Document text
FM 4-15
!^
WAR DEPARTMENT
COAST ARTILLERY#J^
FIELD MANUAL
SEACOAST ARTILLERY
FIBE CONTROL
AND
POSITION FINDING
1
fd
FM 4-15
COAST ARTILLERY
FIELD MANUAL
UNITED STATES
GOVERNMENT PRINTING OFFICE
WASHIN€;T0N: 1910
For sale by the Superintendent or l>ocuments, Waahlnston, D, C, - Price 50 cents
TABLE OP CONTENTS
Paragraphs Pag^
Cmapteb X. General 1-^ 1
Chapter 2. IifotcATioN and Identification of
Naval Targets " 5-9 2
CHAPTEB 3, FtRB-CONTROL AND POSITION PiNDiNG
Systems.
Section I. General 10 7
n. Azimuth measurement- 11-12 8
m. Tracking 13-18 9
IV. Prediction 19-20 14
Chapter 4. Pihing Data 21-25 16
Chapter 5. Displacement,
Section I. General 26-27 21
fc -^ n. Azimuth dlHerence 28-31 21
mky%J jn. Range difference 3^-33 29
IV. Elevation difference 34^-35 31
V. Gun displacement 36-37 36
Chapter 6- Timing of Position Finding System. 38-40 38
Chapter 7. Observation Instruments,
Section I. General 41 41
n. Azimuth instruments 42-^3 41
^I ■ m. Depression position finders 44-47 46
^^M ^ jy Self -contained base instruments^ . 48-61 58
Chapter 8, Reference Numbers 52-^56 72
Chapt^ 9- Plotting Board Accessories.
Section I- Prediction scale 57-58 74
n. Set-forward rule 59-61 75
m. Set-forward chart 62-63 78
IV. Set- forward, scales 64-65 79
V- Targ ^ 66 83
Chapter 10. Plotting Boards.
Section I. Plotting^ board, M1904 ( Whistler -
Hearn) - 67-69 84
V* <'-... r.- n. Mortar plotting boards 70-71 92
^^TSU in. 110^ plotting boards, M1915,
^:^ * M1918, and M3 72-74 93
IV. Plotting and relocating boards,
M1923 (Cloke) and Ml 75-77 98
Chapter 11. Range Correction Devices,
Section I. General 78-79 112
n. Wind component indicator ' 80-^2 116
III, Range correction board Ml 83-86 119
IV, Other models of range correction
^^^^^^ boards 87-90 123
V. Percentage corrector Ml 91-92 124
III
TABLE OF CONTENTS
Chapter 12, Direction Correction Devices, Paragraphs
Section I, General 93
n. Deflection board, M1905 (for
guns) 94-96
ni. Deflection board, M1906 (for
mortars) 97-100
IV, Universal deflection board 101-104
V, Angular travel computor 105-106
VI, Deflection board Ml 107-110
VH. Azimuth adjustment slide rule.., 111-112
Chapter 13. Spotting Systems and Devices,
Section I, General 113-117
II, Earlier types of spotting boards. _ 118-120
m, spotting board M2 121-126
IV, Three-station spotting 127
V, Aerial spotting 128-135
Chapter 14, Fire Adjustment devices.
Section I, Fire adjustment board Ml 13ft-139
n. Bracketing adjustment chart 140-142
Chapter 15. Aerial Fire Control,
Section I, General 143
™ n. Functioning of an aerial Are con-
trol system 144^148
Chapter 16, Emergency One-Station Fire Con-
trol System 149-154
Chapter 17, Pointing Methods and Instruments,
Section I, General 155-158
II, Pointing in elevation 159-168
. Ill, Pointing In direction 169-176
^»M ■ IV, Examples of pointing adjust-
ments 177-180
Chapter 18, Fire Control Communication 181—188
Chapter 19, Organization and Duties of Range
Section and of Other Battery .
PiBE Control Personnel, I
Section I, Range section, general 189-191
II, Duties of range section details 192-194
in. Other battery Are control per-
sonnel 195-197
IV, Training 198-206
Chapter 20, Functioning of Fire Control and
Position Finding Systems.
Section I. General 207
n. Action before target is assigned. 208
III, Action when target is assigned., 209-211
IV, Functioning when using other
equipment 213-215
V, Functioning of spotting system.^ 216
Appendix I, Glossary
II, Construction of charts and scales
for seacoast artillery Are control
instruments
III, Derivation of formulas for spotting
board M2 ^^
IV. Data '__'/.
V, List of references
Index
Page
131
132
281
282
283
287
297
323
325
327
329
IV
FM 4-15
COAST ARTILLERY FIELD MANUAL
SEACOAST ABTIXLExlY
FIRE CONTROL AND POSITION FINDING
(The matter contained herein supersedes sections IV and V.
chapter 1, and chapter 2, part two, volume I, Coast Artillery Field
Manual, February 1, 1933.)
