
AP M112
6.8
6.5
5.9
1
1
2
1
1
2
i
1
3
Best results. Self-
propelled weapon is
desirable.
8-inch how Ml;
M78
5.8
6.5
5.1
1
1
3
1
1-2
4
1
2
5
Excellent penetra-
tion and very ac-
curate.
8-inch gun Ml;
HE M103, fuze
M78
Range
10,000
Range
12,000
Range
15,000
Range
10,000
Range
12,000
Range
15,000
At shorter ranges
(data not available)
probability of per-
foration proportion-
ately greater. Use
should be an exped-
ient
5.2
1
1
2
1
2
3
1
3
5
240-mm how Ml;
HE M114, fuze
M78
Range
8,000
Range
10,000
Range
12,500
Range
8,000
Range
10,000
Range
12,500
When 155-mm gun
is not available. At
these ranges, dis-
persion requires
heavy expenditure
of ammunition.
4.9
1
1
2
1
2
3
1
3
5
Caliber of weapon
57-mm up to 3-inch
90-mm to 4.5-inch
155-mm how to 155-mm gun
♦Diameter of circles used as basis for data:
Diameter of circle Caliber of weapon
240-mm how
3 feet
Ml 4 feet
Diameter of circle
2H feet 8-inch how and 8-inch gun 5 feet
6 feet
9
Notes:
a. HE shell with quick or delay fuze, although not effective against
concrete, is useful for exposing fortifications by blasting away camou-
flage and earth cover. With the anticoncrete fuze, HE shell is very
effective. This fuze, nondelay, should be used for adjustment.
b. After a structure has been breached by AP projectiles, HE shell is
effective in blowing apart the shattered concrete and in producing casu-
alties.
c. HE-AT shell in all calibers (including 2.36-inch rocket) has only
limited effect against concrete.
- d. Fire into embrasures requires flat trajectories. Effectiveness of
AP projectile depends upon high velocity. Firing at close ranges meets
these conditions.
e. Thicknesses perforated are based upon a line of impact normal to
surface. The effectiveness decreases rapidly when the line of impact is
other than perpendicular to the surface. Ricochet will occur when the
line of impact is 20 to 35 degrees and more from the normal. The higher
the striking velocity, the greater this angle may be before ricochet
occurs. After the surface has been chipped, this angle may be still
greater.
16. EFFECT ON ARMOR (HOMOGENEOUS PLATE).
Weapon; Projectile
Thickness of armor perforated by single
round (Angle of impact normal to surface
of armor) (inches)
500-yard
range
1000-yard
range
1500-yard
range
37-mm gun, M3A1, M6; APC-T, M51B1
or M51B2
2.9
2.6
2.2
57-mm AT gun, Ml; APC-T, M86
4.0
• 3.5
3.1
75-mm gun, M3, M6; APC-TM, 61A1
3.3
3.0
2.7
75-mm how, Ml, M1A1, M2, M3; HE-
AT, M66
4 to 4.5
4 to 4.5
4 to 4.5
3-inch gun (TD), 76-mm gun, M1A1,
M1A1C; APC-T, M62A1
£.7
4.3
3.9
105-mm how, M2A1, M3; HE-AT, M67
.5 to 5.5
5 to 5.5
5 to 5.5
155-mm gun, M1918 (mo'tor carriage
M12); AP, Ml 12 (2360 f/s)
6.9
6.7
6.5
155-mm gun Ml; AP, M112 (2745 f/s)
7.6
7.5
' 7.2
2.36-inch rocket; HE-AT, M6A3
4
76-mm gun, M1A2; APC-T, M62A1
4.7
4.4
4.1
76-mm gun, M1A1, M1A1C, M1A2, and
3-inch gun, M5, M7; HVAP-T, M93
8.4
7.3
6.2
90-mm gun, Ml, M1A1, M2, M3; APC-
T, M82 (2800 f/s)
6.4
6.0
5.6
90-mm gun, Ml, M1A1, M2, M3;
HVAP-T, T30E16 (3350 f/s)
11.2
10.0
9.0
10
17. EFFECT ON VEHICLES AND FORTIFICATIONS. Armor -pierc-
ing projectiles or shot and high explosive antitank shell are effective
against armored vehicles and fortifications. Use of this ammunition
is unprofitable against personnel or area targets because of its limited
fragmentation.
18. EFFECT OF HE SHELL.
a. General. The action of the fuze and booster causes the bursting
charge to detonate, driving fragments of metal forward (nose spray),
transverse to the trajectory (side spray), and backward (base spray)
(fig. 5). The side spray consists of a narrow zone of fragmentation.
The nose spray and base spray each form a narrow cone. The initial
velocity of fragments is on the order of 3000 feet per second. This
initial velocity is combined with the terminal velocity of the projectile—
the sum for nose spray, the difference for base spray, and the com-
ponent for side spray. Incomplete detonation (low order burst) breaks
the shell into a few large fragments.
AIR super ricochet MINE
b. Delay fuze.
(1) With delay fuze, the shell has time, before detonation, either
to penetrate and produce mine action, or to ricochet. It is used for
destruction missions which require penetration, and for ricochet fire.
(2) When penetration occurs and the shell is in earth at the instant
of detonation, the fragmentation effect is zero. Penetration into a
bunker or dugout will produce casualities by blast effect, suffocating
gases, and fragmentation. Penetration into a masonry structure which
has been shattered by AP projectiles will tend to blow the shattered
DIRECTION OF FIRE
Figure 5. HE shell bursts.
11
portions apart. Penetration into earth over a dugout may result in
suffocating gases entering the dugout through fissures created by the
detonation. Penetration into a structure built of logs, sand bags, or
similar materials results in the blowing apart of constituent units;
the effectiveness depends upon the amount of high explosive filler.
The use of anticoncrete fuze increases the depth of penetration and
the angle at which penetration may be obtained against reinforced
concrete or heavy masonry targets.
(3) When ricochet bursts are obtained, the effect is similar to that
of an air burst. Factors which determine whether a projectile will
ricochet are:
Angle of impact;
Shape, weight, and velocity of projectile;
Length of delay of fuze;
Condition of surface of ground;
Composition and compactness of soil.
An increase in the angle of impact decreases the tendency to ricochet.
It should be noted that many of these factors cannot be evaluated for
the particular point of impact at the particular time of firing. Hence
ricochet fire must be observed, and another type of fire used if a suitable
percentage of ricochets cannot be obtained.
c. Quick fuze.
(1) With the quick tuze, projectiles burst either at the point of
impact or when only a portion of the projectile has penetrated the
ground. The impact must be on the nose of the present standard
fuzes in order for the quick elements to operate. In these fuzes, the
delay elements will act if the quick element is not activated. The
fragmentation of the projectile is increased by increased angle of impact
and by increased firmness of the ground. The effect is a function of
the fragmentation and the density, size, and velocity of the fragments.
When the projectile passes through foliage, the detonation may occur
in the trees and effectiveness may be either improved or lost, depending
upon the density of foliage. The quick fuze is suitable for use:
(a) In fire against personnel in the open when the angle of impact
is large.
(b) In fire against personnel when neither ricochet nor time fire
can be used.
(c) In fire against material objects, such as trucks, when penetration
is not required.
(d) In firing chemical shell not provided with time fuze.
(2) The relative effectiveness of shell of the various calibers, with
quick fuze, is indicated by the following table:
12
Caliber
Area Covered Effectively (yards)
Radius of Large
Depth
Width
Fragments (yards)
75-mm
10
30
150
105-mm
15
' 50
4.5-inch (estimated)
16
55
155-mm
18
60
550
8-inch
20
80
240-mm (estimated)
25
100
The area covered effectively is considered to be that area in which there
is at least a 50 per cent chance that a man standing will become a
casualty. The area is roughly elliptical.
(3) The wire cutting effectiveness of shell is poor. The employment
of artillery fire to breach wire requires extravagant use of ammunition.
d. Time fuze.
(1) With time fuze, the point of burst is determined by the quadrant
elevation, charge, and time setting (minor variations are caused by
variations in velocity of projectile, density, and temperature of air,
wind, etc.). Should impact occur before action of the time element,
quick fuze action takes place (except with those types of time fuze not
provided with an impact element).
(2) Factors which govern the effectiveness of air bursts against
entrenched targets are:
Number, size, and velocity of fragments;
Height of burst above target;
Horizontal distance of burst from target;
Shape and size of trench;
Direction of fragments.
The direction of the fragments is governed by a combination of the
angle of fall, striking velocity of the projectile, and the initial velocity
of the fragments due to detonation. The side spray fragments are
driven in a zone roughly 15 to 20 degrees in thickness, generally normal
to the trajectory (see fig. 6). This direction is modified by the forward
motion of the shell. The fragments which are driven more or less
upward are ineffective. The fragments which are driven more or less
laterally will be partially effective depending on final velocity, direc-
tion, and other factors. The fragments driven more or less downward
will be the most effective.
13
Figure 6. Effect of burst with time fuze.
(3) For each type of projectile there is a most effective height of
burst. Because of dispersion, it is impossible to secure all bursts at
that height. Some bursts will be lower and some will be higher than
the mean height. For a given range, the probable error in height of
burst is controlled by choice of charge. Range and charge have no
direct influence on the effectiveness of .time fire, but do have a marked
indirect influence on the resulting height of burst probable error.
Therefore, considering ineffective high air bursts and graze bursts, the
most effective mean height of burst is 20 yards.
e. Effectiveness in clearing mine fields. HE shell is ineffective
tor clearing mine fields, regardless of the type of fuze employed. Mines
are not sufficiently sensative to be detonated by shell bursts, except by
direct hits. Artillery fire not only fails to eliminate the mine field,
but increases the difficulty of locating and removing mines by hand,
and increases the difficulty of moving across the field.
19. COMPARATIVE EFFECTIVENESS OF IMPACT, RICOCHET,
AND TIME FIRE WITH HE SHELL.
a. Against personnel in the open, the order of effectiveness is: rico-
chet fire, time fire, high angle fire with quick fuze, low angle fire
with quick fuze, and mine action (zero). This sequence may vary
because of local conditions of soil, terrain, and vegetation. Time fire
cannot be used beyond the range corresponding to the limiting time
14
of functioning of the fuze. When the percentage of ricochets is below
70, time fire is most effective.
b. Against personnel in shallow trenches, the effectiveness of var-
ious types of fire under ideal conditions is in the same sequence and is
governed by the same conditions as in subparagraph a above. How-
ever, it should be noted that range dispersion to the point of burst in
ricochet fire is considerably greater than it is in time fire (because
of irregularities in contour of ground and variations in ricochet dis-
tance), and that for entry into trenches, the angle of approach of the
fragments from ricochets is much less favorable than is that from time
fire. The base spray, effective against personnel in the open, is too
nearly parallel to the ground to enter fox holes. Impact fire is very
ineffective against targets in trenches.
c. Against personnel in deep trenches, time fire is more effective
than ricochet fire and outstandingly more effective than impact fire.
However, under certain conditions of soil and cover, it may be necessary
to utilize the penetration effect of delay fuze!
20. CHEMICAL SHELL.
a. Gas shell is filled with irritant or toxic agents. Action of the
fuze and booster breaks open the shell. Liquid vesicants are most
effective against personnel when sprayed directly and are very effec-
tive against personnel when sprayed on vegetation. Time, ricochet, or
quick fuze action is far preferable to mine action. Many small pro-
jectiles are more effective than a few large ones for attack with liquid -
vesicants. With irritant gases and smoke, quick fuze action is prefer-
able. Only medium and heavy artillery are capable of building up an
effective concentration of these latter agents.
b. White phosphorus produces smoke, incendiary effect, and casual-
ty effect. In all three roles, superquick fuze action is preferable. Be-
low ground, the phosphorus only smoulders. With a burst at medium
height in the air, the particles burn out before reaching the ground;
the smoke rises because of heat produced in burning.
c. Base ejection type smoke shell with time fuze is more effective
as a screening agent than is white phosphorus. On impact the effect
is nearly zero. The action of the fuze and bursting charge ignites
the smoke charges and forces them out of the base of the shell with
a relative velocity of about 200 feet per second. The case continues
along the trajectory, and the smoke charges follow with reduced
velocity. They fall somewhat short of the case, the distance depending
upon height of burst.
15
CHAPTER 4
DISPERSION
21. THE DISPERSION PATTERN. If several rounds were fired from
a piece under conditions as nearly identical as possible, the points of
impact of the projectiles would be dispersed about a point called the
center of impact (fig. 7). The following are characteristics of the
dispersion pattern:
A
. * • • '
:/••*.*♦•• • . -
v, Direction
• . , • ♦ .
— ■ — . . • •
:.••.* • • -
. * • , • • _•_
' of fire"
B
Figure 7. The dispersion pattern; the center of impact is the point
at which line AB intersects the line of fire.
'a. The pattern is roughly elliptical; its center is the center of im-
pact, and its long axis lies along the line of fire.
b. Shots are scattered more in range than in deflection.
c. Shots are grouped more closely toward the center than toward
the edges of the pattern.
d. If a sufficient number of rounds is fired, as many will fall beyond
the center of impact as short of it, and as many to the right as to the
1SPERSION ERRORS.
a. Dispersion errors are errors inherent in the dispersion pattern
(such as\those caused by manufacturers' tolerances and those errors
inherent in^the piece and ammunition); they are the result of varia-
tions of certain elements, from round to round, even though conditions
are as nearly identical as possible. Dispersion errors are generally
beyond control, except that errors due to personnel can be reduced by
careful laying and loading. Dispersion errors should not be confused
with mistakes or constant errors; these are not inherent in the dispersion
pattern. Mistakes cafibe eliminated by care and training; constant
errors can be compensated for by appropriate corrections.
16
b. For practical purposes, the dispersion error of a shot is the dis-
tance from that shot to the center of impact; a dispersion error may be
resolved into its range and deflection components.
23. RANGE PROBABLE ERROR.
a. In figure 8, AB is a line through the center of impact perpen-
dicular to the line of fire. CD is drawn parallel to AB so that there
are as many shots beyond CD as there are between AB and CD. The
distance between these lines (depth of the 25 per cent zone) is the
range probable error, because this error is exceeded as frequently as
it is not exceeded. The value of the probable error is given in the
firing tables and may be taken as an index of the accuracy of the piece.
Center of impact-
A / C
[ 2%
79b
7% 2%'
jDirectiorj
of fire
1 Probable error
Figure 8. The dispersion diagram.
b. If lines are drawn parallel to AB at distances of one probable
error the percentages of shots falling in each subdivision will be approxi-
mately as indicated in figure 8. Eight applications of this interval
(four on each side of the center of impact) will include the dispersion
pattern.
24. FORK. The fork is the change in elevation necessary to move the
center of impact four probable errors. It is sometimes used as a unit
of range (elevation) change in conduct of fire. Its value is given in
the firing tables as a function of the elevation.
25. DIRECTION PROBABLE ERROR. In the dispersion diagram (fig.
8) if the long axis is considered instead of the line AB, the shots to the
right and left of the axis follow rules of distribution similar to those
given in paragraph 23. The direction probable error is one eighth the
width of the dispersion pattern at its greatest width. This value is
given in the firing tables.
26. VERTICAL PROBABLE ERROR. If fire is directed against a ver-
tical plane, the dispersion in this plane follows the same laws as dis-
persion in a horizontal plane (fig. 9). The shots are all contained in
a vertical dispersion pattern and the vertical probable error is one
eighth the height of the pattern. The vertical probable error is the
product of the range probable error and the slope of fall (tangent of
17
the angle of fall). Values for the range probable error and slope of fall
are given in the firing tables.
HORIZONTAL PROBABLE ERROR
Figure 9. Relation of vertical probable error to horizontal
probable error.
27. MISCELLANEOUS ERRORS. Dispersion on a horizontal plane
may be projected on a forward or reverse slope by considering that
slope and the angle of fall. In time fire, the projection of the bursts
(for any particular time setting) on any plane will give the dispersion
pattern in that plane.
28. APPLICATION OF DISPERSION,
a. Location of target with reference to the center of impact.
Consider the pattern of six shots fired under identical conditions
(fig. 10). Four of these (66 2/3 per cent) have been sensed short of a
target and two of them over, the exact location of the target within
the pattern being unknown. For a very large number of shots, 50 per
cent can be expected to fall short of the line AB (range center), and
75 per cent short of the line CD (fig. 8); therefore (assuming linear
interpolation to be correct), 66 2/3 per cent can be expected to fall
short of the line MN, which is two thirds of the way from AB to CD.
The line MN then represents the most probable location of the target.
The rule of computation for precision fire is based on the foregoing
principle.
A
M£ n
Gl
OJ
<r — )HH
Direction
fir
B
ND
Figure 10. Determination of location of target by dispersion
diagram, ,
18
b. Dispersion as seen by a lateral observer. To an observer
conducting fire from a position off the gun-target line, the deviations
caused by range dispersion are very apparent. It is often advantageous
to know the deviations caused by normal dispersion. These can be
computed quickly by using proportional parts of the value of d (par.
127a). Deviations corresponding to less than two range probable er-
rors should be ignored and another round should be fired at the
same elevation.
c. Probability of hitting an area. Considering that range and
deflection errors are measured at right angles to each other, the dis-
persion pattern may, for purposes of computation, be considered as a
rectangle (fig. 11). The distribution of shots throughout this rectangle
can be obtained by applying the dispersion scale along both dimen-
sions. The propability of hitting a certain area within the rectangle
can then be quickly determined as the product of the probability
of a hit for range and the probability of a hit for deflection; also, the
expenditure of ammunition necessary to obtain a given number of
hits in this area (the settings on the piece remaining unchanged) can
be computed.
.02
.07
.16
.25
.02
.07
.16
.25
.25
.16
.07
.02
.0004
.0014
.0932
.0050
.0050
.0032
.0014
.0004
.0014
.0049
.0112
.0175
.0175
.0112
.0049
.0014
.0032
.0112
.0256
.0400
.0400
.0256
.0112
.0032
.0050
.0175
.0400
.0625
.0625
.0400
.0175
.0050
.0050
.0175
.0400
.0625
.0625
.0400
.0175
.0050
.0032
.0112
.0256
.0400
.0400
.0256
.0112
.0032
.0014
0049
.0112
.0175
.0175
.0112
.0049
.0014
.0004
.0014
.0032
.0050
.0050
.0032
.0014
.0004
.25
.16
.07
.02
Figure 11. Rectangle of dispersion.
d. Application of dispersion scale in determing ammunition
expenditures.
(1) The dispersion scale can be of use in determining the proba-
bility of hits on a target of fixed dimensions, with respect to which
the position of the center of impact can be determined. This infor-
mation is useful in estimating ammunition expenditures for destruc-
tion missions. The table below gives probable expenditures per target
hit on a selected target for three weapons of different caliber. For
each caliber the location of the center of impact was assumed, in one
19
case, to be at the target center, and in the other case to be two range
probable errors (e pr ) over or short of the center of the target.
Target: bridge— 10 yards wide, 40 yards long (range).
Range: 18,000 yards.