CHAPTER 1
GENERAL
1. Purpose. — Tlie purpose of this manual Is to provide a
guide for the technical training of the personnel employed
in the determination and application of firing data for sea-
coast artillery.
■ 2. Scope. — ^Hiis manual covers the principles of position
finding, including the design and operation of all instruments
and devices used by the position finding details, and the
pointing details on the guns, as well as the functioning of
that personnel as a whole. In addition, appendix II con-
tains information on the construction of charts and scales
used on seacoast artillery fire control instruments which
will facilitate verification of those charts and scales or con-
struction of new charts and scales when necessary. Data
especially applicable to fire control and position finding for
seacoast artillery are contained in appendix IV. Other use-
ful and more general data are contained in PM 4-155. The
contents of the manual apply to both fixed and mobile sea*
coast artillery. As soon as mobile weapons are emplaced in
position for firing at naval targets, the principles of this
subject, as laid down for permanently fixed weapons, apply.
■ 3. References. — More detailed information on fire-control
instruments and on related subjects niay be found in the
references listed in appendix V.
■ 4. Definitions.— Tliere are certain terms used throughout
the manual the meaning of which should be understood
before beginning a study of the text. Tliese appear in the
glossary, appendix I, which should be read carefully before
proceeding with the study of this manual.
CHAPTER 2
INDICATION AND IDENTIFICATION OP
NAVAL TARGETS
■ 5. General. — Identification, indication, and assignment of
the target are of primary importance. Any system used
must be simple, positive, and universal in its application,
so that when a commander assigns a target there will be in
the mind of the subordinate no doubt as to the target In-
tended. A knowledge of the characteristics and of the ap-
pearance of each of the various types of vessels, both war
and commercial. Is necessary to their ready Identification
by gun pointers, observers, and spotters. The various ves-
sels of war include battleships, battle cruisers, aircraft car-
riers, cruisers, destroyers, submarines, supply ships, fuel ships,
tenders, mine layers, and mine sweepers. They may best
be identified at long range by their silhouettes — the outline
of the solid features of the ship as seen at a distance. Sil-
houettes of warships may be found in pertinent standard
works or In training film slides. Silhouettes for all possible
targets should be prepared and posted In the various sta-
tions of harbor defense commands. Silhouettes are often
classified for convenience by using the number of funnels
and masts as a basis; for example, class 1-2, where the first
digit (I) indicates the number of funnels, and the second
digit (2), the number of masts. (See fig. 1.)
■ 6. Harbor Defense Water Areas. — a. In order that targets
may be indicated it Is necessary that the water areas adjacent
to a harbor defense be subdivided, The method of accom-
plishing this subdivision will vary in different harbor de-
fenses depending upon the geography and hydrography. A
typical method is shown in figure 2. If the harbor defense
shown included forts at one or more of the islands, each fort
would make its own subdivision, and the harbor defense
commander, in assigning a target from his command post
to a groupment or group at one of these Islands, would
2
FIRE CONTROL AND POSITION FINDING
. lil Mi
CLASS 1-1 CLASS t-2 CLASS 1-3
Figure 2. — Subdivision of harbor defense water area.
6-7 COAST ARTILLERY FIELD MANUAL
relocate and indicate the target with respect to the
subdivisions of that fort.
b. (1) In assigning target A, figure 2, to groups on Cor-
regidor Island, the harbor defense commander would indicate
it, TARGET, LiMBONEs; in assigning target B, target, monja.
If there were more than one ship in the Monja subarea it
would be necessary to indicate the target more exactly. |
Thus, target B might be indicated, target, monja right,
directing attention to a particular target toward the right
limit of the Monja subarea. The commander must be as
definite as necessary in his indication of the target. Where
there are several targets of the same type in the same subarea.
the commander may give the approximate azimuth and range
of the particular target, the target in this case being relocated
so that the azimuth and range given will locate the target
with reference to the station or battery to which assigned.