Charge: maximum charge.
Weapon
Range
Probable
Errors
6pr
Deflection
Probable
Errors
epd
Location of CI
Proba
One B
Range
bility of Ol
[it for Ron
Df
taining
nd (%)
Rn and Df
Rounds
Required
for
One Hit
1 55-mm
gun Ml
43
43
9
9
Target center
2 epr (86 yds) over
or short of target
center
23.2
10.6
27.8
27.8
6.S
2.9
16
34
8-inch
how Ml
19
19
6
6
Target center
2 epr (38 yds) over
or short of target
center
61.6
24.5
41.7
41.7
21.6
10.2
6
10
240-mm
how Ml
36
36
8
8
Target center
2 epr (72 yds) over
or Bhort of target
center
27.8
12.9
31.2
31.2
8.7
4.0
11
26
(2) The data above for the 8-inch howitzer, center of impact located
at the target center, is computed as follows:
(a) First determine the probability of a hit for range only. The
target is 40 yards or 40/19 = 2.1 probable errors in length. Two prob-
able errors of the length cover the 25 per cent zones of the range
dispersion scale, the remaining 0.1 probable error falls in the 16 per
cent zones.
% range hits = 0.05 (16) + 1 (25) + 1 (25) + 0.05 (16) = 51.6%.
(b) In like manner, determine the probability of a hit for deflection
only. The target is 10 yards or 10/6 = 1.67 probable errors in width.
The total width of the target falls in the 25 per cent zones of the
deflection dispersion scale.
% Df hits = 0.83(25) + 0.83(25) = 41.7%.
(c) The product of these two probabilities is the probability of a
hit for both range and "deflection.
% hits = .516 x .417 = 21.6%.
(d) The probable number of rounds per target hit is equal to the
reciprocal of the probability of a hit.
Rounds required for one hit = 1/.216 = 4.6 or 5.
(3) Computations of data for the 240-mm howitzer Ml, center of
impact two range probable errors (e pr ) over or short, are made as
follows:
20
(a) As in the preceding example, first determine the probability
of a hit for range only. The distance from the center of impact to the
far end of the target is (2 X 36) + 20 = 92 yards or 92/36 = 2.56
probable errors. One of the 2.56 probable errors covers the 25 per
cent zone, one the 16 per cent zone, and the remaining 0.56 probable
error falls in the 7 per cent range dispersion zone.
% range hits between center of impact and far limit of target =
1 (25) + 1 (16) + 0.56 (7) = 44.9%.
However, a distance of 52 yards (i.e. 72 minus 20) or 52/36 = 1.44
probable errors of the above does not include the target, and probable
hits in this space must be excluded.
% range hits between center of impact and near limit of target =
1 (25) + 0.44 (16) = 32.0%.
% range hits on target = 44.9 — 32.0 = 12.9%.
(b) The target is 10 yards or 10/8 = 1.25 probable errors wide. The
total width of the target falls in the 25 per cent zones of the deflection
dispersion scale.
% deflection hits = 1.25 (25) = 31.2%.
(c) The product of these range and deflection probabilities is the
probability of a hit for both range and deflection.
% hits _ .129 X .312 = 4.0%.
(d) Rounds required for 1 hit == 1 /.040 = 25.
e. Probabilities related to conduct of fire.
(1) PROBABILITY THAT TARGET IS WITHIN BRACKET.
*
Number of Sensings
Probability (%)
At One Limit*
At the Other
1-fork Bracket
2-fork Bracket
4-fork Bracket
1
1
70
85
92.3
1
2
75
89
96
1
3
76
90
97
2
2
85
94.5
99 +
3
3
92.5
98
99 +
(2) PROBABILITY THAT TARGET IS WITHIN ZONE OF
DISPERSION OF CENTER OF 1-FORK RANGE BRACKET. A 1-
fork bracket has been obtained with one sensing at each limit; the
probability that the target is within the zone of dispersion of rounds
fired at the center of the bracket is 96.8 per cent. This probability is
increased as additional verifying sensings are obtained.
21
(3) PROBABILITY THAT CENTER OF IMPACT IS WITHIN
A GIVEN DISTANCE OF THE TARGET WHEN BOTH SHORTS
AND OVERS ARE OBTAINED WITH ONE ELEVATION SET-
TING.
Number of Sensings
Probability (%)
Distance in Probable Errors
In One Sense
In the Other
One
Two
Three
Four
1
1
54
86
98
99 +
1
2
51
86
98
99 +
1
3
44
80
96
99 +
2
2
70
96
99
99 +
3
3
99 +
99 +
99 +
99 +
22
PART TWO
THE FIRING BATTERY
CHAPTER 1
INITIAL LAYING; COMMANDS AND. REPORTS
29. GENERAL FM 6-140 describes in detail the various duties, oper-
ations, and requirements pertaining to the firing battery. Only such
material is included in part two as is required to provide complete
understanding of the part played by the firing battery in the effective
delivery of fire.
30. INITIAL LAYING.
a. The executive lays the battery parallel initially and whenever
he is ordered to record base deflection. (See also pars. 72 and 90b.)
b. When only a general direction of fire has been designated, the
executive lays the battery in that direction on a definite F-azimuth
which is a multiple of 100.
C. At the earliest opportunity, the executive determines the Y-azi-
muth of the initial direction and the referred deflection to any visible
aiming point. He has aiming posts set out and pieces referred. If
he has laid the battery initially on an aiming point, he does not delay
opening fire to have aiming posts set out.
d. An aiming point should be fixed, easily identifiable, with a
clearly defined vertical line, and should be in a convenient direction.
Other considerations being equal, the more distant aiming point is
selected.
31. MAGNETIC METALS. When the needle of an aiming circle is
used, the instrument is set up at the following minimum distances from
objects which will affect the needle:
Yards
High tension power lines 150
Railroad tracks 75
Heavy gun 60
Light gun; telegraph wires 40
Barbed wire 10
23
Steel helmets, small arms, eyeglasses, and other metallic objects which
affect the needle are moved away during use of the needle.
32. MINIMUM ELEVATION.
a. As soon as the position is occupied, the battery executive deter-
mines the minimum elevation (FM 6-140) and reports the amount to
the fire-direction center and battery commander. If this elevation is
a decimal, it is reported to the next higher whole mil.
b. In case it is necessary to determine more than one mimimum ele-
vation in the zone of fire, the executive reports, for example: "F-azi-
muth 4850 to 5200, minimum elevation 55; 5200 to 5650, minimum
elevation 42."
C. A single narrow obstruction, such as. a tree, which will mask
only one piece at a time, is not considered by the executive in computing
minimum elevation. If a piece cannot fire safely, it is called out.
33. REPORTS BY THE EXECUTIVE.
a. As soon as the information is available and can be transmitted
without interrupting fire, the executive reports to the battery com-
mander and to the battalion fire-direction center:
(1) Battery is ready.
(2) Minimum elevation (s) charge (so-and-so) (so much).
(3) Distribution of pieces (to nearest 5 yards): No. 1 (so many)
yards right (left), so many yards behind (ahead of) No. 2; No. 3, etc.
The base line or indicated direction is used as an origin of direction.
When time permits, this report should be submitted as a diagram.
(4) Visible aiming points (so-and-so) (not reported to fire-direction
center). ^
b. When directed, the executive reports, in addition:
(1) Amount, type, lot, and weight of ammunition.
(2) Powder temperature.
(3) Maximum shifts Y-azimuths (so much) to (so much) or BDR (so
much) to BDL (so much). These are the maximum shifts which can
be made without necessitating the movement of the pieces to establish
a new battery front, and within which at least three fourths of the
pieces can deliver fire at and above minimum elevation.
(4) Maximum elevation (when high angle fire is to be used).
34. REPORTS BY OPERATOR.
a. At the first round or salvo, volley, zone, or other series of fire,
the telephone (radio) operator reports to the officer conducting fire;
for example, "On the way," or "4600 on the way," or "No. 1 on the
24
way," as may be appropriate. On completion of the fire commanded,
he reports, "Rounds complete."
b. At the end of each mission, the operator reports to the officer
conducting fire the number of rounds (obtained from the recorder)
expended on the mission.
e. The operator reports immediately any attack which requires
close defense of the battery position or protective measures for
personnel.
35. REPORTING ERRORS IN FIRING. Chiefs of section must report
immediately to the executive all errors that have caused a round to be
fired with improper data. The executive has these errors corrected
and, reports them to the officer conducting fire; for example: "No. 2
fired 20 mils right; has been corrected."
36. CHECKS OF SETTINGS AND CHARGES DURING FIRING.
The executive usually checks settings and layings during lulls in firing
only. When in doubt about the accuracy of the laying of any piece,
he calls that piece out, reports to the officer conducting fire, "No. (so-
and-so) out" and has the necessary checks made. When semifixed and
separate loading ammunition are fired, unused charges are checked
and disposed of as directed by the battery commander. The executive
must at all times use ingenuity and initiative to see that the battery
is laid promptly and that accurate fire is delivered when called for.
25
CHAPTER 2
FIRE COMMANDS AND THEIR EXECUTION
37. SEQUENCE.
a. Pieces to follow commands, special methods of adjustment, and
particular missions.
b. Projectile.
c. Charge.
•d. Fuze. (In time fire, after initial commands, the command for
time immediately precedes the command for elevation.)
e. Direction.
f. Distribution.
g. Site.
h. Pieces to fire.
i. Method of fire.
j. Use of quadrant or elevation scale,
k. Elevation or range.
38. ORIGIN AND TRANSMISSION. Fire commands may originate
with the observer, the computer at the fire-direction center, or the
computer at the battery. They are sent to the executive by the best
available means of signal communication. The executive, or operator
if directed, repeats to the howitzer (gun) sections all commands re-
ceived, except as specifically noted in this manual.
39. NUMBERS. Numbers are announced as illustrated in the follow- .
ing examples:
10— One zero.
25— Two five.
300— Three hundred.
1400— One four hundred.
6000— Six thousand.
3925 — Three nine two five.
4050 — Four zero five zero.
10,000 — One zero thousand.
10,300 — One zero three hundred.
11,000 — One one thousand.
100.7 — One zero zero point seven.
254.4 — Two five four point four.
40. OPENING FIRE.
a. The command to the executive to fire is the command for range
or elevation, the command FIRE, or the command RESUME FIRING.
b. The executive's command to the chiefs of section to fire is the
command FIRE or RESUME FIRING.
C. The command to fire a normal barrage is BARRAGE.
41. HOLDING FIRE.
a. The officer conducting fire may command DO NOT LOAD
immediately preceding the command for range or elevation. DO NOT
LOAD is then part of the command for range or elevation. The com-
mand to the executive to fire (revoking the command DO NOT LOAD)
is a repetition of the range or elevation, or a new command for range
or elevation.
b. The officer conducting fire may give AT MY COMMAND im-
mediately following the method of fire. Then AT MY COMMAND
is part of the method of fire. The executive does not repeat the
command. When pieces are ready to fire, he reports "Battery is ready,"
and fires at the command FIRE. AT MY COMMAND continues in
effect until a command is given for a new method of fire not followed
by AT MY COMMAND.
42. CEASING FIRE.
a. The command CEASE FIRING normally is given by the execu-
tive, but in an emergency may be given by anyone present. At this
command, firing will cease immediately. If the command originated
from the officer conducting fire and the piece is loaded, the executive
reports "No. 1 (or other piece) loaded." If the command originated
at the battery position, a report of this command and the reason for it
is rendered. Firing is resumed at the announcement of range or
elevation.
b. The command SUSPEND FIRING is given only to effect a
temporary halt in firing on a prearranged schedule. At this command
firing is stopped, but settings continue to be altered in conformity
with the schedule. If a piece is loaded, the executive reports to the
officer conducting fire, "No. 2 (or other piece) loaded." Firing in
accordance with the schedule is resumed at the command RESUME
FIRING.
c. Except in continuous fire, a change of data following the com-
mand for range or elevation serves as a signal to stop all fires previously
ordered. Firing is resumed at the new announcement of range or
elevation. In continuous fire, changes in data are so applied as not
to stop the fire or break its continuity.
27
43. PIECES TO FOLLOW COMMANDS. Designated pieces follow
fire commands; for example: BATTERY ADJUST, NO (S). (SO-
AND-SO) ADJUST, RIGHT (LEFT) (CENTER) ADJUST. Pieces
that have not been following fire commands begin to follow when the
officer conducting fire commands BATTERY ADJUST or RIGHT
(LEFT) (CENTER) ADJUST.
44. INITIAL AND SUBSEQUENT COMMANDS.
a. The initial fire commands include all data necessary for laying,
loading, and firing the pieces. Subsequent commands include only
such data as are changed, except that the range or elevation is always
announced. When another observer takes over the conduct of fire dur-
ing a mission, he will, if practicable, continue the mission from the last
round observed or issue complete initial commands if necessary. In
such case the fire-direction center or battery executive should check
closely to determine that the announced commands appear appropriate
to the mission. If advisable, proper authentication may be required
of the new observer.
b. When a change is made in pieces to fire or the method of fire,
or both, the commands for both elements are given. Decreasing or
increasing the number of rounds in a method of fire does not constitute
a change of method.
45. COMMANDS FOR INDIVIDUAL PIECE.
a. When more than one piece is being fired and individual com-
mands are required for each piece, the command for each piece is
preceded by NO. (SO-AND-SO); for example: ELEVATION NO. 1,
350; NO. 2, 340; NO. 3, 330; NO. 4, 320.
b. When more than one piece is being fired, a change for an in-
dividual piece is preceded by the command NO. (SO-AND-SO). A
change for an individual piece is announced and set after any change
of the same element is given for all pieces (par. 100).
46. PROJECTILE. The command for shell is SHELL HE (SMOKE)
(GAS). If more than one type of HE (smoke) (gas) shell is available
at the position, the command is SHELL HE (SMOKE) (GAS), MARK
I (or other type designation).
47. CHARGE. With ammunition that has numbered charges, the
command is CHARGE 4 (or other number). When both green bag
and white bag powder for a given charge are at the position, the com-
mand for charge is followed by GREEN (WHITE) BAG. With am-
munition of supercharge, normal charge, and reduced charge, the
28
command for charge is SUPERCHARGE, NORMAL CHARGE, or
REDUCED CHARGE. If more than one type of normal charge is
available, the command' NORMAL CHARGE M8 (or other designa-
tion) is given.
48. FUZE.
a. The command for percussion fuze is FUZE QUICK (DELAY).
When two types of quick fuze are available, the command FUZE
QUICK is given for the type of fuze generally used (M48, M51). For
the other type of fuze, the command FUZE QUICK M54 (or other
designation) is given.
b. The command for time fuze is CORRECTOR (SO MUCH),
TIME (SO MUCH); or CORRECTOR (SO MUCH), FUZE RANGE
(SO MUCH). (Note: Corrector fs not announced unless fuze setter
is used.) The command for a change in corrector setting or time
setting is a new command for corrector or time. A command for fuze
range is required only in initial commands; subsequently the command
for range covers both fuze range and range. By prearrangement within a
battalion, all time corrections may be included in time settings, and
the command for corrector omitted; with this prearrangement, the
corrector scale (if any) is set habitually at 30.
c. In time bracket fire, when fuze range for the charge used does
not appear on the fuze setter, firing battery personnel determine the
time setting corresponding to the announced range or fuze range (see
TM 9-524 and TM 9-526). The executive then commands TIME
(SO MUCH).
d. When a ladder is to be fired with time fuze, the officer con-
ducting fire announces all times consecutively. The executive initially
repeats only the first time commanded and subsequently repeats the
second time after the first round has been fired, and so on.
49. DIRECTION. The battery may be laid initially by: a F-azimuth,
a base angle, an aiming point and a deflection, a target and a lead, an
airplane, or a high air burst.
50. PARALLEL SHEAF— RECIPROCAL LAYING.
a. General. A piece is laid reciprocally on an instrument as fol-
lows: the 0-3200 line of the instrument is established in direction; the
operator, using the upper motion, turns the vertical hair to the sight
of the piece, reads the azimuth and micrometer scales, and (subtracting
3200 mils if necessary) announces the reading. Using this reading for
29
a deflection and the instrument as
an aiming point, the gunner lays
the piece. By this method an in-
strument may be laid reciprocally
on a piece or on another instru-
ment; a piece may be laid recipro-
cally on another piece. If time per-
mits, reciprocal laying is repeated
until successive readings are within
1 mil (fig. 12).
* b. Form sheaf parallel. The
command to the executive is, ON
NO. 2 (or other piece) FORM
SHEAF PARALLEL. The execu-
tive does not repeat the command.
He forms the sheaf parallel by re-
ciprocal laying.
c. On No. 2 lay parallel. The
base piece is laid for direction; the
executive may command, for exam-
ple: ON NO. 2 LAY PARALLEL.
The gunner of the base piece lays
the other pieces reciprocally.
Gun
Instrument <§
Note:
Angle \, the reading on the in-
strument, is equal to angle 2, the
deflection on the sight.
Figure 12. Reciprocal laying.
51. IT-AZIMUTH (COMPASS) (fig. 13).
a. The command to the executive is COMPASS (SO MUCH). The
executive does not repeat this command.
b. The executive sets up an aiming circle away from magnetic
metals (par. 31) and in a place when it can be used as an aiming point
for all pieces (fig. 13). The executive:
(1) Subtracts the announced F-azimuth (angle 1) from the declina-
tion constant of the aiming circle (adding 6400 to the declination con-
stant if necessary).
(2) Sets the remainder (angle 2) on the azimuth and micrometer
scales of the aiming circle.
(3) Releases the compass needle and centers it with the lower motion.
(The 0-3200 line of the instrument now coincides with the announced
F-azimuth.)
30
t
I
Figure 13. Laying the battery on a Y -azimuth, using
the aiming circle.
(4) Lays each piece reciprocally (par. 50). His commands are, for
example: AIMING POINT, THIS INSTRUMENT, DEFLECTION
NO. 1, 3091; NO. 2, 2738; NO. 3, 2369; NO. 4, 2045.
(5) Commands, for example: AIMING POINT, AIMING POSTS,
REFER.
c. If he has a compass but does not have an aiming circle, the
executive sets up the compass away from magnetic metals and in a place
where it can be used as an aiming point for the base piece. The
executive:
(1) Measures the K-azimuth to the sight of the base piece.
(2) Subtracts the announced F-azimuth from the Y-azimuth which
he has measured (adding 6400 if necessary).
(3) Using the remainder as a firing angle and the compass as an
aiming point, lays the base piece.
(4) Lays the other pieces reciprocally on the base piece.
(5) Commands, for example: AIMING POINT, AIMING POSTS,
REFER.
52. BASE ANGLE. The command to the executive is BASE ANGLE
(SO MUCH). The executive does not repeat this command. He sets
up an instrument on the orienting line where it can be seen by all
pieces. The executive sets the base angle on the azimuth and microm-
31
eter scales of the instrument and, using the lower motion, sights along
the orienting line. The 0-3200 line of the instrument is now parallel
to the direction in which the pieces are to be laid. He then lays the
pieces reciprocally (par. 50).