Thus, target B might be indicated, target, monja right, _ m \
AZIMITTH 135, RANGE 6,600. ^^%
(2) Another method of relocating a target is by reference
to an oriented grid which has been superimposed on maps of
a water area or subarea. A system which has been used isi
one in which large squares which are lettered are subdivided I
into smaller squares which are numbered. The grid system]
has the advantage that the target appears in the same area
from all stations, and its apparent location does not depend
on the point of view of each particular observer. Typical
target indications using such a system would be, target, ai4
and target, B26. ^ M
■ 7. Contents of Commands. — Commands employed in indi- -^"
eating and assigning a target to subordinate units contain
the following elements which should be given In the order
indicated:
a. Units^-The unit or units to which the command is ad-
dressed, as ALL GROUPS, GROUP 2, ALL BATTERIES, Or BATTERY
SMITH. This element alerts the unit or units addressed. It
is omitted when unnecessary, such as by the commander of ^^^
the lowest unit (the battery) in commands to his unit.
b. Target.— The word target quickly informs the com-
mander of the lower unit that a target is about to be desig-
nated or assigned.
FIRE CONTROL AND POSITION FINDING 7-B
c. Water area. — The name of the water area or subarea or
the letter and number of the square in which the target is
located.
d. Designation of target. — This element of the command
will include such of the following information as may be
necessary:
(1) For an isolated ship, the type, as battleship.
(2) For a ship which is part of a formation, the type, unit,
formation, and the number of the ship in the formation, as
CRUISER DIVISION, LINE, SHIP NO. 2. (Ships are numbered in
column from the leading ship; if not in column, from the
starboard (right) ship of the formation with reference to the
direction in which the formation is headed.)
(3) The classification as to funnels and masts (not
usually given for war vessels).
(4) In night operations, the number of the covering
searchlight, as in beam of no. a.
(5) The direction of movement of the target, as coming in,
GOING OVT, MOVING FROM LEFT TO RIGHT, OF MOVING NORTH.
(6) Any other description necessary for prompt and posi-
tive identification.
e. Designation of position finding system and statiOTis to be
used. — ^This element of the command is given, by the battery
commander when a system other than the normal system
is to be employed, for example, vertical base, b -second (the
secondary base end station).
/. Track. — The command track is given by a battery of-
ficer to position finding personnel to initiate the operation of
tracking. (See par. 13.)
g. Other commands. — Such additional commands for firing
or other action as may be appropriate. (If appropriate,
commands given by the battery commander may be preceded
by the command battery attention.)
■ 8. Examples of Commands. — The following are examples of
commands (dashes indicate pauses to allow for the repeti-
tion of the command by receiving personnel) :
TARGET : OCEAN VIEW _ ; BATTLESHIP,
COMING IN ; IN BEAM OF NO. 6 : COM-
MENCE FIRING WHEN IN RANGE.
8-9
COAST ARTILLERY FIELD MANUAL
TARGET . : LYNNHAVEN, RIGHT ; OIL
TANKER, CLASS 1-3. GOING OUT ; ALTERNATE
BASE, B' — B* ; (observers, spotters, and gun pointers
report "on target*') ; TRACK.
TARGET ; MARIVELES, RIGHT ; DE-
STROYER DIVISION, LINE, MOVING NORTH :
■ SHIP NO.l ; VERTICAL BASE B' ; (observers,
spotters, and gun pointers report "on target*') ; TRACK.
TARGET ; B-20 ; AIRCRAFT CARRIER,
MOVING WEST ; TRACK.
Note. — Grouping of naval vessels Into units and their tormations
for maneuver and for battle are covered In FM 4-6.
■ 9. Observing and Aiming Point, — ^The observing and aiming
point for observers, gun pointers, and spotters should be
some prominent feature of the target with which the vertical
wire of the telescope can be readily alined. Unless other-
wise designated by the officer assigning the target, the ob-
I serving and aiming point will be as follows:
I a. For vessels having one funnel — that funnel.
I^_ b. For vessels having two funnels — the rear funnel.
P^» c. For vessels having three funnels — the center funnel.
fc^ d. For vessels having four funnels — the third funnel,
f— e. For vessels having masts but no funnels — the appropri-
I ate mast selected according to the plan illustrated in a. b, c,
\ and d above for funnels.
/. For other vessels— the point designated by the officer
assigning the target.
CHAPTER 3
PIBE CONTROL AND POSITION FINDING SYSTEMS
Paragraphs
Section I. General 10
II. Azimuth measurement 11-12
m. TYacfclng 13-18
IV, Prediction 19-20
Section I
■ GENERAL
■ 10. General.— a- The function of a fire control and posi-
jtion finding system is to furnish data in the proper form
for use in pointing the guns of a battery for firing at a target.