53. AN AIMING POINT AND A DEFLECTION.
• a. The command to the executive is AIMING POINT (SO-AND-
SO), DEFLECTION (SO MUCH).
b. The executive may accomplish the laying by computing a shift
from a previous laying, by repeating the command and computing
individual shifts to lay the battery parallel, or by laying his instrument
or a designated piece on the deflection commanded and laying the
remaining pieces by reciprocal laying.
54. TARGET AND A LEAD. The command for a target and a lead
and the execution of the command are prescribed in the appropriate
service of the piece manual and in FM 6-140.
55. AN AIRPLANE OR HIGH AIR BURST. No specific command is
prescribed. The executive may lay the battery initially for direction
by sighting with an instrument on an airplane or high air burst over
the target area.
56. CHANGES IN DIRECTION. The command is RIGHT (LEFT)
(SO MUCH), or BASE DEFLECTION RIGHT (LEFT) (SO MUCH).
57. DISTRIBUTION.
a. The command for distribution is ON NO. 2 (or other piece)
OPEN (CLOSE) (SO MUCH).
b. For handling irregularities in distribution resulting from the
emplacement of pieces in staggered positions, see paragraphs 95-100.
58. SITE. The command for site is SITE (SO MUCH). The com-
mand for a change in site is UP (DOWN) (SO MUCH).
59. PIECES TO FIRE. The command to fire all pieces is BATTERY.
The command to fire a pair of pieces is RIGHT (LEFT) (CENTER),
indicating the right (left) (center) pair of pieces. The command to
fire a piece or any other combination of pieces is NUMBER (S) (SO-
AND-SO). The command FIRE AT WILL directs all pieces to fire.
32
60. METHODS OF FIRE. Methods of fire are: salvo fire, volley fire,
continuous fire, single piece, by piece at my command, fire at will,
ladder fire, and zone.
61. SALVO FIRE.
a. The command for a salvo is RIGHT (LEFT), whic"h indicates
the flank from which the pieces are to be fired successively. The com-
mand to change the normal interval of 2 seconds is AT (SO MANY)
SECONDS, given after the RIGHT (LEFT). The command AT (SO
MANY) SECONDS continues in effect until the method of fire is
changed or another interval is commanded.
b. The executive gives the command FIRE when he sees that the
pieces will be ready to fire in turn; each piece is then fired at com-
mand of its chief of section. If a piece is obviously in error or is very
slow, the executive calls the piece out, has the remaining pieces fire,
and reports to the officer conducting fire, for example: "No. 3 did not
fire."
62. VOLLEY FIRE.
a. The command for .volley fire is (SO MANY) ROUNDS. Fire
is opened at the executive's command FIRE, given immediately after
the range or elevation, unless a command for holding fire is prescribed.
Each designated piece fires the specified number of rounds, as rapidly
as is consistent with accuracy, without regard to other pieces.
b. The command for a specific time interval is (SO MANY)
ROUNDS AT (SO MANY) SECONDS, or (SO MANY) ROUNDS
PER MINUTE.
63. CONTINUOUS FIRE. The command for continuous fire is CON-
TINUOUS FIRE RIGHT (LEFT) AT (SO MANY) SECONDS. If
fire is by single piece, RIGHT (LEFT) is omitted. Continuous fire,
when executed by more than one piece, is a succession of salvos, the
pieces being fired consecutively at the interval designated in the com-
mand. CONTINUOUS FIRE remains in effect until the method of
fire is changed or until the command CEASE FIRING is given.
Changes of data are applied so as not to stop the fire or break its
continuity.
64. SINGLE PIECE. The command is NO. (SO-AND-SO). The exec-
utive repeats the command and gives the command FIRE when he
sees that the piece is ready.
38
65. BY PIECE AT MY COMMAND. The command is BY PIECE AT
MY COMMAND. The executive repeats this command. When the
battery is ready to fire, he reports, "Battery is ready." When each
command to fire is received, he commands NO. (SO-AND-SO) FIRE.
66. FIRE AT WILL. The command is TARGET. (SO-AND-SO), FIRE
AT WILL. If a method of close defense has been prearranged, the
command is simply FIRE AT WILL.
67. LADDER FIRE. The command to the executive is LADDER. It
is followed by three ranges 300 yards apart, or by ELEVATION
(QUADRANT)- and three elevations 3 c's apart. The executive has
the designated piece fire one round at each of the three ranges or
elevations, in the sequence of their announcement. At the command
REPEAT LADDER, the same ranges or elevations are fired.
68. SHIFTING FIRE. When the width of the target is too great to
be covered with an open sheaf, it should be attacked by successive shifts.
The number of sheafs required is determined by dividing the width of
the target by width of area covered by an open sheaf (par. 90a). If
this result is fractional, the next greater whole number is used. The
amount of each shift is determined by dividing the difference between
the width of the target and the width of area covered by an open sheaf,
by one less than the number of sheafs required to cover the target. The
result is converted to mils at the target range.
69. ZONE.
a. When the elevation scale or gunner's quadrant is to be used,
the command is ZONE (SO MANY) MILS. It is followed by ELEVA-
TION (QUADRANT) (SO MUCH)— the elevation (quadrant) for
the center of the zone. The executive has the designated pieces fire
at five elevations, in the sequence: center elevation, the elevations
differing from the center elevation by the announced number of mils
in any order, and the elevations (to the nearest mil) midway between
the center elevation and the other two. For example, if the command
is ZONE 10 MILS, ELEVATION 190, the executive has the designated
pieces fire at 190, 200, 180, 185, and 195.
b. When the range scale is to be used, the command is merely
ZONE. It is followed by the range for the center of the zone. The
executive has the designated pieces fire at the center range, the
ranges 100 yards over and short of the center, and the ranges 50 yards
over and short of the center.
70. QUADRANT. ELEVATION SCALE. RANGE SETTING. The com
mand for the use of the gunner's quadrant is QUADRANT, for the
34
use of the elevation scale, ELEVATION. The command for range is,
for example: 4800. QUADRANT or ELEVATION continues in ef-
fect until a different method of laying for range is announced. The
command for elevation is, for example: ELEVATION 178. The com-
mand SAME ELEVATION may be given, but only when more than
one piece is firing and the pieces are laid at different elevations. When
the officer conducting fire transmits ranges, and the appropriate range
drum is not in position on the pieces, firing battery personnel convert
each command for range to an elevation using tabular firing tables
or GFT (TM 9-524 and TM 9-526).
71. PREARRANGED FIRES.
a. Written data for prearranged concentrations, schedules, and bar-
rages usually are sent to the executive by data sheet (figs. 141 and 147).
These data are kept up to date with latest available corrections.
b. Data for barrages are furnished by fire-direction center on data
sheets, if time permits, but may be transmitted to batteries by telephone
or radio when commands are to be executed at once. Any special
instructions regarding the firing, such as shifts, fires to be repeated, and
rates of fire, appear in the REMARKS column.
c. The normal barrage may be started by the piece sentinels or
by the command BARRAGE. When not firing other missions and
unless otherwise directed, the battery is kept laid on its normal barrage.
72. RECORDING BASE DEFLECTION.
a. Before base deflection is recorded the battery must be laid par-
allel (except as authorized in subparagraph b below). The executive
may have base deflection recorded after the initial laying or after a
change of aiming points. The officer conducting fire may order base
deflection recorded at any time; for example, after registration.
Base deflection will not be changed thereafter except on command of
the officer conducting fire. The command is RECORD BASE DE-
FLECTION. Only one base deflection is on record at any time; upon
the command RECORD NEW BASE DEFLECTION, any previous
base deflection is discarded.
b. In case the officer conducting fire desires to have base deflection
recorded with the pieces laid other than parallel, he adjusts the sheaf,
and then commands AS LAID, RECORD BASE DEFLECTION.
c. When the officer conducting fire desires to verify deflection later,
he commands RECORD INSTRUMENT DIRECTION, followed
by commands for corrector, time, and elevation, on completion of the
base point (check point) registration. These commands are based
35
on the results of the registration. The executive fires an air burst at
a site which will surely allow the burst to be seen above the crest in
front of the battery. The 0-3200 line of the instrument is placed on
the burst with the lower motion. Having marked the position of the
instrument with a stake, referred to an object, and recorded the read-
ing, the executive reports "Instrument direction recorded (base point)
(check point No. 1)."
c"
if)
<-*-
o
73. MEASURING COMPASS (fig. 14).
a. The officer conducting
fire may command MEAS-
URE COMPASS. The exe-
cutive does not repeat the
command. He sets up the
aiming circle away from mag-
netic metals, where it can be
used as an aiming point for
the base piece, and with 0-
3200 line approximately in
the direction of fire. The
executive lays his instrument
reciprocally on No. 2 (or
base piece). Using the upper
motion, he then centers the
needle. He subtracts the read-
ing of the scales from the
declination constant (plus
6400 if necessary) and reports
"Compass (so much)."
b. If the executive has a
compass instead of an aiming
circle (or as an alternate
method for the aiming cir- Figure 14. Measuring the compass.
cle), he:
(1) Places the compass away from magnetic metals and in a place
where it can be used as an aiming point for the base piece.
(2) Measures the F-azimuth to the sight of the base piece.
(3) Commands NO. 2 (base piece), AIMING POINT THIS IN-
STRUMENT, MEASURE DEFLECTION.
(4) Subtracts the resulting deflection from the F-azimuth which he
has measured. With compass on the left of the base piece, he adds
6400 if necessary to get a positive number. With compass on the right
Aiming circle
Pass—
of the base piece, he adds 3200 if necessary to get a positive number
or if the difference was less than 3200; he subtracts 3200 if the dif
ference was greater than 3200.
(5) Reports "Compass (so much)."
74. MEASURING BASE ANGLE. The command is MEASURE BASE
ANGLE. The executive does not repeat the command. With the base
piece laid on base deflection he sets up his instrument on the orienting
line where it can be used as an aiming point for the base piece with
the 0-3200. line of the instrument approximately in the direction of
fire. The executive lays his instrument reciprocally on No. 2 (or base
piece). Using the upper motion he then sights along the orienting
line. He reports the reading of the azimuth and micrometer scales
as "Base angle (so much)." The base angle is never greater than 3200
mils.
75. MEASURING DEFLECTION. See the appropriate field manual
on service of the piece.
76. REPORTING ADJUSTED COMPASS. After an adjustment, the
officer conducting fire may command REPORT ADJUSTED COM-
PASS. Upon receiving this command the executive checks the sight of
the adjusting piece and reads the deflection. The executive then de-
termines the difference between this" deflection and the initial deflec-
tion, applies it to the F-azimuth on which the piece was previously
laid, and reports "Adjusted compass (so much)." Note that an increase
(decrease) in deflection causes a decrease (increase) in compass.
77. REPORTING ADJUSTED DEFLECTION. After an adjustment of
the base piece, the officer conducting fire may command REPORT
ADJUSTED DEFLECTION. The executive does not repeat the com-
mand. He compares the deflection on the sight of the base piece with
the base deflection and reports "Base deflection right (left) (so
much)."
78. EXAMPLES OF FIRE COMMANDS. In the examples which fol-
low, a particular weapon is indicated in most cases, but the commands
are applicable, in general, to all calibers.
a. Precision adjustment for registration and for recording base
deflection and instrument direction for observed fire chart; 105-
mm howitzer.
37
(1) Commands:
BATTERY ADJUST
SHELL HE
CHARGE 4
FUZE QUICK
COMPASS 1450
SITE 305
NO. 2 ONE ROUND
ELEVATION 210.
(2) The executive does not repeat COMPASS 1450. After repeating
the commands for ammunition, he uses one of the methods described
in paragraph 51. He then repeats the other commands, and adds FIRE.
(3) To record instrument direction on the completion of the regis-
tration, the command is:
RECORD INSTRUMENT DIRECTION
TIME 11.2
ELEVATION 218.
(4) The executive requires at least 20 mils site to observe a burst
above the mask. He commands for example:
TIME 11.2
SITE 340
NO. 2 ONE ROUND
ELEVATION 218.
An instrument with 0-3200 line in the direction of fire is set up near
the base piece; the executive commands FIRE.
He moves the cross hairs to the burst, refers to an object, and reports
"Instrument direction recorded."
b. Check deflection from recorded instrument direction; 105-
mm howitzer.
(1) Commands:
CHECK DEFLECTION, CHECK POINT NO. 2
CHARGE 4
BASE DEFLECTION RIGHT 280
TIME 15.7
ELEVATION 312.
(2) The executive lays his instrument on the check point by use
of the previously recorded instrument direction. He gives a command
for site that will enable him to observe a burst above the mask. He
commands for example:
38
NO. 2 ADJUST
SHELL HE
CHARGE 4
CORRECTOR 30, TIME 15.7
BASE DEFLECTION RIGHT 280
SITE 350
NO. 2 ONE ROUND
ELEVATION 312
FIRE..
The burst is observed 7 mils right of the vertical cross hair. He applies
a correction of left 7 to the base deflection shift and reports "Ad-
justed deflection check point No. 2, base deflection right 273."
(3) At the fire-direction center, the reported adjusted deflection is
compared with map data' and new corrections determined.
e. Shift from base deflection, zone fire; 105-mm howitzer.
(1) Commands:
BATTERY ADJUST
SHELL HE
CHARGE 5
FUZE DELAY
BASE DEFLECTION RIGHT 120
ON NO. 2 CLOSE 3
SITE 307
BATTERY ONE ROUND
ZONE 7 MILS
ELEVATION 268.
(2) If chiefs of section are to fire the zone, the executive repeats
all commands and adds FIRE.
(3) If the executive is to fire the zone, he repeats all of the com-
mands except ZONE 7 MILS, and adds FIRE. After the volley at 268 is
fired, he commands the next elevation and FIRE; for example:
261 FIRE
275 FIRE
271 (or 272) FIRE
264 (or 265) FIRE.
39
d. Use of aiming point; 75-mm howitzer.
(1) Commands:
BATTERY ADJUST
SHELL HE
CHARGE 3
FUZE DELAY
AIMING POINT, SMOKESTACK, LEFT FRONT
DEFLECTION 2840
ON NO. 3 CLOSE 4
SITE 295
CENTER RIGHT
ELEVATION 270.
(2) The executive repeats all commands and at the proper time adds
FIRE.
(3) To change data after the first salvo, the commands may be:
RIGHT 20
286.
e. Shift from last target and change to time shell; 105-mm how-
itzer.
(1) Commands:
SHELL HE
CHARGE 3
CORRECTOR 30, TIME 18.4
LEFT 60
ON NO. 2 OPEN 2 (to allow for difference in range from last
target)
SITE 315
CENTER RIGHT
ELEVATION 405.
The executive repeats all of the commands and at the proper time
adds FIRE.
(2) To change data after the first salvo, the commands may be:
LEFT 15
DOWN 5
TIME 19.6
435.
The executive repeats all commands and at the proper time adds
FIRE.
40
f. Laying by base angle and recording base deflection without
adjustment.
(1) Commands:
BASE ANGLE 1460
RECORD BASE DEFLECTION.
(2) The executive converts the above commands, thus:
AIMING POINT, THIS INSTRUMENT
DEFLECTION NO. 1, 463; (and so on)
AIMING POINT, AIMING POSTS
REFER
RECORD BASE DEFLECTION.
(3) The executive reports "Base deflection recorded."
g. Precision adjustment on a check point; 155-mm howitzer.
NO. 2 ADJUST
SHELL HE
CHARGE 4, GREEN BAG
FUZE QUICK
BASE DEFLECTION LEFT 140
NO. 2 ONE ROUND
ELEVATION 285. '
The executive repeats all of the commands and adds FIRE.
h. Bracket adjustment; 1 05-mm howitzer, M2. The officer con-
ducting fire announces range. Ricochet fire has been found imprac-
ticable in the area. A graphical firing table is being used at the
battery position to convert fuze range to time, and range to elevation.
(1) Commands:
BATTERY ADJUST (The battery has been firing a mission con-
ducted through the fire-direction center.)
SHELL HE
CHARGE 6
FUZE RANGE 6600
BASE DEFLECTION LEFT 120
ON NO. 2 OPEN 3
SITE 310
CENTER RIGHT
6600.
41
The executive commands:
(The battery has been following commands, and HE shell with
charge 6 has been in use.)
TIME 20.6
BASE DEFLECTION LEFT 120
ON NO. 2 OPEN 3
SITE 310
CENTER RIGHT
ELEVATION 318.
At the proper time he adds FIRE.
(2) To change data after the first salvo, the commands may be:
RIGHT 30
UP 10
6200.
The executive commands:
RIGHT 30
UP 10
TIME 19.1
292.
At the proper time he adds FIRE.
i. Firing for a center of impact to be located by an air ob-
server; 155-mm howitzer, Ml. •
(1) The officer conducting fire commands:
NO. 2 ADJUST
SHELL HE
CHARGE 5, GREEN BAG
FUZE QUICK
BASE DEFLECTION RIGHT 260
NO. 2 FOUR ROUNDS
DO NOT LOAD, ELEVATION 380.
The executive repeats all of the commands.
(2) When the air observer directs the battery to fire, the officer
conducting fire commands: 380. The executive repeats the command
and adds FIRE.
42
j. Firing a ladder; 105-mm howitzer.
(1) The officer conducting fire commands:
NO. 2 ADJUST
SHELL SMOKE
CHARGE 4
CORRECTOR 30
TIME 12.9, 11.6, 10.3
BASE DEFLECTION
SITE 300
NO. 2, LADDER
ELEVATION 297, 269, 242.
(2) The executive repeats the commands as given except TIME 11.6,
10.3; LADDER; and ELEVATION 297, 269, 242. For these he sub-
stitutes NO. 2, ONE ROUND, ELEVATION 297, FIRE; TIME 11.6,
ELEVATION 269, FIRE; TIME 10.3, ELEVATION 242, FIRE.
Note: The command ELEVATION may be given for each round to
avoid errors.
k. Transfer of fire at a range of 16,930 yards; 155-mm gun;
fuze quidc, time on target; battery front 200 yards, uniform.
(1) Commands:
BATTERY ADJUST
SHELL HE
SUPERCHARGE
FUZE QUICK
BASE DEFLECTION RIGHT 312
BATTERY THREE ROUNDS
DO NOT LOAD, QUADRANT 280
• • • •
TIME ON TARGET, three minutes from . ... NOW, QUAD-
RANT 280. ■
(2) The executive determines the time of flight to be 35 seconds.
He repeats all commands except "TIME ON TARGET, three minutes
from .... NOW," and announces the second QUADRANT 280 so
as to coordinate the time of loading to insure that the projectile will
not be in the bore longer than 30 seconds prior to firing. He gives the
command FIRE at the appropriate time, from the count by fire-
direction center.
43
PART THREE
Paragraphs 79 to 169, inclusive, pages 44 to 150, inclusive, are
superseded by Change 2, FM 6-40, 20 February 1947, which will
be found at the back of this manual.
PART FOUR
SURVEY
CHAPTER 1
SURVEY PRINCIPLES
Section I. BASIC PRINCIPLES
170. GENERAL
a. Whereas the observed fires of a battery can be maneuvered with-
out a chart, a firing chart or an observed fire chart is essential for
maneuvering the observed fires of battalions and larger units. A fir-
ing chart is necessary for all unobserved fires.
b. A firing chart is a map, photomap, or grid sheet on which are
plotted to a known scale the relative horizontal and vertical locations
of the base point, check points, targets, batteries, and any other data
necessary for the preparation of fire.
c. The effectiveness of artillery fires and the amount of ammuni-
tion which must be expended in order to insure effective fire are af-
fected by the relative accuracy and completeness of survey. Every ef-
fort must be made to augment survey by the following:
(1) Vertical photographs.
(2) Oblique photographs.
(3) Stereoscopic pairs (or strips).
d. The purpose of field artillery survey is to determine the hori-
zontal and vertical locations of points on the ground in order that
they may be placed on the firing chart, and to provide a means of
orienting the pieces on the ground. The accuracy of the chart should
be checked by firing as soon as possible. (See par. 72 for recording di-
rection thus established.)
e. An observed fire chart is a chart on which the location of points
and the orientation of the pieces have been determined by firing,
rather than by survey.
171. OBSERVED FIRE CHART. Observed fire charts which are based
on registration are used before the firing chart is completed. When
151
no topographical information is available, an arbitrary location on a
chart is selected as the base point. The registration data of each bat-
tery, converted to back azimuth and range, are then plotted from the
base point. When a map or photomap is available and the base point
can be identified on it by inspection, the map or photomap is used
as the observed fire chart; the battery positions may be located either
by inspection or by plotting back azimuth and range. The observed
fire chart is used for massing observed fires based on an adjustment
by one of the batteries. As soon as survey is completed, the observed
fire chart is replaced by the firing chart in order that unobserved and
surprise fires may be executed.
172. FIELD ARTILLERY SURVEY. Field artillery survey is not an end
in itself, but is rather a means to an end— namely, furnishing to the
fire-direction center such information as will enable the fire-direction
center to determine data necessary to place the fire of any or all pieces
of the unit on any point within range. All survey should be performed
with a definite goal and should be based upon a carefully formulated
plan. It should be planned to preclude wasted effort. Survey is not
concluded as soon as the firing chart is complete; it is never ending
and ceases in one area only to commence in another. Survey is con-
tinued in order to check and improve earlier survey, augment target
locations, locate alternate positions, and extend common control.
173. THE BASIS OF THE FIRING CHART.
a. The type of firing chart used depends upon the amount of topo- .
graphical information available in the form of maps and photomaps.
When a map or photomap of suitable scale is available, it is used as
the firing chart initially. For convenience, a copy of the survey (on
a map) may be reproduced on a grid sheet, which may then be used
as a firing chart. The accuracy of the map or photomap is checked by
survey as soon as possible (par. 294). If the map or photomap is proved
inadequate by the survey, the grid sheet becomes the firing chart, with
details transferred from the map or photomap to the grid sheet by
restitution when necessary. When the only map available has a scale
too small to permit use of the map as a firing chart, basic informa-
tion, upon which to initiate a grid sheet survey, may be taken from
the map. This basic information should be coordinated with and
supplemented by control information obtained from the higher sur-
vey echelons in the area. This procedure will permit delivery of fire
on targets reported by coordinates on the small scale map.
b. When an available map or photomap has been checked by survey
or firing and found to be accurate, it is suitable for use as a firing
152
chart, and a large part of the survey will have been accomplished by
the map maker or. camera. In this case, the firing chart is constructed
by locating on the existing survey the base point, check points, targets,
batteries, and other- critical points. When maps or photomaps are not
available, the field artillery surveyor must perform the entire survey,
locating, in their relative positions on a blank grid sheet, the points
needed for the firing chart. This process, which in the final analysis
is the making of a map with incomplete coverage of the target area,
is slow and laborious as compared with survey for use on a map or
photomap.
c. The ideal equipment for use in firing observed and unobserved
fires— ranging from use of a single gun to employment of masses of
artillery— includes all of the following: a first class map of a scale not
smaller than 1/50,000 (and preferably 1/25,000) which was constructed
by accurate survey; a mosaic prepared from recent photographs; cur-
rent stereoscopic intelligence photographs of the target area; and ob-
lique photographs, if procurable. The completeness of available photo-
graphic coverage is influenced by weather, planning, and availability
of airplanes.
174. SURVEY OPERATIONS INVOLVED IN CONSTRUCTION OF
A FIRING CHART. To construct a satisfactory firing chart, any in-
formation shown on the chart must be supplemented by additonal in-
formation obtained by survey. To obtain this information some or
all of the operations listed below will be necessary:
a. Determination of any basic chart information which is necessary
to tie the survey to the chart or which will facilitate the survey by mak-
ing use of control afforded by the chart. Such basic information in-
cludes chart scale, chart location and altitude of ground points, and
chart azimuth of ground lines.
b. Determination of relative horizontal location of batteries, targets,
and observation posts.
e. Determination of relative vertical location of batteries, targets,
and observation posts.
d. Determination of base angles, in order to lay the guns in a known
direction.
e. Provision for uniform declination of instruments.
175. DIVISION OF SURVEY OPERATIONS.
a. The survey operations listed above logically break down into
three phases which are performed by details as follows:
(1) TARGET AREA SURVEY. The detail conducting the target
area survey determines the relative horizontal and vertical location
153
of the base point, check points, targets, key terrain features, and ob-
servation posts. The location of points in the target area will in
most cases be determined by the use of a target area base, which is
oriented by a line of direction to a reference point preferably in the
target area. This method of orientation insures the proper tying to-
gether of all points in the target area. Target area survey is contin-
uous throughout all operation.
(2) POSITION AREA SURVEY.
(a) The detail conducting the position area survey determines the
relative horizontal and vertical locations of the batteries; establishes
the orienting line, or lines, on the ground and determines their direc-
tion; and computes the base angles.
GROUND OPERATIONS
NOTE: (q) Angles 1, 2, 3,ond 4
are angles measured in order
to tie target area and position
area to reference point. If
division control were available,
reference point would be a
point furnished by division. The
AS base is oriented by an
instrument reading to the
reference point .
(b) Some points whose
locotions have been determined
"are not plotted because they
are of no value to fire direction
personnel .
CHECK
[POINT I BASE
-IPOtNT-
[PU1N
- CHECK
IPOINT
INO 2
GRID SHEET
Figure 48. Battalion survey when a grid sheet is used.
154
(b) The survey should be run to the base piece of each battery or
to the staked location selected for occupation by the base piece. In
order not to delay the survey when information on the location of the
base piece is not available at the time the survey is performed, con-
trol may be run to a point near the center of the battery position area
and a stake placed at this point. On occupation of position, the exe-
cutive will report to the fire-direction center the azimuth, distance,
and difference in altitude from this stake to the location of the base
piece of his battery, in order that the true position may be plotted on
the firing chart.
The orienting line is materialized on the ground by means of stakes.
Short stakes should be placed on the orienting line, over which the bat-
CROUND OPERATIONS
NOTE: (o) Angles 1, 2, 3, and 4
are angles measured in order
to tie target area and position
area to reference point. If
division control were available,
reference point would be a
point furnished by division. The
AS base is oriented by an
instrument reading to the
reference point.
(b) Some points whose
locations hove been determined
ore not plotted because they
are of no value to fire direction
personnel.
■
.REFER
Ipoint
i
ENCE
CHE
CK
NT BA
' Ipo
i
,POt
INO
i
SE
NT
CHECK
■ POINT
(NO 2
i
NT
— r
i
_lc_ -
i
-j*. -r
PHOTOMAP
Figure 49. Battalion survey when a photomap is used.
155
tery executives can place their instruments; tall stakes are placed on
the orienting line within sight of each of the short stakes to furnish
the battery executives a means of sighting along the orienting line.
It is desirable that the stakes to be sighted on should be located at
least 200 yards from the stakes over which the instrument is placed.
Only one limitation must be observed in the selection of an orienting
line: the line must pass conveniently near the battery which it serves.
The direction of the orienting line with regard to the direction of fire
is immaterial. The decision to establish one orienting line, or two, or
three for a battalion is based partly on considerations of accuracy (a
single orienting line is more accurate because fewer angles are meas-
ured) but to a greater extent on time and feasibility. Frequently, the
GROUND OPERATIONS
NOTE-" (a) Angles 1, 2, 3, and 4
are angles measured in order
to tie target area and position
area to reference point . If
division control were available,
reference point would be a
point furnished by division. The
AS base is oriented by an
instrument reading to the
reference point.
(b) Some points whose
locations have been determined
are not plotted because they
are of no value to fire direction
personnel .
Figure 50. Battalion
.REFER
[point
i
ENCE
CHf
CK
NT iBA
1 IPO
i
,POl
[NO
i
E
NT
CHECr
|POINT
INO 2
i
NT
— r
i
lc
i
BATTLE MAP
when a baffle map is used.
156
establishment of two or more orienting lines in preference to a single
orienting line, will result in a great saving of time. It may be physically
impossible to establish a single orienting line which will pass conveni-
ently close to all batteries. The position area survey detail computes
the base angle for each battery.
(3) CONNECTION SURVEY. The detail conducting the connec-
tion survey establishes the control required to tie the target area and
position area together.
b. The determination of basic information is not assigned to any
particular detail because its determination will vary with the type of
firing chart used. Provision for uniform declination will usually be
made by higher echelon than battalion; if not, the battalion survey of-
ficer must include it in his plan and assign it to the detail that can
handle it with the greatest facility.
c. Figures 48, 49, and 50 illustrate battalion survey operations with
various types of charts as the basis of the firing chart.
176. EFFECT OF REGISTRATION ON SURVEY OPERATIONS.
a. Registration does not affect the actual survey operations neces-
sary to establish a firing chart except in so far as orientation of pieces
NOTE:
a. Grid sheet. Targets located with
respect to base point by survey. Pieces
located with respect to each other by
survey. Pieces located with respect to
base point and oriented by survey.
b. Photomap or battle map. Base
point and targets located by inspec-
tion or survey. Pieces located by in-
spection or short traverse. Pieces tied
to target area by photomap. Pieces
oriented by survey.
Figure 51. Registration prohibited.
and speed of opening fire are concerned. If registration is permitted,
the pieces may be oriented by firing; whereas, if registration is pro-
hibited, it is necessary to orient the pieces by survey. If time permits,
it is desirable that the pieces be oriented by survey even when registra-
tion is permitted, and the data obtained from registration used to de-
termine corrections to apply to the surveyed data.
b. Although registration does not affect the amount of survey neces-
sary to establish a firing chart, it will affect the order in which the sur-
vey operations are performed. Priority is given to target area and po-
sition area survey inasmuch as the connection between the two can be
established by firing if necessary.
c. Figures 51, 52, and 53 illustrate the relation between registration
and survey.
POSITION
i
44h
AREA
NOTE:
a. ^ Grid sheet. Targets located with
respect to base point by survey. Pieces
located with respect to each other by
survey. Registering piece located with
respect to base point and oriented by
firing. Nonregistering pieces oriented
by means of surveyed orienting line.
b. Photomap or battle map. Base
points and targets located by inspec-
tion or survey. Pieces located by in-
spection or short traverse. Pieces tied
to target area by photomap. Regis-
tering piece oriented by firing. Non-
registering pieces oriented through
surveyed orienting line.
Figure 52. Registration limited.
158
NOTE:
a. Grid sheet. Targets located with
respect to base point by survey. Pieces
located with respect to base point and
oriented by firing.
b. Photomap or battle map. Base
point and targets located by inspec-
tion or survey. Pieces located by in-
spection or short traverse. Pieces tied
to target area by photomap. Pieces
oriented by firing.
O
o
TARGET p)
AREA q W
BASE
POINT
\ o
Figure S3. Registration unlimited.
Section II. ECHELONS OF SURVEY
177. SURVEY ECHELONS.
a. General. Although the flow of survey control is from higher to
lower units, the subordinate unit never waits for this control before
commencing its survey. Survey can be initiated on assumed control and
converted to common control when common control is made available.
b. Corps.
(1) The corps topographic engineers are normally the highest echelon
from which artillery survey sections obtain control. These engineers
furnish control to the corps observation battalion or, in some cases,
work in conjunction with the observation battalion in extending this
control to divisions and separate battalions.
(2) The corps observation battalion normally furnishes common
control to the artillery units of the corps. This is done by providing
159
each division and separate battalion with the coordinates, altitude, and
ground location of two intervisible points; or the coordinates, altitude,
and ground location of one of these points, and a F-azimuth to the
other point. One of these points is in the division or battalion position
area, and the other is preferably in the target area. The observation
battalion, in addition to furnishing this control, frequently coordinates
with lower echelons the survey to be performed, in order to prevent du-
plication of survey effort.
The coordination and extension of control is accomplished through
a corps survey information center established by the observation battal-
ion. A record is maintained there of all survey control available in the
corps sector. All requests for control should be made to the survey in-
formation center, and all survey data determined by units of the corps
should be reported to it. Since the degree of accuracy of survey varies
within the several echelons of survey, the source of any control ob-
tained at the survey information center should be considered.
c. Brigade. The brigade survey is the same as that performed by
division artillery. See subparagraph d below.
d. Division artillery. The purpose of division artillery survey is to
place all the artillery of the division on the control which is common to
all the artillery with the largest unit involved in the sector, in order that
fire from its battalions can be massed either as a unit or in conjunction
with the fire of other divisions and reinforcing artillery. To extend
common control within the division, division artillery furnishes each
battalion the coordinates, altitude, and ground location of a point in
the battalion position area, and the F-azimuth of a line from that point
to a point (preferably in the target area) visible from the first. Instead
of furnishing a F-azimuth between the two points, the coordinates, alti-
tude, and ground location of the second point may be furnished. Al-
though the division artillery survey section establishes a division artil-
lery survey information center where this control will be available, it is
the responsibility of the division artillery commander, usually delegated
as a duty of the division artillery survey officer, to ascertain that the
control reaches each battalion. Division artillery also assists the battal-
ions in transferring to common control by furnishing any available sur-
vey information in addition to that listed above.
e. Group and regiment. Neither the group nor the regiment per-
forms any survey.
f. Battalion. The battalion is primarily interested in the construc-
tion of a firing chart which will permit the maneuver of the fire of the
battalion and its batteries, and the delivery of unobserved fires.
g. Battery. Battery survey is an integral part of the battalion sur-
vey. Batteries will rarely perform an independent survey.
160
Section III. RESPONSIBILITY OF COMMANDERS FOR SURVEY
178. GENERAL. The commander is responsible that his unit be able
to deliver the most effective support at all times. In order to facilitate
the delivery of such support, the commander must:
a. Foresee the need for, plan for, procure, keep up to date, and dis-
tribute maps and photomaps, in order that the best possible chart is
available for use as a firing chart.
b. Augment survey by obtaining proper coverages by aerial photo-
graphs, thus permitting the fixing and locating of targets beyond the
normal means of survey by ground observation.
c. Continually seek observation for his unit. To exploit to the full-
est the possibilities of artillery, all artillery units must not only have the
ability to mass their fires but must also be able to observe the fires and
apply any corrections which will increase their effectiveness.
d. Strive to obtain permission for his pieces to register. The in-
creased effectiveness in fires obtained through registration should be
impressed on supported commanders.
e. Establish a standing operating procedure and conduct training
so that with minor changes this procedure will fulfill the requirements
of most situations. Such items as assumed control on the grid sheet,
loading charts for survey personnel, use of battery personnel, etc., read-
ily lend themselves to standardization in a standing operating pro-
cedure.
f. Be familiar with the survey principles outlined in chapter 8, part
four.
Section IV. SURVEY ELEMENTS IN THE FIELD ORDER
179. GENERAL. The artillery commander should not attempt to form-
ulate the detailed survey plan. His survey officer is a specialist in this
field and will formulate the detailed survey plan. The commander,
however, should discuss with the survey officer the possible survey solu-
tions so that a plan will be drawn up which will provide the most sat-
isfactory results for the situation at hand.
180. SURVEY ELEMENTS IN THE FIELD ORDER. The following in-
formation, vital to the survey officer in formulating his plan, must be
furnished by the commander.
161
a. Situation. It is essential that the survey officer be familiar with
the situation, to include:
(1) Mission of the battalion.
(2) The status of registration.
(3) Time available for survey. The commander should always allow
the maximum possible time for survey inasmuch as the time available
will determine the relative accuracy of the firing chart.
(4) Normal and contingent zones.
b. Firing chart to be used. The decision as to which chart to use
will depend upon topographical information available, time available,
and the mission.
e. Available survey control furnished by higher echelons. The
commander should inform the survey officer where and when survey
control is to be made available. If no control is available, arrangement
must be made for assumed control.
d. Availability of additional survey personnel.
e. Designation of base points and checks points. The base point,
reference points, and check points should be pointed out on the ground
and definitely identified by the survey officer and any other personnel
who might be called upon to register on them. Such points should be
identifiable on an air photograph. Sufficient check points must be des-
ignated throughout the target area to insure rapid and accurate mass-
ing of artillery.
f. Location of observation posts and fire-direction center. The
locations of possible observation posts must be identified on the ground.
The general location of the fire-direction center must be known to
survey personnel.
g. Location of position areas.
162
CHAPTER 2
GRID SYSTEMS AND PLOTTING
Section I. FIRE CONTROL GRID
181. DESCRIPTION.
a. Military maps habitually have a rectangular grid superimposed.
On battle maps the distance between adjacent grid lines ordinarily rep-
resents 1000 yards or 1000 meters. A grid o£ this type is known as a fire
control grid; the term implies that the azimuth and scale are accurate.
b. The grid lines are numbered in accordance with the grid system
to which the map conforms. Military maps may be provided with a
standard grid for a particular theater of operations, or with an arbitrary
grid used for a single map or for a limited area.
182. COORDINATES.
a. Writing coordinates. The distance in yards of any point east
or west of the zero Y-line is the X-coordinate, and the distance north
or south of the zero X-line is the Y-coordinate. In writing coordinates,
the X-coordinate is written first and the F-coordinate last, with a dash
between, and the whole included in parentheses, thus: (804.729-
1286.684). A decimal point is used to mark the division between thou-
sands and hundreds of yards.
b. Designation of sheet. The name of the sheet of the map is part
of the designation of a point by coordinates; when the identity of the
map is clear to all concerned, its designation need not be given.
c. Abbreviated coordinates.
(1) If location to the nearest 10 or nearest 100 yards only is desired,
or if the measurements cannot be made with greater accuracy, the digits
indicating units or tens, respectively, may be omitted. Thus the coor-
dinates of a point may be written:
(804.729-1286.684) to the nearest yard.
(804.73-1286.68) to the nearest 10 yards.
(804.7-1286.7) to the nearest 100 yards.
(2) It seldom is necessary to give more than two digits left of the
decimal for each coordinate. The coordinates for the point given above
then would be (04.729-86.684).