In seacoast artillery, the guns must be pointed at a moving
target. The Ideal system would furnish firing data instan-
'taneously and continuously. With the present standard
plotting room and data transmitting equipment, the operation
is neither Instantaneous nor continuous. There is a lapse of
time between the instant an observation is taken on a target
and the instant the guns are fired with the firing data that
were calculated as a result of that observation. This interval
t is called the "dead time." Its length depends on the time
necessary to calculate the firing data with the desired
i accuracy and apply them to the guns.
b. In a 3 -inch rapid fire battery, case I or case II pointing
(par, 158) is used. The ranges and times of fiight are short
and the dead time is negligible. The problem of determining
firing data is comparatively simple, a self-contained range
finder and a gun sight being used.
c. Fbr a battery of 6-lnch caliber or larger the operation of
determining firing data for a moving target may be divided
into the following steps:
(1> Tracking— which includes observing and plotting suc-^
cessive positions of the target.
(2> Location of set -forward point — which consists of pre-
dicting the future position of the target, that is, its predicted
position at the end of the predicted time of flight.
7
/.
II 10 u
10-11 COAST ARTILLERY FIELD MANUAL
(3) Relocation — which consists of determining the range
and direction of the future position of the target from the
directing point.
(4) Calculation of firing data — which consists of convert-
ing the relocated data into corrected firing data for use in
pointing the guns.
d. Excessive dead time would afford the target undue op-
portunity to avoid the fire by maneuvering. On the other
hand too short a dead time would not permit performance of
the necessary operations with suitable accuracy. (See also
par. 39.) A satisfactory fire-control and position finding sys-
tem must be based on the principles of simplicity of method
and Of operation^
e. The three standard systems of position finding in use by
seacoast artillery are the horizontal base, vertical base, and
self -contained base systems. In all of these systems the pro-
cedure is similar. They differ only in the method of locating
the target in tracking. At least two standard systems are
usually made available for each battery. The standard sys-
tems may be supplemented by alternate systems consisting of
different combinations of elements of the standard systems.
The personnel of a battery should be trained and prepared
to use all of the standard systems and alternate systems.
Section II
Azimuth Measurement
■ 11, Angular System. — In all standard position finding sys-
tems, one of the elements of the data measured in locating
the position of the target is called the "azimuth." Azimuth
is the horizontal angle measured in a clockwise direction
from a selected reference line, passing through the position
of the observer, to the horizontal projection of the line of
sight from the observer to the objective (in this case, the
target) . P\5r seacoast artillery, the reference line is a hori-
zontal line parallel to the true south line at the origin of
the coordinates. (See *TM 4-225.) Any Instrument which
will correctly measure horizontal angles will measure
azimuths.
* See appendix V-
FIRE CONTROL AND POSITION FINDING
12-14
■ 12. ANGULAR Units. — The angular unit of measurement of
all horizontal angles for all seacoast artillery, except as stated
below for certain 155-mm guns, is the degree, an angle which
is one three -hundred- sixtieth (1/360) part of a circle.
Azimuths expressed in degrees are measured to the nearest
one one-hundredth (0.01) of a degree. The angular unit of
measurement of all horizontal angles for 155-mm guns which
have not been modified to use the degree system (see note)
is the mil, an angle which is one sixty-four-hundredth
(1/6400) part of a circle. Azimuths expressed in mils are
measured to the nearest mil. Thus, a degree Is equal to
17.778 mils, and 9 degrees is equal to 160 mils. Fbr practical
purposes, in small angles, a mil may be taken as the angle,
which intercepts an arc (or chord) equal to one one-thou-
sandth (1/1000) of the range; for example, at 10,000 yards one
mil intercepts approximately 10 yards.
Note. — As rapidly as funds permit, all sighting and other equip-
ment for seacoast artiUery using the mil as the azimuth unit
will be replaced with new or modified equipment using degrees
and hundredths.
Section III
TRACKING
■ 13. Principles Common to All Systems. — The first step
in all position finding systems is the location of the position
of the target with respect to the observation stations of the
battery. This operation is called "tracking** and consists
of locating at regular intervals of time (see note) by observa-
tion from one or more stations, successive positions of the
target, and plotting those positions on a plotting board. The
time interval between successive observations is called the
"observing interval" and is 15 to 20 seconds in length. The
observing intervals are indicated by TI (time interval) bells
or buzzers which sound simultaneously in all stations of the
battery.
Note. — In aerial fire control (ch. 15), the observing Intervals
are Irregular, are longer than 20 seconds, and are not marked by
TI bells or buzzers.