163
(3) If the point is fixed within an area 10,000 yards square, only one
digit need be given before the decimal point of each coordinate. The
coordinates of the point given above would be (4.729-6.684); to the
nearest 100 yards (4.7-6.7). If a point is designated by abbreviated co-
ordinates, the decimals and the dash may be omitted, thus (4767).
These are known as hectometric coordinates.
803
804
805
806
1289
1288
1287
1286
1285-
1284
8.
o
o
O-
684-^
o
O-
o
o
o-
729
500
i i i i I i il
1000
J_L
I I I I I I III I
500 \ 1000
1500 2000
o
I— o
o
-O
o
o
-o
-o
>^ 7-684
1500 2000
729
803 804 805 . 806
Figure 54. Plotting a point from coordinates on a normal grid.
164
183. PLOTTING A POINT FROM COORDINATES.
a. Normal grid. (See fig. 54.) To plot a point whose coordinates
are (804.729-1286.684), place the zero of the scale on F-line 804.000, and
the 1000-yard point on F-line 805.000. Holding the scale about one
square above the approximate location of the point, mark 729 yards
with a fine-pointed pencil or plotting needle. Place the scale about one
square below the approximate location of the point, repeat the opera-
tion, and connect the two marks with a fine, light line. This will be
a F-line passing through the point. In a similar manner determine the
X-line passing through the point. The intersection of these lines is the
desired point. If the plotted point is to be used a number of times, the
intersection of the lines is pricked with a fine needle to prevent erasure,
and the lines in the vicinity of the point are accentuated with a soft
pencil. These accentuations do not extend to the point.
Figure 55. Plotting a point from coordinates when grid lines are
closer than normal.
b. Grid lines closer than normal. (See fig. 55.) Plot the point as
before, inclining the scale so that zero is on one grid line and the 1000-
yard point is on the other. The point then will be plotted* in its true
relation to the grid, as the digits after the decimal point express the
proportional part of the distance between grid lines.
c. Grid lines more distant than normal. (See fig. 56.) Measure
the distance between the grid lines and find the difference from normal.
The proportional part of this difference is added to a measurement.
165
Figure 56. Two methods of plotting points from coordinates when
grid fines ore more distant than normal. In method b, the scale
should be reversed to check accuracy of first plot.
For example, if the distance between grids measures 1020, the difference
from normal is 20 yards, and the proportional part of this difference for
a 400-yard measurement is 400/1000 of 20, or 8 yards. The 400-yard
measurement is scaled as 408 yards. Similar results can be obtained by
inclining the scale so that zero is on one grid line and the 2000-yard
point is on the other. The yards to be plotted are multiplied by 2, and
that distance scaled. In the above example, in plotting the X-coordi-
nate, the 400-yard measurement would be scaled as 800 on the inclined
scale. .
184. MEASURING THE COORDINATES OF A POINT. Coordinates
are measured in the same manner as they are plotted except that the
distance is read directly between the point and the grid line. Write
the number shown at the top or bottom of the F-line west of the point;
place a decimal point, and write the distance of the point from this
F-line. Place a dash, then the number shown at the right or left end
of the X-line below the point; place a decimal point and then write the
distance of the point from this line. Inclose the whole in parenthesis.
If abbreviated coordinates are desired, make the measurements to the
nearest 10 or 100 yards, depending upon the approximation desired. If
the grid lines are not standard distances apart, the measurements are
made as in plotting points.
185. USE OF THE COORDINATE SCALE. The method of plotting
described in paragraph 183 is the accurate method used for survey and
166
for the plotting of target locations for prearranged (schedule) fires.
However, it is rather slow when the rapid massing of fires on targets of
opportunity or on transient targets is necessary. With a coordinate scale,
such as shown in figure 57, targets may be plotted more quickly and
with sufficient accuracy. The scale should be so placed that its hori-
zontal scale is in coincidence with a grid line. When the coordinate
scale is used in designating or in plotting targets, no attempt should be
made to correct for grid lines that are closer together or farther apart
than is normal, since the targets will usually be area targets.
To determine the coordinates of a point (fig. 57), the intersecting
grid lines forming the lower left hand corner of the grid square in
which the point is located are first indicated; then the two numerical
coordinates are read to the right and up, in that order, in the same
manner as for the fire control grid.
6 7 8
Figure 57. Method of using the coordinate scale.
186. MEASURING AND PLOTTING ANGLES WITH A PROTRAC-
TOR.
a. General. The center of the protractor must be placed exactly
over the vertex of the angle, and the base exactly over one side of the
angle. For greater accuracy, measure the angle from both sides of the
T67
protractor and take the mean of the readings. For example, measure
first with the arc to the right of the center; then with the arc to the
left of the center. The difference, if any, between the reading will be
small. The mean of the readings is used,
b. Measurement of a IT-azimuth.
(1) ORIENTING THE PROTRACTOR FROM A F-LINE. (See
fig. 58a.) The F-azimuth of a line can be measured by using its inter-
section with a F-line as the vertex. The protractor is placed so that the
clockwise angle, F-line to given line, is read. If the F-azimuth is greater
than 3200, the proper relation of the measured angle to 3200 or 6400
must be determined.
(2) ORIENTING THE PROTRACTOR FROM AN X-LINE. (See
fig. 58b.) The F-azimuth may also be measured by using the intersec-
tion of the line with an X-line. Place the center of the protractor over
the intersection, and the 1600-mil graduation of the protractor on the
X-line; the reading of the line is the F-azimuth. If the F-azimuth is
greater than 3200 mils, the proper relation of the measured angle to
3200 or 6400 must be determined.
Orienting the protractor from a Y-line Orienting the protractor from on X-line
Figure 58. Method of measuring Y -azimuth with a protractor.
168
e. To draw a line of given azimuth through a point. If the point
is on either an X-line or a 7-line, the line is drawn in the same manner
as described above for measurement of F-azimuth. If the point is not on
a grid line, the line may be drawn in either of the following ways:
(1) (See fig. 59.) The protractor is placed with its center exactly
over the point, and the base (straight edge) of the protractor roughly
parallel to either an X or Y grid line. Rotate the protractor about the
point until an X-line (y-line) cuts off the same length of arc on both
ends of the protractor. (In figure 59, the X-line cuts 165 mils of arc
from each end of the protractor.) The base of the protractor is now
parallel to the X-lines (F-lines). A line of given azimuth (or back azi-
muth) is drawn by marking a point at the circumference of the protrac-
tor and drawing a line through the given point and the marked point.
In figure 59, the line drawn has a F-azimuth of 5635. Care must be
taken to add or subtract multiples of 1600, depending on the quadrant
of the azimuth in question.
Y-AZI
\ 5635
MUTH
!*
\
,700 '800 ts 0(
* w «C7°o
/
\
V%
v\
1650
poop; 8' §^
^165 irf
DOiOe .
1 llll
TfnTm'
tr,,rfl,r
flu?
«
i 1 1
TImTm
SBi , °°| 01
Figure 59. Method of drawing line of given Y-azimuth through
a point.
(2) (See fig. 60.) The protractor is placed with its center over a grid
line adjacent to the point. The protractor is then revolved until the
grid line passes through the angle giving the desired azimuth. The pro-
tractor is held in this position and moved along the grid line until the
straight edge of the protractor passes through the point. The line of
desired azimuth is drawn by marking a point at the straight edge of the
protractor and drawing a line through the marked point and the given
point.
169
^600 trr
(6400-
5800)
ZIMUTH
5800 tK\
— "^J*
'^,0 2*>° 2 T0 ° K
c
00 ?500 ?,L 7
s'
x * *
— v — %^
V
%.
V \
f-tf
/ \
\
— +- — i
i-P
fjj
/ 3
V'*°\
....
Figure 60. Alternate method of drawing line of given Y-azimuth
through a point.
187. MEASURING AND PLOTTING DISTANCES WITH THE PLOT-
TING SCALE.
a. Measuring. The most accurate method of measuring distances
is with the plotting scale.
b. Plotting.
(1) After the direction of a line has been established on the chart
(par. 186) its length may be plotted with the plotting scale.
(2) A more accurate method of plotting a definite line for distance
and direction is to plot its extremities, using coordinates. Frequently
the coordinates of the ends of a line will be too close together to pro-
vide a good base for drawing the line. In order to get points that are
more widely separated but on the same line, determine the differences
in the X and Y coordinates of the two points, multiply these differ-
ences by the same figure, and apply the products to the coordinates of
the first point-the result is the coordinates of a third point which,
when plotted, will lie on an extension of the line between the first two
points. Example (fig. 61): the coordinates of point M are (860.200-
170
860 861 862 863
1291 M= —
864
M
1290-
1289
1288
1287
N'l
Figure 61. Method of plotting a line by coordinates.
1290.900), and the coordinates of point N are (860.500-1290.500). It is
desired to plot the line MN. The points when plotted are too close to-
gether to allow a line to be drawn between them satisfactorily. To get
points that are more widely separated the procedure outlined above is
followed:
Coordinates Point N =
Coordinates Point M =
Dx _
Dy _
Multiply by same figure
Apply to Point M
Coordinates N'
860.500-
1290.500
860.200-
1290.900
+.300
-.400
10
10
+3.000
-4.000
860.200
1290.900
863.200—1286.900
The point N' is plotted and the line MN' is drawn. MN' has the same
direction as MN.
171
188. MEASURING AND PLOTTING ANGLES AND DISTANCES
WITH THE RANGE-DEFLECTION FAN.
a. General.
(1) When several angles and distances are to be plotted or measured
using one point and one line of direction (and when great accuracy
is not necessary), the procedure is greatly facilitated by the use of the
range-deflection fan. The range-deflection fan is of particular value
in the fire-direction center for use in determining deflection shifts
and ranges, and for plotting targets. The range-deflection fan usually
has three scales for measuring horizontal angles, each scale capable
of measuring angles up to 500 mils. The space available on the fir-
ing chart will govern the selection of the scale to be used. If more
than one scale falls on the chart, the scale most distant from the vertex
should be used. The vertex of the fan is always placed against a pin
in the point of origin. The pin should be slightly inclined toward
the direction of measurement.
(2) All scales of the range-deflection fan should be checked with scales
known to be accurate. Fans with inaccurate scales should be replaced;
however, small errors may be corrected or, for short periods of time,
compensated for by means of a K. Subsequently, the charts or maps
upon which the fan is used should be checked from time to time with
the fan. For example: originally the distance between two points on
the firing chart scales 6620 yards; later, due to stretch, the distance
between the same two points measures 6680 yards. Corrections for
the charts may be determined by periodically testing the maps in both
directions. This procedure is particularly applicable to the firing charts
of long range artillery.
b. Measuring. (See fig. 62.) Assume that three points, A, B, and
C, have been plotted on the chart. Jt is desired to measure the distance
from A to C and the angle between the lines AB and AC. With the
vertex of the fan at A and one side running through B, a fine line is
drawn along that side of the fan. The line should extend from ap-
proximately 1 inch short of to 1 inch beyond the selected deflection
scale. An inverted arrow is placed on this line 1/8 inch beyond the
selected scale. With the edge of the fan against a pin in C, the distance
AC is read opposite the pin (8100) and the angle is read on the de-
flection scale opposite the arrow (240 rfi). The normal use of the range-
deflection fan is measurement of shifts. In the example above, the
shift from B to C, vertex at A, is left 240. (Note that the left side of
the fan is against the pin in C. For a left shift the left side is always
against the pin for a right shift, the right side.)
172
Figure 62. Method of measuring or plotting a distance and an
angle with the range-deflection fan.
c. Plotting. (See fig. 62.) The procedure for plotting an angle and
a distance is very similar to that used for measuring an angle. Assume
in the situation above that points A and B have been plotted on the
chart. It is desired to plot C 240 mils left of the AB line at a distance
of 8100 yards from A. An extension of the AB line is made as described
above. The fan, with vertex at A, is moved until the extension of AB
cuts the fan at 240 mils right of the left edge. With the fan in this
position, C is plotted at the left edge of the fan at a distance of 8100
yards.
Section II. POINT DESIGNATION GRID.
POLAR COORDINATES, AND POLAR PLOTTING
189. POINT DESIGNATION GRID.
a. The printing of accurate fire control grids on photomaps is im-
practicable because pf distortion and the difficulty of reproducing a
photo to a desired scale. Therefore, an arbitrary grid, known as the
point designation grid, is usually used. This grid has no relation to
the actual scale or orientation of the photo; it serves only for point
or target designation and normally is not suitable for measurement
of distance or azimuth. If the 1/20,000 scale is to be used for deter-
mining and plotting the coordinates of points, a point designation grid
of 1.8-inch grid squares is the most convenient. Depending upon the
scales that are current, other dimensions for the grid squares may be
more desirable; for example: for a scale of 1/25,000 graduated in
173
yards, use 1.44-inch grid squares, and for a scale of 1/25,000 graduated
in meters, use 1.575-ihch grid squares.
Figure 63. Wide angle photo with point designation grid. Note
collimation ticks.
b. The point designation grid may be printed on the photo (as is
the case with the wide angle photo, figure 63). For ungridded photos,
a transparent template with the grid printed on it may be used; it is
essential that all who use this procedure place their templates in ex
actly the same position on the photo.
c. The coordinate scale (fig. 64) is always used to plot points or to
measure coordinates on a chart having a point designation grid.
190. POLAR COORDINATES AND POLAR PLOTTING. Points may
be designated by an angular measurement from a determined direction
and a distance from a known point. The angular measurement and
distance are known as polar coordinates. The angular measurement
may be made from Y-north or clockwise from a line fixed by two
174
K
Figure 64. Method of reading coordinates with coordinate scale.
The point indicated may be read KN 86 or KN 7763, according
to degree of accuracy sought.
known points. The distance may be measured with any predesignated
scale. If the distance is measured with one scale and plotted with
another, the conversion is accomplished as outlined in paragraph 196.
The point of origin, the direction from which to measure the angle,
and the scale to be used must be prearranged. The procedure of plot-
ting a point from polar coordinates is known as polar plotting.
175
CHAPTER 3
AERIAL PHOTOGRAPHS
Section I. VERTICAL PHOTOGRAPHS
191. GENERAL.
a. Air photos furnished to the field artillery will be reproductions
of either individual verticals or obliques, or of mosaics assembled from
individual wide angle verticals. Air photos are of great value to the
field artillery in reconnaissance and in locating and designating tar-
gets. In some cases, air photos may also be used as firing charts. When
a map or a grid sheet is used as the firing chart, air photos are used to
supplement them for horizontal and vertical locations.
b. See FM 21-26 for interpretation of air photos.
192. WIDE ANGLE VERTICAL. The wide angle vertical is an air
photo taken with the camera plate as nearly horizontal as practicable.
The wide angle camera has a 6-inch focal length and an angle of view
of approximately 70 degrees across the square dimension of the photo
and approximately 90 degrees across the diagonal. Wide angle verticals
normally are taken in a series of overlapping photos.
193. TILT AND RELIEF.
a. General. If a flat piece of terrain is photographed vertically,
the result is a map that is perfect in all planimetric detail (fig. 65).
However, the vertical air photo is subject to distortion of detail due
principally to tilt of the camera and to relief of the terrain photo-
graphed.
b. Tilt. If the camera is not level at the moment the photo is taken,
the scale of the photo will not be uniform. In figure 66, it is evident
that a horizontal line of a certain length near A will appear longer on
the photo than a line of the same length near B, since A is nearer the
camera than B. When the tilt is small, as is the case in a carefully made
photo, the resulting errors are negligible for artillery work. In a series
of overlapping photos taken on a single flight, excessive tilt of one
photo is apparent if its center is materially out of place with reference
to the line of centers established by the other photos. Field artillery
176
PLATE
GROUND
Figure 65. Relation of photo to ground.
units are not equipped to remove tilt from photos. If tilt is large
enough to distort the photo materially, the photo will be almost useless
for survey purposes.
Figure 66. Effect of tilt.
177
c. Relief. The second important
sidering figure 67a as any vertical
section through the axis of the
lens, it is seen that C will be re-
corded in its true position, the cen-
ter of the photo, regardless of its
altitude. With reference to a hori-
zontal datum plane MN, the object
A at a greater altitude will record
as an object located at A'; simi-
larly B will record as at B'. These
displacements are radial from or to-
wards the center, as shown in fig-
ure 67b. For a given altitude of
the airplane, the amount of dis-
placement varies directly as the
horizontal distance from C and the
height above or below C. Note
that directions of the radial lines,
CA and CB, are not changed by
the displacements of A and B. The
relief distortion of any particular
point varies inversely as the alti-
tude of the airplane. The amount
of distortion can be found by solv-
ing the following proportion (fig.
68):
d/D = h/H where d = dis-
placement correction in yards radi-
ally toward (from) center of photo.
h = height in feet of ground
above (below) ground at center ol
photo.
D = distance in yards from
center of photo to point to be cor-
rected.
H = height in feet of camera
lens above center of photo (alti-
tude of airplane). Since it usually
is not practicable to determine' the
altitude of the center of the photo,
an altitude, usually that of the cen-
ter of the target area, is assigned as
178
source of error is relief. Con-
Vertical section showing ground
Horizontal projection on chart
Figure 67. Displacement due
to relief of ground.
Figure 68. Determination of
relief distortion.
the mean datum plane and.used as the basis for determining distortion
corrections.
d. Example. (See fig. 69.) An air photo taken at an altitude of
25,000 feet is to be used as the battalion firing chart. The base point
has been assigned an arbitrary alti-
tude and is to be used as the mean
datum plane. By survey, point A
has been determined to be 180 feet
above the base point. The dis-
tance from point A to the center
of the photo is scaled as 4000 yards,
d 180
4000 25,000
d = 28.8 or 29 yards. Point A
will be plotted 29 yards toward the
center of the photo from its photo
location.
Figure 69. Example of correc-
tion for relief distortion.
194. DIRECTION IN PHOTOS. The effect of relief is the displace-
ment of images radially from or toward the center of the photo. The
effects are shown in figure 70. The points a and b are on higher
ground and the point d is on lower ground than the center of the
photo. In the figure, a, b, and d represent the true locations of these
points, whereas a', b', and d' represent the photo locations. The lines
a'd and a'b' are not true direction lines, whereas ca', cb', and cd'
are true, and b'd' is approximately true. It follows that the directions
of lines passing through or near the center of an average vertical photo
are substantially true. However, lines passing well away from the cen-
b'
Figure 70. Effect of relief displacement on direction and scale.
179
ter and joining points of different altitudes whose images lie in the
outer field of the photo may show excessive errors when the relief is
considerable. If the altitude from which the photo was taken is known,
the error may be corrected by a replot of the points to the same datum
plane (par. 193c).
195. DETERMINING SCALE OF PHOTOS.
a. From figure 65, it is evident that the approximate scale of the
photo can .be expressed by the formula: RF (representative fraction)
= focal length divided by height. Hence the scale of the photo and
the area covered by the photo depend upon the focal length and the
height. For example, if a 7-inch X 9-inch photo is taken at a height
of 20,000 feet with a camera of 6-inch focal length, the scale is .5/20, d00
or 1/40,000, and the area covered is (7 X 40,000) X (9 X 40,000) in-
ches, or about 8000 X 10,000 yards.
b. The scale of the photo as determined above is unsatisfactory.