■ 14. Horizontal Base System. — a. Description. — (1) In
the horizontal base system, the target is located by the method
9
14 COAST ARTILLERY FIELD ftlANUAL
of intersection used in surveying in which the direction of
the target from two known points is determined. In the
triangle involved, one side and the two adjacent angles are
known. The solution is made graphically on the plotting
board. The system retjuires a base line on the ground, the
azimuth and length of which have been accurately deter-
mined by surveying (see* TM 4-225) ; two observation sta-
tions, one at each end of the base line, in each of which is
mounted an instrument for measuring azimuths; a plotting
board; and the necessary communication lines,
(2) The plotting board represents to scale the field of fire
of the battery. On it are located to scale in their proper
relation to each other the observation stations, the basei
line, and the directing point (the point for which the firing J
data are to be determined). Figure 3 illustrates the rela-
tion between the installations in the field and the set-up on ]
the plotting board.
(3) The observation station nearest the directing point f
is usually called the primary station. The station at the '
other end of the base line is called the secondary station* i
The base line of a horizontal base system is called "right-
handed" or "left-handed,*' according to whether the sec-
ondary station is to the right or to the left of the primary j
station, as viewed from behind the base line facing the field \
of fire.
<4) The base line for a horizontal base system should con-
form to the following principles:
(a) Its length should be from one-fourth to one-third of
the maximum range to be measured by the base line,
(b) Its direction should be approximately perpendicular
to the center line of the field of fire to be covered by the
base line,
<c) The base end stations should have suflftcient height
above sea level to afford a field of view to seaward beyond
the maximum range to be measured. (See app, IV.)
b. Operation. — The observers at the base end stations sight '
and follow with the vertical cross wires of their instruments
the target assigned by the battery commander. At the sound-
ing of each TI bell, the observers stop following the target
♦See appendix V.
10
FIRE CONTROL AND POSITION FINDING
14
with their instruments while the readers read the azimuths
and then resume tracJUng. Each reader is equipped with a
telephone head set connecting him to an operator, called an
"arm setter," in the plotting room. There the successive ob-
FiGUBE 3, — Relation between plotting board and field of fire.
servations are plotted on the plotting board. The plotting
board has an arm corresponding to each of two observation
stations with a means of setting each arm in azimuth.
Each arm setter sets his arm to the azimuth read by the
corresponding reader. The point of intersection of the arms
11
14-16 COAST ARTILLERY FIELD MANUAL
represents the position of the target at the instant the ob-
servations were taken. This point is marked by the plotter.
The operation is repeated at the sounding of each successive
TI bell. The points are called "plotted points." A line joining
the plotted points represents the track or path of the target.
■ 15. Vertk^al Base System — a. Description. — In the vertical
base system, the target is located by the offset method used
in surveying, in which the direction and distance of the
target from a known point are determined. The direction
is determined by reading the azimuth as in the horizontal
base system. The distance is determined by the depression
angle method which involves the solution of a vertical right
triangle of which one leg is the desired range, the other leg
is the effective height of the observation instiniment above
the target, and the hypotenuse is the line of sight from the
observer to the target. The known angle is the complement
of the angle between the hypotenuse and the known side,
corrected for refraction. It is the angle through which the
line of sight must be depressed from the horizontal to inter-
sect the target and is called the depression angle. (See par.
44 and FM 4-10.) The triangle is solved mechanically by
the observation instrument called a '^depression position
finder." This system requires but one observation station, the
azimuth and range to the target being read from the same
instrument.
b. Operation. — ^The observer tracks the target in azimuth
with the vertical cross wire as in the horizontal base system.
At the same time he tracks the target in range with the
horizontal cross wire. In the plotting room, only one arm
of the plotting board is used. The azimuth and range are
received from the reader at each sounding of the TI bell.
The arm setter sets the arm in azimuth and repeats the range
to the plotter who marks the point at that range by means
of range graduations along the edge of the arm.
■ 16. Self-Contained Base System — a. Description. — In the
self-contained base system, the target Is located by the offset
method as in the vertical base system. The direction is deter-
mined by reading the azimuth as in the other systems. The
range is determined by means of a self-contained range finder
12
FIRE CONTROL AND POSITION FINDING
16-18
of either the coincidence or the stereoscopic type. The prin-
ciples of operation of these instruments are discussed In
section IV, chapter 7.
b. Operafiort.— The operation of tracking with this system is
similar to that with the vertical base system except that
azimuths are usually read from a separate instrument.
I While it Is more difficult to read ranges as the TT bell sounds
in this system than in the vertical base system, observers
can be trained to furnish ranges regularly on or sufficiently
near the Instant the TI bell sounds.