It may be inaccurate for any of the following reasons:
(1) Because the altimeter may not have been set accurately at zero
for the area photographed.
(2) Because of barometric variations which affected the altimeter.
(3) Because of shrinkage of negative and print.
c. (1) The basic method of determining the scale of a photo is to
determine the relation between a photo distance and the corresponding
ground distance. For example, the photo distance between two points
is 5.40 inches and the ground distance between the same two points is
3100 yards (111,600 inches). The scale is determined by solving the
equation 5.40 : 111,600 = 1 : x. From the equation, x — 20,667, and
the scale is 1/20,667.
(2) The points between which the distance is measured should be
selected carefully. They should be well defined both on the photo
and on the ground; they should be far apart and near the average
ground level, so as not to introduce material error through distortion.
In flat terrain, points separated by 2000 yards are satisfactory. In gen-
eral, the greater the distance between the points, the more accurate is
the scale. They should be approximately equidistant from the center
of the photo and chosen so that the line joining them passes near the
center of the photo. A more accurate value of the scale of the photo
can be determined by averaging the scales determined by two such
lines roughly perpendicular to each other.
(3) Figure 70 shows the effect of relief distortion on the scale. The
lengths of the radial lines ca', cb', and cd' are not accurate for the
datum plane through c. Points a and b have been moved outward
and point d inward by relief distortion; thus, for the datum plane
180
through c, the lengths ca' and cb' are too great, and the length cd is
too small. To construct an accurate chart for the datum plane through
c, a', b', and d' should be replotted (fig. 70) at a, b, and d respectively
(par. 193). Except in comparatively flat terrain, it is necessary to
consider the possibility of relief distortion when selecting points.
(4) When determining the scale of a wide angle photo taken from
a known altitude, it is desirable to select points of about the same
altitude, or, if the altitudes differ materially, to replot one or both
points to a selected datum plane. In case the points have the same
altitude, the scale is determined for the datum plane of these points;
for example, 1/20,300 at 1600 (feet altitude). If the locations of the
points have been replotted to a selected datum plane, the scale deter-
mined is the scale for the selected datum plane.
(5) A scale determined by using .one point in the target area and
the other in or near the position area usually gives the best results
because, while the scale obtained may not be the best for the photo as
a whole, it is relatively the most accurate for distances from the posi-
tion area to the target area.
196. CONVERTING PHOTO AND TRUE MEASUREMENTS. Photo
measurements may be converted to true measurements (and true to
photo) as follows:
a. By relation to any convenient scale. For example, the true
ground distance between any two points identified on a photo has
been determined to be 1500 yards. Using any convenient plotting scale,
the distance between the same two points on the photo measures 1800
units. This relation is shown by the following equation:
Photo measurement 1800
True measurement 1500
Using the above equation, the conversion of photo measurements to
true distances (and the reverse) may be accomplished quickly and ac-
curately with a slide rule by setting the photo distance on the C scale
over the true distance on the D scale (fig. 71). Using the measure-
ments given above, the reading of 1800 on the C scale is placed over
the reading of 1500 on the D scale to set the rule for conversion. To
convert a photo measurement of 2100 units to true distance, locate 2100
on the C scale and the true distance, 1750, will appear directly below it
on the D scale.
b. By use of the graphical firing table. When a photo is used as
a firing chart, the conversion of photo measurements to firing data is
accomplished automatically by use of the graphical firing table.
181
Figure 71. Method of using slide rule for conversion of photo
measurements to true distances.
197. TYPES OF MOSAICS. An uncontrolled mosaic is compiled by
any one of several techniques involving matching of detail. A con-
trolled mosaic is compiled by fitting the images of control points over
their locations plotted on a control sheet; sometimes the prints must
be rephotographed to bring them to the average scale, or must be recti-
fied if they are appreciably tilted.
198. ASSEMBLY OF MOSAICS.
a. General. Normally, mosaics will be assembled and reproduced
by the engineers, and the reproductions distributed to the field artil-
lery. Situations may arise,, however, when it will be necessary for field
artillery to assemble their own mosaics. Whenever possible, this assem-
bly should be made by the radial line method described in TM 5-230,
since this gives the most accurate results. . A somewhat simpler method
which gives fair results appears below. This method is applicable
when the overlap is less than the 60 per cent required for the radial
line assembly method.
b. Assembly of strip mosaic. The first step in assembling a mosaic
is to assemble 4wo photos as a strip. Any two overlapping photos may
be used.
(1) Locate the photo centers from marginal information (collima-
tion marks). (Centers may also be located by the intersection of the
two diagonals.)
(2) Secure one photo to the board on which the mosaic is to be
assembled.
(3) Orient the second photo relative to the first by matching detail.
(4) Draw a line joining the two centers, C-l and C-2, and extend
it across that portion of photo 1 which was overlapped by photo 2
(fig. 72). This line is the approximate common radial line of the two
photos. The purpose of determining such a radial line is to permit the
selection of two suitable orientation points on or near this line that
182
i — 7
\ r
\ *
/ A
COMMON RADIAL
LINE
are identifiable on both photos.
The reason for using points along
a common radial line is that if the
points are distorted they will he
distorted along the common radial
line, and the direction of a line
established by the two points will
not be affected materially by the
distortion. If there is sufficient ov-
erlap, the actual centers are used.
(5) The two points selected
should be as near as possible to the
Figure 72. Method of deter-
mining common radial line.
radial line, on the same side of the radial line, and as far apart as
possible. In figure 73, two points would be a and b, labeled a 1; b 1( and
a 2 , b 2 on photos 1 and 2 respectively.
COMMON RADIAL
Figure 73. Selection of orientation points.
(6) Draw jthe line a x b x on photo 1, extending it on to the board
(fig. 74). Draw the line a 2 b 2 on photo 2, extending it to the edges of
the photo.
(7) On photo 1 determine the mid point between a x and b ± and mark
it on the photo. On photo 2 determine and mark the mid-point be-
tween a 2 and b 2 . Place photo 2 over photo 1 so that the mid-point of
a 2 b 2 is over the mid-point of a x b lt and the line a 2 b 2 coincides with
the extension of a, b., on the board (fig. 75). The photos are now
oriented and located. Photo 2 is secured to the chart. Additional photos
may be added in the same manner if only a strip mosaic is to be as-
sembled.
183
Figure 74. Orientation points on both photos connected by lines.
c. Assembly of area mosaic. If the mosaic is to be an area mosaic
rather than a strip mosaic, the procedure after assembly of the first two
photos is as follows:
(1) Determine the center of photo 3.
(2) Orient photo 3 by matching detail with photos 1 and 2.
Figure 75. Photos oriented and located by bringing the lines into
coincidence and matching the mid-points.
(3) Draw lines joining the center of photos 3 with the centers of
photos 1 and 2. These lines are approximate common radial lines to
be used in selection of orientation points.
(4) Select two points on photo 3 that can be identifed on photo 1.
The factors controlling the selection of these points are identical with
the factors that controlled the selection of the orientation points for
assembly of photos 1 and 2. In figure 76, the points are d and e,
184
labeled d 1( e^ and d 3 , e 3 on photos 1 and 3 respectively. In the same
manner, select two points on photo 3 that appear on photo 2. In figure
76, these points are f and g, la-
beled f 2 , g 2 and f 3 , g 3 on photos
2 and 3 respectively. Frequently, it
will be necessary to raise one photo
slightly in order to find desired
points that are covered by over-
lap; this procedure is more advis-
able than trimming away part- of
the photo before it has been ascer-
tained exactly where the trim line
will fall.
ERROR DIVIDED
r p
1
T 2
Yc-2 '
Figure 76. Selection of orien-
tation points on three photos
and connection of points by
lines.
(5) Draw lines d x e ± (fig. 76)
and f 2 g 2 , extending them across
photos 1 and 2 and well past their
point of intersection. Draw lines
d s e 3 and f 3 g 3 entirely across photo
3.
\ 7 ®
A+c-3
/I \
/ \u
- J-l-
A
C-1
©
I — ^ —
©
Figure 77. Three photos ori-
ented by matching points of
intersection and bringing the
lines into coincidence.
(6) Place the intersection of d 3 e 3 and f 3 g 3 on photo 3 over the in-
tersection of dj e x and f 2 g 2 (fig. 77). Orient photo 3 on photos 1 and
2 by causing lines d g e 3 and f 3 g 3 to coincide with lines d 1 e 1 and f 2 g 2
respectively. If perfect coincidence is not obtained, the error is divided.
Photo 3 is fastened to the board.
(7) Additional photos may be assembled in the same manner as
photo 3.
d. When the entire mosaic has been assembled, the excess of each
photo is trimmed away in such a manner as to preserve as much of
the center of each photo as is possible.
185
199. RESTITUTION.
a. General. Restitution is the process of determining the map or
chart locations of features appearing on air photos. The methods
discussed in this section apply to vertical air photos. The basic
principle of accurate restitution is the assumption that all angles
measured at the center (principal point) of a given photo with less
than 3 degrees of tilt are true angles and that this is the only point
on the photo where this is the case. Any method of restitution which
makes use of this principle, using overlapping photos (radial line),
gives accurate results whereas any method which is based upon other
angles or single photos will provide less accuracy, depending upon
the amount of relief distortion of the given photo. Section II describes
methods of using oblique photos for this purpose.
b. Radial line method.
(1) USE. This method is used to restitute a point appearing in the
overlap of two aerial photographs taken at different camera positions
when the tilt of the photos (from vertical) is 3 degrees or less.
(2) PROCEDURE.
(a) Identify on each photograph three control points whose chart
locations are known A different set of points may be selected for each
photograph or the same identical points may be used. The points
selected should be well out from the center of each photo and so
distributed that the rays drawn from them to the photo centers pro-
vide 'good three-ray intersection.
(b) Place tracing paper over the firing chart and prick the chart
locations of the control points to the tracing paper.
(c) Draw rays from the center of each photo to the photo location
of the control points.
(d) Place the prepared tracing paper over one photo so that the
rays (drawn in (c) above) on the photo pass through corresponding
control points on the tracing paper.
(e) Draw a ray on the tracing paper from the photo center to the
point to be restituted.
(f) Repeat (d) and (e) above using the second photo and the same
tracing paper.
(g) Intersection of rays gives the tracing paper location of point to
be restituted.
(h) Orient tracing paper over firing chart and transfer point to
be restituted to the chart.
(3) For a detailed discussion of the radial line method, see TM
5-230.
186
c. Fire control data sheet.
(1) GENERAL. Under certain conditions when a controlled mosaic
is not available, a suitable substitute is furnished in the form of a
gridded plot, to an appropriate scale (on the theater grid), of the
principal point of each of a number of vertical photos. This plot is
usually prepared by the engineers of the higher echelons (army or
base section) and provides a means for determining the true coordi-
nates of any point which can be identified on each of two adjacent
overlapping verticals. A discussion of how this grid plot is prepared
appears in TM 5-230.
(2) USE. The gridded plot in (1) above may be in the form of an
acetate sheet on which appears the theater grid with the principal
points of the photos of the basic cover plotted in their true location.
Any other control points which are available may also be shown.
Alternately a list of principal point coordinates may be supplied.
Having the grid positions of the photo principal points and knowing
that the principal point is angle true, it is a simple matter to inter-
sect any desired feature appearing on the photos and thus to obtain
its true coordinates. It is a variation of the radial line restitution
method described in subparagraph b above.
NOTE: The photos obtained for use with this plot will \.-,ually have
the course lines on them; if not, this line may be put <» as follows:
If the principal point p 2 of photo 2 falls, exactly on a |> ; ece of detail,
then its position on photo 1 can be identified easily ..'i.l the course
line p x p 2 drawn as a fine line. Unfortunately, in m?ny cases it will
be in the middle of a field or other open space. The procedure in this
case is (see fig. 78):
1 |
± I
Figure 78. Placing the course fine on photos.
(a) Identify p 2 approximately on photo 1 at p' 2 and aline a straight-
edge on this point and principal point p x .
187
(b) Score, with a needle, a small part of the course line at p 1
(shown dotted). A small error at p' 2 would not affect this part of
the line.
(c) Now, instead of trying to identify principal point Pj on photo
2, it is easier to select some piece of detail which lies exactly on the
short scored line through p r Join this selected position to p 2 and the
course line of photo 2 is correctly established.
(d) The course line of photo 1 is fixed by choosing, in the vicinity
of p 2 , a point which lies on the course line of photo 2, by identifying
it on photo 1, and by joining it to the principal point p .
a
r— (--, ..
;
X?
1
1
1
~7
;
/
/
/
10
d — -
/ /
1 91 /
1—0.
i Vi
^ ^ 93
—i
/
/
/
/
/
94
: — j
1
i
/
/
/
/
/
1
1
i
1
1
1
/
/
/
/
b
Figure 79. Method of using fire control data sheet with photos
to determine true coordinate of a point: a. Two adjacent over-
lapping vertical photos; b. Fire control data sheet on acetate with
theater grid, showing principal points.
188
(3) ILLUSTRATIVE EXAMPLES. In order to obtain the map
coordinate of a ground point, A, whose image position on photo 91
is a-91 and on 92 is a-92 (fig. 79), the procedure is as follows:
(a) With an acetate grid. Place photo 91 under the acetate with
its principal point (91) directly under the plotted position of this
point," and the course line coincident with the plotted course line
91-92. Draw a ray from a-91 to the principal point. Now place photo
92 under the acetate in the same manner in its proper position, and
draw a ray from a-92 to the principal point of 92. The intersection
of these rays is the map location of point A, and is corrected for
nearly all tilt and relief distortion.
(b) With the principal points plotted directly on the firing chart.
If a list of the coordinates of principal points, instead of a gridded plot
on acetate, is provided by the engineers of higher echelons (see c(2)
above), these points may be plotted directly on, the firing chart, and
course lines drawn to connect them. Targets may be restituted to the
firing chart from photos of the basic coverage by the following method:
7. Place a piece of tracing paper oi acetate over the plot on the
firing chart and draw the course line 91-92, taking care to mark the
principal points accurately so that the distance 91-92 is the same on
the plot and on the tracing paper.
2. Orient this tracing over photo 91, 91 of the trace over the princi-
pal point of the photo, and the course line of the trace over the photo
course line 91-92. With the trace in this position, draw a ray from a-
91 to the principal point. Note that the distance 91-92 may differ on
each photo, and on the trace.
3. Place the trace on photo 92, 92 of the trace over the principal
point of 92, and aline the course line with the photo course line 92-91.
Draw ray from a-92 to 92, intersecting the first ray in a.
4. Replace the trace on the firing chart, in the same position as in
1 above, and prick through the position of a.
(c) Points very close to the course line.
1. If the point to be intersected lies very close to the course line,
the intersection of the rays will be at too great an angle for accuracy,
and the following procedure must be used:
(i) Such a point is b, figure 79a. On each photo drop a perpendicu-
lar bb' to the course line, as shown.
(ii) On the trace or acetate, figure 80, draw 91-92' at about 30
degrees to the course line. Mark 91-b' equal to 91-b' from photo
91 (fig. 79a), and mark b'-92' equal to b'-92 of photo 92.
189
(iii) Join 92'-92, and draw b'b" parallel to 92'-92, cutting the base
in b". Erect the perpendicular b"b. Now draw either ray 91b or 92b
(or both) as before; the intersection
of either with the perpendicular
establishes b.
2. If the point to be located
is on the course line, the same pro-
cedure is followed except that no
perpendicular is used; b and b'
will coincide on the photos and b"
will be the location of the point
on the trace.
d. Polar plot method.
(1) USE. This method is used to locate targets when only a single
vertical of the area is available or when there is insufficient overlap
lor radial line restitution. Since the angles used are not radial, inac-
curacies from relief and tilt may be introduced.
(2) PROCEDURE. Two or more well separated points whose chart
locations are known are identified on the photo. A line is drawn be-
tween these two points on both the phot6 and the chart. The line
on the photo is extended so as to enable shifts and distances to be
measured from either of the points with a range-deflection fan (par.
188b). The line on the chart is extended so as to enable shifts and
distances to be plotted from either of the points with a range-deflec-
tion fan (par. 188c). The difference in scale between the photo and
the chart is considered by measuring the photo and chart distances and
by setting up a relation between the two on the slide rule (par. 1 96).
When a point is to be restituted from the photo to the chart, the photo
shift and distance are measured from whichever known point will give
the smallest angle and the largest distance. To reduce errors of relief
and tilt, the base should be chosen so as to pass close to the center of
the photo, and the base ends should be at about the same altitude.
The photo distance is converted to chart distance by the use of the
established relationship. The desired point is plotted on the chart with
the same measured shift and the true distance.
(3) EXAMPLE (fig. 81). The chart location of points A and B,
which are identifiable on the photo, have been determined. It is de-
sired to restitute points to the firing chart from the photo. The AB
line is drawn on the photo and on the chart and is extended as neces-
sary to allow the range-deflection fan to be used with vertex at either
Figure 80. Method of locat-
ing a point close to course
fine.
190
A or B. The distance between A and B is measured on the photo
(assume 4000 yards) and on the chart (assume 5200 yards). The photo-
chart relationship is set up on the
slide rule by placing 4000 on the
C scale over 5200 on the D scale.
To transfer point T from the photo
to the chart, the fan is placed with
its vertex at B on the photo, and
the shift from the BA line to T (R
405) and the distance from B to
T (3980 yards) are measured. Us-
ing the slide rule on which the
photo-chart relationship is set up,
the photo distance (3980 yards)
is converted to chart distance
(5150 yards). The fan is then
placed with its vertex at B on the
chart, and T is plotted 405 mils
right of the BA line at a distance
of 5150 yards from B. The trans-
ferred location of T is in error
due to errors in the relative photo
location of T with respect to the
Figure 81. Polar plot method rest i tution points. This may be
of restitution. corrected as in paragraph 193c.
(4) ALTERNATE POLAR PLOT METHOD. In the example
above, the transferred location of T would include errors introduced
by erroneous photomap locations of the restitution points A and B.
A method of restitution, applicable to individual photos, which par-
tially eliminates these errors is as follows:
(a) At least three points whose chart locations are known are iden-
tified on the photo. The photo center is determined.
(b) An overlay is made from the photo showing the three points and
the photo center.
(c) Using the overlay, the location of the photo center is resected
(par. 246b) to the chart.
(d) When the photo center has been located on the chart, the pro-
cedure for transferring points is the same as in subparagraph d (2)
above except all measurements on the photo and all plotting on the
chart are performed with the vertex of the fan at the photo center.
FIRING CHART
191
ZOO. STEREOSCOPIC COVERAGE. Normal methods of restitution
do not include adequate means for determination of vertical control.
Stereoscopic coverage augments and aids in the determination of rela-
tive altitudes and aids photo interpretation. See FM 21-26.