■ 17. Alternate Base Lines and Alternate Stations. — For
batteries employing the horizontal base system, several alter-
nate base lines frequently are provided in order that use
may be made of the base line allowing the greatest accuracy
under existing conditions of visibility, target position, and
target course. Figure 4 illustrates a set-up in which B^ — B-
and B' — B^ are alternate base lines, all stations of which are
accurately located. Those stations of the horizontal base
system which have sufficient height of site are usually pro-
vided with depression position finders lor use in a vertical
base system, thus offering a choice of the most advantageous
system and stations.
■ 18. Emergency Systems. — ^Emergency systems possess fea-
tures of reduced accuracy that are acceptable only under
emergency conditions and are for use when all the normal
241701 <^— 40 2
13
18-19 COAST ARTILLERY FIELD MANUAL
systems break down or are put out of action. Possible emer-
gency methods include use of data determined from a station
outside the battery — either a group command station or the
directing point of an adjacent battery — and their conversion
to suitable firing data by means of range difference and azi-
muth difference charts; use of aerial observation to deter-
mine initial data and to determine adjustment corrections
thereto; and estimation of data from the guns by means of
comparison with the known ranges and azimuths of refer-
ence points, such as buoys, in the field of fire, with subsequent
adjustment as a result of observation of fire. An emergency
one-station fire control system and an emergency aerial
fire control system are described in chapters 16 and 15, re-
spectively.
Section IV ^^
1
'i?^y
PREDICTION
■ 19. Location op Set-Porward Point. — The point for which
firing data are calculated is called the "set-forward point."
This point must be located on the expected path of the target
and far enough in advance of the last plotted point to allow
for the travel of the target during the time that will elapse
between the moment the last observation was taken and the
instant of the impact of the projectile. In order to locate the
set-forward point, then, three things must be determined —
the expected path of the target, the elapsed time (D, and
the rate of travel (R) during that time. The rate multiplied
by the time will give the desired distance along the expected
path of the target from the last plotted point to the set-
forward point. This information cannot be determined ex-
actly but may be approximated to a satisfactory degree of
accuracy from the plotted positions of the target by assum-
ing that the target will continue to travel during the time
T in the same direction and at the same speed as it did dur-
ing the last observing interval. The expected path of the
target will then be a prolongation of the plotted path, and
the rate R will be the yards of travel during the observing
interval divided by the interval in seconds. The time T con-
sists of the dead time plus the time of flight. The necessary
14
FIRE CONTROL AND POSITION FINDING
19-20
amount of dead time fox' the system is selected In advance.
The only requisite Is that the time allowance be sufficient
for the performance of all the operations required. The guns
are usually fired as the bell sounds, in which case the dead
time is the same as, or is some multiple of, the observing in-
terval. (For a further discussion of the timing of a posi-
tion finding system, see ch. 6.) The time of flight depends
on the range to the set-forward point and may be determined
quite closely by a series of successive approximations, but
the usual procedure is to use a time of flight corresponding
to the range to the last set-forward point. This introduces
■ PREDiCTEO PATH OF TARGET
rfldVEL DURtfiC Tmt OF FLtGHT.^ >- ' ' '
TRAVEL DumrtO DEAD TiME^
TRACK OF TARGET,
forwardpout
PREDtCTEO POINT
PLOTTED POmTS
Figure 5. — Diagram of various positions of target.
only a small error If the predicting Interval is reasonably
short. In practice, this prediction is done with some. form
of prediction scale or set-forward device that eliminates
mathematical calculation. (See ch. 9.)
■ 20. Location of Predicted Point, — Sometimes (in mortar
fire only) the predicted positions of the target at the end of
the dead time (at the Instant the gun is to be flred) are
plotted on the board. These points are called "predicted
points." A comparison of the location of a predicted point
with that of the corresponding plotted point serves as a
check on the accuracy of location of the predicted point. A
better method of checking is described in paragraph 205.
15
CHAPTER 4
FIRING DATA
■ 21, Elements of Uncorrected Fiking Data (fig. 6) . — The
set-forward point having been located on the plotting board,
a direction and a distance must be determined that may be
used for or may be transformed into suitable data for the
actual pointing of the gun. These data are called "uncor-
rected firing data."
a. It is obvious that a gun must be pointed in direction.