Section II. OBLIQUE PHOTOGRAPHS
201. GENERAL.
a. The mil-gridded oblique pictures the terrain substantially as it
would be seen by an observer through an instrument having a fully
graduated reticle and which was located at the point occupied by the
camera at the instant the photo was taken (fig. 82). The vertical and
horizontal grid lines have graduations with which horizontal and
vertical angles may be read; these, in effect, make the mil-gridded
oblique another survey instrument. If successive plumb points repre-
sent ends of a base, and the gridded photos include a reference point
the location of which is known, the principles of long base intersection
may be employed to transfer points from mil-gridded obliques to fir-
ing charts or maps. 1
When mil-gridded obliques are used, targets normally are located
with such accuracy that fire for effect may be initiated immediately.
When fire is placed on the target, if errors occur which are traceable
to the obliques only, surveillance of the first mission will permit the
determination of a K which will compensate for errors in the control
and in the plotting from the obliques. Corrections may be improved
as subsequent missions are fired.
b. Coordinates and altitudes of plumb points (geographic location
of camera at the instant the picture was taken) and reference points
will normally be furnished by the corps observation battalion or by
division artillery headquarters if the division is operating separately.
However, using units should be capable of determining these critical
data.
c. Technical information. For details of construction and uses of
grids, corrections for tilt and dip, and corrections for misplaced grids,
see appendix VI.
202. HORIZONTAL LOCATIONS.
a. Orientation of photographs. Orientation is accomplished by
determining the chart locations of plumb points and the chart direc-
tions of center lines.
192
193
(1) PLUMB POINTS. The location of the plumb point may be
determined by: a vertical photo taken simultaneously with the oblique;
intersecting the position of the airplane at the instant the picture is
taken; and/or resection. Plumb points of oblique photos taken from
observation posts may be located by ordinary survey methods.
(a) Simultaneous vertical and oblique. Whenever possible, it is
desirable to have a vertical photo taken simultaneously with the
oblique. If the photo is taken with the optical axis of the camera truly
vertical, the center of the picture is the plumb point. If, however,
for any reason, the camera deviates from the vertical, corrections, as
described in appendix VI, must be applied to locate the plumb point
accurately on the vertical photo.
Once the photo location of the plumb point has been determined,
its ground location must be identified; normally, this will be easy to
do (because of the altitudes at which these photos are taken, verticals
of relatively large scale will result). The plumb point is tied to the
chart by survey which may consist of only a short traverse if the line
of flight was over the position area. Since it is possible to occupy its
ground position, this method of locating plumb points gives the great-
POINTS DISTANCES FROM THE PLUMB POINT
< and 2 As near as possible and in the corners of the photo.
3 In the middle distance (4000 to 6000 yiris).
4 As far away as possible, at least 8000 yards.
With poor visibility, 7000 yards minimum.
Figure 83. Selection of control points.
194
est accuracy. Survey, even for several successive plumb points, need
not be a serious problem if control is available at various separated
points along the line of flight.
(b) Intersection. Occasionally, the most feasible method of locat-
ing the plumb point will be to triangulate its position by taking simul-
taneous observations on the airplane, using a base the location of
which is known. Communication is necessary. Each observer tracks
the airplane in its flight until the instant the picture is taken, at which
time the tracking stops and the instrument reading is recorded. Sig-
nals between the pilot or cameraman and the terrestrial observers
must be prearranged. This method is limited to use with low per-
formance aircraft so that satisfactory accuracy may be obtained.
(c) Resection.
1. Principle. The tracing paper method of resection (par. 246b) can
be used to determine the chart locations of plumb points. Instead of
being measured with an instrument, the angles are read directly from
the oblique photo. Because the front covered by an oblique is limited,
care must be exercised in choosing control points.
2. Selection of control points. Three or four control points should
be selected; more afford greater accuracy. Their location on the chart
must be known or determined. The
points should be situated on the
photo approximately as shown in
figure 83.
The result of a good selection of
control points is shown plotted in
figure 84.
If three control points are used,
greater accuracy is secured if the
center point is at a greater range
than the near points. However,
care must be taken that the points
selected are not on or near the
circumference of a circle passing
through the plumb point, a con-
dition under which the tracing
paper method fails. In open coun-
try with numerous landmarks,
ideal conditions can readily be
obtained. In heavily wooded coun-
Figure 84. Confroi points try or in terrain which has no out-
on a chart. standing landmarks, control points
may have to be selected in friendly areas only.
195
Accuracy of resection will suffer in proportion to the extent of de-
parture from ideal selections.
3. Use of the resector. A resector (substitute for tracing paper) is a
thin piece of celluloid, 18 inches square and frosted on one side to
permit marking by pencil. It can be used repeatedly. A permanent
center line about 15 inches long is scratched on the resector and two
tiny pin holes are pricked as shown in figure 85a. A resector is merely
a convenience; tracing paper may be used.
In performing a resection for the photograph shown in figure 82,
only the coordinates for deflection are needed. With a range-deflection
fan, a ray is laid off in pencil, as shown in figure 85 b, and labeled
Control Point 1. In a similar manner, rays are laid off for control
points 2, 3, and 4. The resector should then resemble figure 85c, and
should carry a photo identification number or letter. The resector
must now be placed over the chart containing control points 1, 2, 3,
and 4, and is manipulated until the appropriate rays lie directly over
their respective control points (fig. 85d). The chart location of the
plumb point is recorded by pricking through the hole of the resector
at which the vertex of the fan was placed.
(2) CENTER LINES. Since horizontal angles on the mil-gridded
oblique are read from the center line, plotting is facilitated if the
same line is drawn on the chart.
When the simultaneous vertical or intersection methods are used
to locate the plumb points, the center line may be placed on the chart
by using a single control point.
Example: The reading to the control point is L 103. The range-
deflection fan, with vertex at the plumb point, is placed on the chart,
and an angle of 103 mils is laid off to the right of the control point.
The center line is drawn on the chart along the right edge of the fan.
If the resection method of locating the plumb point is used, the
center line is recorded on the chart by pricking through while the
resector is properly oriented for transferring the plumb point (see
(c) 3 above).
Each plumb point and center line must be labeled with its photo
number or letter designation, and the center lines are marked with
identifying arrows, as in fire direction technique.
b. Transposition of points from obliques to chart.
(1) DESIGNATION OF POINT.
(a) Oblique coordinates are announced as "Photo 58, LI 56042." The/
photo number designates the photo from which the reading was
taken. The letter "L" or "R" indicates a reading left or right of the
center line. The first three figures represent the horizontal angle from
196
PIN HOLE
PIN HOLE
r~
"7
i
/
/
CONTROL
/
DfM KIT 1
rUI IN 1 1
i
\
/
/
/
/
/ R-D
/ FAN
\
/
\
/
/
/
\
1
f^PIN
CONTROL
POINT 4
CONTROL
POINT 3
CONTROL!
POINT I
CONTROL
POINT 2
PHOTO 4-1
CONTROL
POINT 3
CONTROL
POINT 1
\.
I CONTROL
POINT 4
/'
CONTROL
POINT 2
PLUMB
POINT
c d
Figure 85. Tracing paper resection to locate plumb points and
center lines.
197
the center line. The last three figures represent the vertical angle from
the zero line.
(b) There are two ways whereby an observer may designate a point
to be plotted.
1. If identification of the point on the photographs is . difficult, the
observer should, if he has more than one photo, send complete co-
ordinates from at least two.
2. Where the point is easily identified, the observer need only desig-
nate the target location on one picture. Fire-direction center will
choose one or more additional photos on which the same target is
recognized. This latter arrangement is preferable because the observer
need concentrate on locating the target on only one photo. Generally,
the location will also be more accurate since, fire direction personnel
may be expected to have a better opportunity to select photographs
which 'will give the best intersection.
60 61
+ t
l
CONTROL
POINTS
5
B
I
PHOTO
59
* C _L
PHOTO
60
I A
I
T
PHOTO
61
Figure 86. Firing chart showing plumb points and center lines.
198
The target will often appear on several adjoining photographs.
Targets should be indicated from photographs as widely separated
as possible.
(2) PLOTTING THE POINT. Figure 86 is an example of a fir-
ing chart for oblique photographs. It shows the location of the bat-
teries, plumb points, the center lines, and the control points used in
placing center lines for each photo. For convenience, the firing chart
for the obliques can be a duplicate of the firing charts used for mis-
sions reported by other means or used for planning purposes.
Targets are located from oblique photographs by the plot of the
intersection of two or more lines of sight. Example: Using the sample
firing chart (fig. 87), assume that an observer sent in the following
message, Photo 59, R210118, infantry platoon with heavy weapons,
request battalion, fire for effect. Fire direction personnel identified
the same point on photo 60 as L 60, and on photo 61 as L 305. The
target would be plotted as in figure 87.
59 6*0 61
Figure 87. Location of a target on the firing chart.
With a range-deflection fan, a ray is drawn 210 mils right of the
center line of photo 59, and another ray is drawn 305 mils left of the
center line of photo 61. The intersection of these two rays marks the
chart location of the target, which should be checked by plotting a
199
ray from a third photo (photo 60). The plotting of the location of
the target is long base intersection. Any point identifiable on two
obliques can be located quickly in a similar fashion.
C. Checks. All work performed in connection with locating points
horizontally should be checked to insure accuracy. Any feature ap-
pearing on several photographs may be used for checking accuracy.
The angular shift to the point on each picture is plotted on the chart.
All rays will intersect at a point if the coordinates have been correctly
read and if the locations of the plumb points and the directions of the
center lines are correct.
An inaccurate orientation or plumb point plot may seem to check
perfectly if but one plot of a known point is used in making the check.
Therefore, the check should be applied to at least three points. If
good intersections are obtained for three features, one in the fore-
ground, one at midrange, and one at extreme range, the resection may
be considered correct.
All plumb points and center lines must be checked. If one terrain
feature does not appear on all photographs, suitable combinations of
photographs must be chosen so as to provide a complete check. A
triangle of error which does not exceed 50 yards is allowable.
203. DETERMINATION OF ALTITUDES.
a. Altitude of the camera. When gridded obliques are used in
connection with maps having suitable vertical control, map altitudes
are. generally preferable to those computed by the following method.
The altitude of the camera may be determined from the altitude of
the terrain appearing in the photograph. Assume that a house, being
used for a division artillery control point, appears at a vertical angle
of 100 mils in photo 59, and its distance on the firing chart from the
plumb point of photo 59 is 3500 yards. Figure 88 shows the relations
that exist. From 'the mil formula, W = tfi X R> the vertical interval
between the house and the camera is determined to be 350 yards. The
altitude of the house, as established by the division artillery survey
section, may be known or assumed to be 300 yards. The altitude of
the camera is, therefore, computed to be 650 yards.
b. Altitude of the target. The altitude of a target is computed
from an oblique photograph after the altitude of the camera has been
determined. The difference in altitude between the camera and the
target is determined as in figure 88. To establish the altitude of the
target, this distance is subtracted from the altitude of the camera. In
paragraph 202b (2), an observer sent a message, Photo 59, R210118',
infantry platoon with heavy weapons, request battalion, fire for effect.
The location of the target was plotted, in figure 87 from the deflection
200 1
VERTICAL
|< R = 3.5 »»
Figure 88. Determination of the altitude of the camera.
coordinate given, and a similar coordinate was obtained from a com-
panion photograph. To illustrate the use of the vertical angle 118.
assume the target plotted 3200 yards from the plumb point of photo 59.
The vertical interval is 378 yards, and the altitude of the target is
650 — 378 = 272 yards. This system of determining altitudes is pre-
dicated on the fact that the horizon line is sufficiently clear to enable
the photographer to place the grid properly.
204. OBSERVED FIRE CHART.
a. The use of mil-gridded obliques to supplement an observed fire
chart is accurate and may be convenient where survey is slow. The
simultaneous vertical can be used expeditiously only when the locating
of the plumb point can be included in the position area survey. Locat-
ing the plumb point by intersecting the position of the airplane at the
instant the picture was taken would be practicable only when a sur-
veyed base had been established.
b. If a simultaneous vertical was taken with the oblique or if the
position of the airplane was intersected at the instant the picture was
taken, one check point visible on the oblique photograph is selected
and registered upon. If the plumb point is to be resected, three or
more check points, visible in the oblique photographs, are selected and
registered upon. Registrations may be conducted from the air or from
observation posts. Site should be stripped from the registrations. The
check points are plotted on the observed fire chart from the adjusted
data and are then used as control points for orienting the photo-
graphs. The control points should be selected as for a surveyed chart
(par. 202a (l)(c)). Fires will be more accurate inside the pattern of
the control points than outside. In the case of targets which when
201
plotted fall outside the pattern of control points, surveillance of the
fires should be provided. This method may be refined, if time and
circumstances warrant, by stripping all determinable corrections from
the registered data and by using the results in lieu of survey.
205. FIRING WITH ONE OBLIQUE PHOTOGRAPH.
a. If an oblique photograph i is taken approximately over the bat-
tery position, the oblique photograph grid can be used to measure
deflection shifts directly in mils, and to estimate ranges. The height
of the camera above the plumb point can be obtained from the alti-
meter reading in the airplane at the time the photograph was taken
(fig. 88). This distance in yards, divided by the vertical angle to the
target as measured by the grid, gives an approximate range. The first
target taken under fire is used as the base point, and deflection shifts
are measured from it on the oblique photograph. The adjusted range
is compared with the range computed from the photograph to deter-
mine whether the correction is positive or negative.
b. As an example of the use of this method, assume that the first
target taken under fire is at (R020191) in the oblique photograph
(par. 202b). The actual range fired is 3700, and the photograph was
taken at an altitude of 667 yards. The computed range is 667 -4- 191 =
3.5 or 3500 yards. The next target is at (L064222). The computed
range is 667 222 = 3.0 or 3000 yards, and the deflection shift is left
84 (R 20 to L 64). The basic data to open fire is BDL 84, 3200. The
deflection correction and sheaf centering are included in the registra-
tion; 200 yards of range are added to correct the range disparity found
in registering on the base point. This method is approximate and is
suited to country that is flat or rolling, but not to mountainous country;
it, insures excellent data, but immediate fire for effect is seldom jus-
tified.
206. LOCATION OF TARGETS AT NIGHT. By the use of mil-gridded
obliques taken simultaneously from surveyed observation posts which
form an adequate base for intersection, targets may be located at
night. Anv camera, the focal length of which is known or can be de-
termined, is suitable. High speed panchromatic film gives best results.
Orientation is accomplished by setting up two aiming posts with
lights in the field of view of each camera; the surveyed direction to
each aiming post from the camera is predetermined. Communication
between the observation posts and careful prearrangement are neces-
sary in order that the opening and closing of the camera shutters can
be synchronized. The length of exposure depends on the activity in
the sector and the assigned mission. Registration of many flashes on
202
one pair of photographs makes identification of the same target on
both obliques difficult. The grid used on the photos is the center
section of the plate grid whose construction is described in appendix VI.
The grid is oriented to the photograph when the horizontal and ver-
tical axes of the grid are placed over the horizontal and vertical axes
of the photo. Targets are plotted on the firing chart as described in
paragraph 202b (2); the aiming post lights are used as control points.
203
CHAPTER 4
SURVEY EQUIPMENT AND ITS USE
Section I. PRINCIPAL INSTRUMENTS
207. PRINCIPAL INSTRUMENTS. The principal instruments used
for survey operations are the tape, the aiming circle, the battery com-
mander's telescope, the transit, the altimeter, and the military slide
rule. Detailed description of the aiming circle is given in TM 6-220.
Detailed descriptions of the tape and the transit are given in TM 5-235
Section II. THE TAPE AND TAPING
208. THE TAPE AND ACCESSORIES.
a. Tape. Field artillery survey sections are equipped with 300-foot
tapes and 100-foot tapes. Both types of tapes may be graduated through-
out their length into feet and tenths, or only the foot marks may be
shown, and one foot on both or on one end of the tape divided into
tenths. The graduated foot or feet may be included within the 100 or
300 feet, or they may be outside the 100 or 300 feet.
b. Accessories. Each tapeman should be equipped with a plumb
bob and cord and a notebook. The head tapeman should have a set
of 11 pins.
209. PERSONNEL.. Two individuals, called head and rear tapemen,
are required to determine distances by taping.
210. ' TAPING ON LEVEL GROUND. The head tapeman with the
300- or 100-foot end of the tape in his hand, after setting one pin at the
starting point and checking to see that the remaining 10 pins are on
his ring, starts along the course to be. taped. The tape is allowed to
drag on the ground, the rear tapeman watching it only to prevent its
snagging. Just before the whole length of the tape is drawn out, the
rear tapeman calls, "Halt," at which the head tapeman turns and
straightens the tape on the true line (par. 212). When the tape is on
the true line, the rear tapeman calls, "Pull," and the head tapeman
204
pulls on the tape until it is properly taut (par. 212). The rear tapeman
guides the zero exactly over the starting point and calls, "Stick." At
this instant, the head tapeman sets his pin to correspond with the 300-
or 100-foot mark of the tape and responds, "Stuck." When both tape-
men have checked for accuracy, the rear tapeman pulls his pin. Both
now. proceed, the rear tapeman giving the preliminary "Halt" signal
as his end of the tape approaches the pin just set by the head tapeman.
The tape is lined up and stretched, the front pin is set, and the rear
pin pulled on signal, as described for the first tape length. This process
is repeated until the head tapeman has set his last pin, at which time
he calls, "Tally." The rear tapeman goes ahead, counting his pins as
he goes, and, if there are 10, transfers them to the head tapeman who
also counts them and replaces them on his ring. This exchange is
known as a tally and means that 10 tape lengths have been measured
from the starting point or from the last tally. Both tapemen record
each tally in their notebooks. A similar check of pins may be made at
any time by remembering that the sum, omitting the one in the ground,
should be 10. Frequent checks detect the loss of a pin and the conse-
quent loss of a tape length. The rear tapeman does not give his pins
to the head tapeman on a mere check of pins. When the end of the
course is reached, if the last measurement is not an even tape length,
the head tapeman continues past the station until the usual signal of
"Halt." He then returns to the station. The manner of determining
the last measurement, if it is not an even tape length, will depend upon
the type of tape being used:
a. If the tape is graduated in tenths of a foot through its entire
length, the rear tapeman holds zero over the pin and the head tape-
man reads the feet and fraction direct.
b. If only the first foot, that is from zero to 1, is graduated in tenths
of a foot, the rear tapeman holds the 1-foot mark of the tape opposite
his pin, and then slacks off sufficiently to allow the head tapeman to
bring a whole foot mark into coincidence with the front pin. Then
the head tapeman reads the whole number of feet and the rear tape-
man reads the fraction. The fraction read is subtracted from the whole
number of feet to determine the last measurement.
c. If an additional foot, that is, a foot beyond the zero, is graduated
in tenths of a foot, the rear tapeman holds the zero about even with
the pin. The head tapeman then pulls the tape until an even foot is
opposite his pin and reads the feet. The rear tapeman reads the tenths
of a foot and the fraction is added.
When the last measurement has been made, the rear tapeman goes
forward, counts his pins, and gives them to the head tapeman. The
length of the course is the sum of the whole tape lengths plus the
205
length of the last measurement. Both tapemen compute and enter the
length of the course in their notebooks. Only the results of their com-
putations are checked one against the other.
211. TAPING ON SLOPING GROUND.
a. General. In measuring on a slope there are two ways of getting
horizontal distance: to measure along the slope and to correct this
measurement by calculation, known as slope taping; or to hold the
tape horizontal and to determine the horizontal distance directly,
known as breaking tape. The methods of marking points and record-
ing distances in taping on slopes is identical with taping on level
ground.
b. Slope taping. In this method, the tape lies on the ground and
the slope distance is measured. The angle of slope is measured with
an instrument. The taped distance multiplied by the cosine of the
angle of slope is equal to the horizontal distance. TM 5-236 provides
tables which reduce the amount of computations. This method is
suitable only for gradual and evenly sloping terrain.
c. Breaking tape. In this method, the head tapeman goes forward
the entire length of the tape, dropping the tape approximately on line.
He then comes back toward the rear tapeman until he reaches, a point
at which a fractional part of the tape when held level is not above
shoulder height. The fractional part should be a multiple of 10 feet.
On steep slopes, the taping must be done in short sections. In measur-
ing downhill, the head tapeman sets a pin at the point on the ground
beneath the plumb bob. He keeps his finger at the fractional reading
on the tape until the rear tapeman comes up. In measuring uphill,
the process is reversed, the rear tapeman holding his point on the tape
over the point on the ground by means of the plumb bob, while the
head tapeman sets his pin. In both cases, every time the rear tapeman
takes over a point from the head tapeman, the rear tapeman gives the
head tapeman a pin to replace the one in the ground which represents
only a fractional part of the tape length. This exchange continues
until a full tape length has been measured, at which time the rear tape-
man retains the pin.
212. ALINEMENT, TENSION. AND SAG. •
a. Alinement. Alinement of the tape during the measuring is per-
formed by the tapemen without help from the instrument man. If the
next station has been selected, it is marked by the rodman; the rear
tapeman lines the head tapeman in by eye. When the course, to the
next station is selected before the station is established, the direction
of the course should be toward some unmistakable object. The rear
tapeman then lines the head tapeman in on the selected course. Dur-
206
ing the crossing of low ground, it may be necessary for the head tape-
man to line himself in with the rear tapeman and the last station.
Lining tapeman in with an instrument is an unnecessary refinement.
b. Tension and sag. With the tape resting on the ground, a pull
of approximately 20 pounds must be exerted. When the tape is sus-
pended in the air for plumbing, the tension should be increased to
counteract the sag; an unsupported length of about 100 feet requires
a pull of about 25 pounds. Unsupported lengths greater than 100 feet
introduce errors in measurements and should not be used.
213. TRAINING TAPEMEN.
a. General. Tapemen should be carefully trained. Prescribed
methods should be rigidly enforced. Tapemen must exercise constant
vigilance to avoid errors and blunders. The most common blunders
are: misreading the tape; failing to record a complete tape length;
failing to consider errors in the length of the tape caused by repairs
of the tape.
b. Don'ts for tapemen.
(1) Don't jerk the tape.
(2) Don't pull the tape when it is kinked.
(3) Don't let vehicles run over the tape.
(4) Don't bend the tape sharply around corners.
(5) Don't split hairs in lining in.
(6) Don't allow the chaining pin to be disturbed.
(7) Don't pull the pin until you are sure that it will not be needed
again.
(8) Don't break tape oftener than necessary. Each break slows up
the work and introduces another chance for error.
(9) Don't fail to wipe the tape clean and dry before putting it away.
(10) Don't forget that methodical procedure is speedy and prevents
errors.
Section III. THE AIMING CIRCLE AND BATTERY
COMMANDER'S TELESCOPE
214. THE AIMING CIRCLE. In survey, the aiming circle is used to
measure horizontal angles, to measure limited vertical angles, and to
measure azimuths by magnetic needle.
215. BATTERY COMMANDER'S TELESCOPE. The battery com
mander's telescope is designed primarily for observing artillery fire.
However, since means are provided for the measurement of both hori-
zontal and vertical angles to the nearest mil, it may be used in lieu of
207
a transit or aiming circle for survey work, as the methods of employ-
ment are similar. This instrument can be used to good advantage in
survey work, particularly on a target area base because its greater opti-
cal power will aid in the location of targets.
a. It is not comparable to the transit in accuracy but it does have
some definite advantages over an aiming circle. They are:
(1) Ability to measure vertical angles of 300 mils above or below
the horizontal plane.
(2) Greater optical power and wider field of vision.
(3) Four different filters to aid in observation under various climatic
conditions.
(4) Usefulness in fox holes, trenches, etc. (only the lens shows above
ground).
b. Its disadvantages are:
(1) It is heavier than the aiming circle and therefore more difficult
to move rapidly.
(2) It has a relatively high silhouette (unless dug in).
216. MEASURING HORIZONTAL ANGLES. To obtain accurate,
measurements of horizontal angles, the spherical bubble must be cen-
tered and the following precautions taken:
a. When a setting is made, the last movement of the vertical hair
should always be in the same direction, usually clockwise. If the object
is overrun, the hair is moved well back of the object and moved up
to it again. This procedure eliminates the effect of lost motion. As a
check after a measurement is made, the hair is brought back to the
origin. If the index varies materially from the proper reading, the
measurement is thrown out. Angles should be read twice to prevent
gross errors of 10 or 100 mils. Reading an angle more than once is
known as making a multiple reading.
b. The size of the possible error is reduced if the instrument is
read to. the tenth of a mil, by interpolation between the mil gradua-
tions on the micrometer scale.
C. The most accurate horizontal reading is obtained by repeating
the measurement three times cumulatively, and taking the average. For
example: the angle between two points, A and B, is to be measured
cumulatively three times. With the scale set at zero, the line of sight-
ing of the instrument is directed at A with the lower motion. The
angle to B is then measured with the upper motion (assume a reading,
of 205.2 mils). The instrument is again directed at A with the lower
motion, without changing the reading, and the angle to B measured
again (assume a reading of 411 mils). The angle is measured a third
208
time in the same manner (assume a reading of 615.9 mils). This value,
615.9 mils, is divided by the number of readings to give 615.9/3 or
205.3 mils.
217. ACCURACY. In one reading of an angle with an average aiming
circle, or battery commander's telescope, carefully operated as described
above, an error as large as 1 mil will not be unusual. This instrument
error is unpredictable. However, ordinarily it will be reduced by
cumulative readings. The cumulative readings should be limited to
three. Readings in excess of three do not increase accuracy because
operator errors become excessive.
218. MEASURING VERTICAL ANGLES. For reliable work in meas-
uring vertical angles, the aiming circle or battery commander's tele-
scope must be accurately tested and its correction constant determined.
219. CHECKING THE LEVEL LINE OF THE AIMING CIRCLE OR
BATTERY COMMANDER'S TELESCOPE. The level line of the in
strument may be determined by either of the following methods:
a. If the altitudes of two points and the distance between the points
can be determined, the true vertical angle may be computed. The in-
strument is set up at one of the points and the vertical angle to the
other point is measured. To determine the correction constant, sub-
tract the measured angle from the computed angle. The correction
constant is applied algebraically to all measured vertical angles. The
accuracy of the correction can be increased by using the mean of cor-
rections obtained on a number of points.
Example: The computed vertical angle from A, where the instru-
ment is set up, to B is —(-15 mils. The vertical angle from A to B as
measured with the instrument is -[-12 mils. The correcti'on constant of
the instrument is -)-3 mils which will be added to all vertical angles
measured with this instrument.
b. The alternate method which requires no known altitudes is as
follows: two stations are established on the ground. 75 to 100 yards
apart. At each station a stake with a flat smooth top surface is driven
or a stone with a flat top surface is seated firmly in the ground. The
instrument to be calibrated is set up over station 1 and leveled with the
objective lens slightly in rear of a pole held in a vertical position rest-
ing on the stake or stone. The height of instrument is then marked
on the pole at the same height as the center of the objective lens, by
determining the radius of the objective lens and applying it in the
proper direction to the height determined for the top or bottom of
the objective lens. The pole is then taken to station 2 and held verti-
cally on the stone or stake there. The angle of site to the height of
209
instrument mark is then measured with the instrument and noted.
Clarity of sighting is improved if the edge of a card or other well defined
straight edge is held horizontally in contact with and just below the
pencil mark on the pole. The instrument is set up over station 2 and
the entire precedure is repeated, step for step. (CAUTION: The
height of instrument as set up at station 2 must be marked on the pole'
and used as the sighting point when the pole is set up at station 1.)
If the algebraic sum of the two angles of site measured equals 0, the
instrument is in adjustment and no correction need be applied to ,
readings taken with it; for example:
Site measured at station 1 = -f-5 vfi
Site measured at station 2 = — 5 rfi
Algebraic sum =0
If the algebraic sum of the two angles of site is other than 0, the instru-
• ment is in error by one half of this amount. The sign of the error is
the same as the sign of the algebraic sum of the two angles of site. The
correction is applied in the opposite direction; for example:
Site measured at station 1 = -f-5 tfi
Site measured at station 2 = —8 ifi
Algebraic sum = — 3 yfi
Error (-3/2) " = -1.5 tfi
Correction = -f- 1.5 tfi
The correction is applied to each angle of site read thereafter, unless
the instrument contains an angle of site micrometer which can be ad-
justed to the. true setting.
220. CENTERING THE NEEDLE. For precise work, the average of
several trials is taken as follows:
a. Set the scales at zero and center the needle with the lower motion.
b. Using the upper motion, bring the line of sighting to some well
defined object and note the scale reading.
c. Repeat the operations three to six times and take the average
of the readings.
d. Set this average on the scales and lay on the object. The 0-3200
line of the instrument will be on compass north.
221. DECLINATION.
a. A declination constant is the clockwise angle between F-north
and compass north; in other words, the F-azimuth of compass north.
This constant is recorded for any instrument equipped with a magnetic
needle; the constant for any one instrument may vary in different
localities; in any one locality the constant will vary slightly for differ-
ent instruments.
210
b. When practical, a declination station is established for deter-
mining the declination of instruments. The point chosen for the sta-
tion should afford a view of at least one distant, well defined point
with a direction of known Y-azimuth; additional points are desirable
as a check. The Y-azimuths may be determined by the application of
the known grid declination for the area to the true azimuths deter-
mined by astronomic methods, by computing the azimuth of the distant
point, or by measuring the Y-azimuths on a battle map. If the grid
declination is unknown, all instruments should be declinated to true
north or if true north cannot be determined, all instruments may be
declinated to a magnetic azimuth determined by measurement with
one aiming circle. In the absence of an established declinating station,
an instrument may be declinated for a particular locality on any line
of known azimuth.
C. Set up the aiming circle over the declinating station and level
the instrument carefully. Set the scales at zero and center the needle.
With the upper motion, turn to the point of known azimuth and
record the reading. Repeat this process three times and subtract the
mean of these readings from the known Y-azimuth (adding 6400 mils
to the Y-azimuth if necessary). The result is the declination constant
of the instrument. If more than one point of known Y-azimuth can be
seen, readings are made for each point, the computation repeated for
each, and the mean of the differences taken as the declination con-
stant.
d. If an aiming circle is to be declinated in one locality for use in
another locality, follow the procedure shown below.
(1) On the map of the area where the instrument is to be declinated
is found the information shown in figure 89a. From this the following
is determined:
The Y-azimuth of magnetic north is 348° 45', or 6200 mils
This would be the declination constant of ah
instrument having no error. *
Declination constant of aiming circle deter-
mined as above 6184 mils
Instrument correction of aiming circle —16 mils
(2) On the map of the area where the instrument is to be used is
found the information in figure 89b. From this is determined the
Y-azimuth of magnetic north which would be 6° 45' or 120 mils, which
would be the declination constant of an instrument having no instru-
ment error.
(3) The declination constant of the instrument for the new locality
would be 120 mils minus 16 mils or 104 mils.
211
DECLINATION INDICATOR FOR
LOCALITY WHERE INSTRUMENT
IS TO BE DECLINATED
6°45'
DECLINATION INDICATOR FOR
LOCALITY WHERE INSTRUMENT
IS TO BE USED
Figure 89. Marginal information taken from map. to be used in
the declination of an instrument.
222. LAYING ON A GIVEN YVAZIMUTH. To place the 0-3200 line
of the aiming circle on F-north, set the declination constant on the
scales and, using the lower motion, center the needle. To place the
0-3200 line of the instrument on a line of designated F-azimuth, sub-
tract this azimuth from the declination constant (with 6400 added when
necessary), set the remainder .on the scales, and, using the lower motion,
center the needle.
223. MEASURING IT-AZIMUTH. To measure the F-azimuth to a
point, set the declination constant on the scales and, with. the lower
motion, center the needle. With the upper motion, bring the vertical
hair to the point, and the reading on the scales is the F-azimuth.
Section IV. THE TRANSIT AND ACCESSORIES
224. USE OF THE TRANSIT. The transit is used in field artillery
survey for measuring horizontal and vertical angles and for measuring
distances by stadia. In battalions having only one transit, the transit
normally is used for position area or connection survey. In heavy artil-
lery survey, practically all angles are measured with the transit.
212
225. TRANSIT SCALES.
a. General. The transit has two scales which are used in conjunc-
tion to determine angular measurements. These scales are the main
scale and the vernier.
b. Horizontal scales. The main scale is a complete circle graduated
primarily from to 360 degrees. There are two sets of numbers on the
main scale. The numbers of the inner set increase from to 360 in a
clockwise direction. The numbers of the outer set increase from to
360 in a counterclockwise direction. The main scale turns -only with
the lower motion of the transit. The vernier is an arc rotating edge to
edge with, and inside of, the main scale. It is an auxiliary scale used
for reading fractions of the smallest division of the main scale, and its
graduation is dependent upon the graduation of the main scale. The
vernier rotates with the telescope.
c. Vertical scales. The vertical scale differs from the horizontal
scale in the following respects: the main scale is inside of the vernier;
the main scale rotates with the telescope while the vernier is fixed;
the largest numeral on the main scale is 90 degrees. Vertical scales
are graduated to read to 1 minute. They are read in exactly the same
manner as are the horizontal scales of the 1-ininute transit (par. 226).
d. Types. The two transits commonly used in field artillery survey
are the 1-minute transit, which can be read to 1 minute, and the 20-
second transit, which can be read to 20 seconds.
226. READING HORIZONTAL ANGLES ON THE 1-MINUTE TRAN-
SIT (fig. 90). The main scale of the 1-minute transit is graduated to
30 minutes, and the vernier to 1 minute. To read the angle when the
measurement is complete, read the main scale opposite the center of
the vernier. If the center of the vernier falls between two main scale
graduations, the graduation giving the smaller angle is read. In figure
Figure 90. Scales of the 1-minute transit.
V
90 the main scale reading (clockwise angle) is 39° 30'. The vernier is
read by continuing in the same direction as the main scale reading (as
indicated by the arrow in figure 90). A further aid in determining
which side of the vernier to read is furnished by the numerals on the
213
scales. The side of the vernier on which the numerals slant the same
way as those read on the main scale is the correct side. To read the
vernier, follow along the scales until a graduation on the vernier is
found that coincides with a graduation on the main scale. Read the
value of this graduation on the vernier (in figure 90 it is 4'), and add
it to the main scale reading. The complete reading in figure 90 is 39°
30' -4- 4' = 39° 34'. To read the counterclockwise angle, the same pro-
cedure is followed. The counterclockwise reading in figure 90 is 320°
+ 26' = 320° 26'.
227. READING HORIZONTAL ANGLES ON THE 20-SECOND TRAN-
SIT (fig. 91). The main scale of the 20-second transit is graduated to
15 minutes. The vernier is graduated in minutes and each minute is
graduated to 20 seconds. The procedure in reading the 20-second
transit is the same as in reading the 1 -minute transit. The main scale
is read opposite the center of the vernier. In figure 91 the clockwise
main scale reading is 90° 15'. The correct side of the vernier is selected
as for the 1-minute transit, the coinciding graduations found, and the
value read from the vernier. In figure 91 the vernier reading, clock-
wise, is 11' 40". The complete clockwise reading for figure 91 is 90° 15'
+ 11' 40" = 90° 26' 40". The counterclockwise reading in figure 91
is 269° 30' 3' 20" = 269° 33' 20".
Figure 91. Scales of fhe 20-second fransif.
228. SETTING UP THE TRANSIT.
a. When setting up the transit, place one of the tripod legs in ap-
proximately the correct position with reference to the station mark;
then manipulate the other two legs so that the plumb bob is brought
approximately over the mark, and, at the same time keep the leveling
head approximately level. On hillsides, one tripod leg should be up-
hill, the other two downhill. Keep the tripod bolt nuts sufficiently
tight so that they will just sustain the weight of the legs when the instru-
ment is lifted. Press the tripod shoes firmly into the ground to insure
rigidity. If the plumb bob is nearly over the mark, final centering may
be accomplished by moving the shifting plate after loosening the level-
ing screws.
214
b. When leveling the instrument, turn the plates so that each level
of the plate is parallel to a pair of diagonally opposite leveling screws.
Great care should be exercised when leveling. Initially, all screws
should have contact with the plate; one or more loose screws will cause
the plate to tip. The screws must not be so tight as to injure the in-
strument and strain the metal. To level, grasp one pair of opposite
screws between the thumbs and forefingers and turn so that the thumbs
move toward each other or away from each other, thus tightening one
screw and loosening the other. The motion of the two screws should
be uniform to prevent binding; one screw descends as fast as the other
ascends. After one bubble has been brought nearly to the center of its
tube, the other bubble is centered in a similar manner. Instead of get-
ting one bubble centered exactly, it is better to get both bubbles ap-
proximately centered, after which one bubble and then the other may
be exactly centered. When both bubbles are exactly centered, turn the
plate through 180 degrees. Any error in the plate levels will then be
evident. To correct the error, move the bubbles toward the center one
half of the deviation. After the instrument is leveled, check the plumb
bob to see that it has not been moved from the mark during the level-
ing process.
229. TO MEASURE A HORIZONTAL ANGLE.
a. With the instrument set up over the station at which the angle
is to be read, set the zero of the vernier opposite the zero of the hori-
zontal circle, using the upper clamp and tangent screw to bring them
to coincidence. Using the lower motion, point approximately at the
first object by looking over the top of the telescope. Move the tele-
scope until the vertical cross hair is very nearly on the point, clamp the
lower plate by means of the lower clamp thumb screw, and set exactly
on the point by using the lower clamp tangent screw. The line of
sight is now on the first object. To measure the angle, loosen the upper
clamp, turn the telescope to the second point, set approximately on
the point, clamp the upper plate, and set the vertical cross hair exactly
on the point by means of the upper tangent screw. The angle is then
read by means of the vernier, which was set at zero. Never overrun
the point in bringing the vertical cross hair upon it. Bring the
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