This may be accomplished by either direct or Indirect
methods. If the target can be seen from the gun, the gun
sight may be used. The sight may be pointed at the target
and the gun set to diverge from the line of sight by the
amount of the angular travel during the time of flight and
fired at the expiration of the dead time. If the target cannot
be seen from the gun, the gun is pointed in azimuth at the
azimuth of the set-forward point and fired at the expiration
of the dead time as before. In the first method, used in
case i and in case n pointing, the desired element of the firing
data Is the uncorrected defiectlon; in the second method, used
in case III pointing, the desired element is the uncorrected
azimuth. From figure 6 it may be seen that in both cases
the gun is pointed in the same direction.
b. In addition to being pointed in direction, the gun must
be pointed so that the projectile will fall at the desired dis-
tance from the gun. This may be done by varying the angu-
lar elevation of the gun and, since the horizontal may be
readily established, the elevation is measured from the hori-
zontal. This is called the ^'quadrant elevation." If the
relation between the range and the quadrant elevation can
be established, the range to the set- forward point may be
used as the other element of the uncorrected firing data.
(This range elevation relation is published in firing tables by
the Ordnance Department.) Hence, the other element of
the uncorrected firing data is the uncorrected range. It is
the same for all cases of pointing.
16
1
FIRE CONTROL AND POSITION FINDING
22
■ 22, Determination of Uncorrected Firing Data, — a. Case
III pointing. — In case HI pointing, the uncorrected range
and the uncorrected azimuth may be read from the plotting
board by bringing the gun arm up to the set-forward point-
&. Case II pointing. — In case n pointing, the uncorrected
range, being the same as for case in, may be read from
the plotting board as before. The uncorrected deflection is
17
22-24
COAST ARTILLERY FIELD MANUAL
the angular travel of the target during the time of flight.
To obtain it, there must be some means of determining the
rate of angular travel of the target with respect to the di-
recting point. That rate being known, it may be multiplied
by the time of flight to the set-forward point. The range
to the set-forward point having now been determined, the
time of flight used in this operation is that corresponding^
to that range, as given in the flring tables. The rate of an-
gular travel is determined from data obtained on the plotting
board. The multiplication is performed graphically on either
the deflection board or a special device called an angular
travel computor. The functioning of these instruments is
discussed in chapter 12.
■ 23. Necessity for Corrections for Nonstandard Bal- ^
LisTic Conditions. — ^In order to compare the results of firings
held at different times and places and take into account the
conditions that actually exist at the time of flring, the range ^
elevation relation is constructed for certain assumed bal-
listic conditions called "standard." Conditions at the battery
at the time of a flring very seldom are exactly the same as
those which are considered standard. Therefore it is neces-
sary to consider and correct for those nonstandard conditions.
To meet this problem, the flring tables include, in addition
to the data for standard conditions, tables of differential
effects by means of which necessary corrections may be
made.
■ 24. Corrections to Range. — a. Corrections to the range j
for the following nonstandard conditions are ordinarily made '
in the plotting room:
(1) Variations in muzzle velocity (including temperature
of powder).
(2) Variations in atmospheric density.
(3) Variations in atmospheric temperature (elasticity).
(4) Height of site (including tide). (See note at the end
of this paragraph.)
^ (5) Wind.
(6) Rotation of the earth (for long range guns) .
(7) Variations in weight of projectile.
18
FIRE CONTROL AND POSITION TINDING
24
b. These corrections are determined by a range correction
board and are applied to the uncorrected range by an instru-
ment called a ^'percentage corrector," the result being the
firing range (or firing elevation) which is sent to the guns.
Figure 7 (vertical projection) is a graphical representation of
the application of corrections to the uncorrected range. In
^^M this projection ballistic conditions were assianed
^^^^ that it was necessary to lay the gun on point
to hit the target at the set- forward point.
PiGiTRE 7, ^Elements of corrected firing data.
order
Note, — For fixed seacoast batteries In which each gun is laid
m range by means of a range disk, the height of site of each gun
jiibove the datum plane (mean low water) is known, and the
19
24-25
COAST ARTILLERY FIELD MANUAL
correction is incorporated in the graduations on the range disk
on the gun. In such cases the correction for tide only Is made
in the plotting room. For mobile artillery which is pointed In
range by setting elevations, and for guns equipped with an elec-
trical data transmlBston system, the height of site correction is
not made on the pointing equipment and therefore the correction
for both height of site and tide must be made in the plotting
room.
■ 25. Corrections to Azimuth or Deflection. — a. To the
azimuth or to the deflection shown in flgure 6, corrections
for drift and for the following nonstandard conditions are
ordinarily made in the plotting room :
<1) Wind.
<2) Rotation of the earth (for long range guns) .
b. These corrections are determined and applied to the
uncorrected azimuth or deflection by a deflection board, the
result being the flring azimuth (or flring deflection) which
is transmitted to the guns. Figure 7 (horizontal projection)
shows a graphical representation of tJie application of these
corrections to the uncorrected flring data.
CHAPTER 5
DISPLACEMENT
Paragraphs
S£CTio>rl. General 26-27
11. Aglmutn difference ^ 2S--31
in. Range difference ^ 33-33
IV, fiHevatlon difference ■ 34^-35
V, Gun displacement 36-37
i^ Section I -^— if Jb^^"^
GENERAL ' ^
■ 26, Definitions, — See glossary, appendix I, for pertinent
definitions. The following terms should be understood before
proceeding with the study of this chapter: relocation, azimuth
difference, range difference, directing point, gun displacement,
gun parallax, gun difference, elevation difference,
■ 27, Relocation. — In all the standard systems that employ
the plotting board, relocation is performed mechanically on
this instrument. It is accomplished by establishing the posi-
tion of the directing point In the proper relation to that of
the other points on the board and providing means for read-
ing the azimuth and the range from the directing point to
the target. However, it is sometimes necessary or desirable to
relocate independently of the plotting board. Furthermore,
it is often necessary, after having data referred to the direct-
ing point, to determine corrections to apply to these data in
order to use them at other locations. The methods described
in sections TI to V, inclusive, are intended for use in these
latter cases.
Section II
AZIMUTH DIFFERENCE
■ 28. APPROXIMATE Formulas, — In situations similar to that
shown in figure 8, where the triangle formed is either right
21
28^30
COAST ARTILLERY FIELD MANUAL
or isosceles, and for values of the parallax angle of less than
400 mils, the relationship is
AB
Parallax (degrees) =57-7-^;
or
AB
Parallax (mils) = 1,000 -7-=,
Figure 8. — Azimuth difference (parallax), approxttnate formulas,
■ 29. General Formula, — For practical purposes the formula
below is satisfactory for general use. In figure 9, A is a
point from which the range and azimuth to T are known.
It Is desired to find the parallax angle pi=BTA), having
given the azimuth of AB and the displacement d.
I But
p Therefore
sin p sin BA T
AB ^ BT
AT^BT (approximately)
,^Bsin BAT
^
^> - AT
Angle BAT Is obtained from the known azimuths of AT
and AB.
¥
Figure 9, — Azimuth difference (parallax), general formula.
■ 30. Azimuth Difference Chart, Type 1. — a. General. — The
chart, figure 10, is actually a graphical solution of the gen-
eral formula given in paragraph 29. It consists of equally
spaced horizontal lines labeled in azimuth differences within
an azimuth circle, and a rotating arm graduated in a par-
ticular manner with ranges. The device is operated simply
22
FIRE CONTROL AND POSITION FINDING
30
by setting the movable arm to the azimuth to the target and
reading the azimuth difference from the horizontal line
opposite the range.
b. Example. — Construct a graphical chart for the determi-
nation of azimuth differences for a point B when the ranges
and azimuths to the target from a point A are known and
the field of fire is from 100* to 290 ^ The azimuth from A
to B is 280*" and the distance AB is 100 yards. Since at any
particular azimuth the azimuth difference Is greatest when
the range is shortest, the size of the chart required may
be limited by selecting as the minimum range to be covered
a range as great as practicable. POr this example the mini-
mum range is assumed to be 3,000 yards.
In figure 10 the horizontal lines are drawn first. Any
convenient uniform spacing is used. There must be enough
lines to accommodate the maximum azimuth difference.
Since the azimuth difference for a given range is a maximum
when the angle BAT is 90% the number of lines required
is determined by a solution of the general formula, using that
23
30
COAST ARTILLERY FIELD MANUAL
value of the angle and the value of the minimum range
already selected. This solution is
p — sin"
^ AB sin BAT
AT
,100 sin 90°
~^"' 3000
^^^^ =sin"^ 0-03333.
Therefore p=1.91'*.
In figure 10 the horizontal lines are spaced at intervals
each representing 0.10* of azimuth difference and lines up
to 2.00' will be sufficient in this case. In practice the lines
would be spaced at intervals representing 0.05*. To determine
ZERO ZERO
FicuHE 11. — Determination of sign of azimuth differences.
which azimuth differences are plus and which are minus, a
simple sketch should be made. For the example given, figure
11 shows the situation. Prom this figure it can be seen that
the azimuth of BTt is less than the azimuth of ATi and
the azimuth of BT^ Is greater than the azimuth of AT,.
Plotting of other assumed target positions will show that
the azimuth differences for all target azimuths above line
AB (that i5, between target azimuths 100* and 280') are
negative and those for all targets below AB (that is, between
target azimuths 280" and 100^) are positive. Figure 10 Is
marked accordingly.
The azimuth circle i5 next drawn and graduated. It will
be noted that the azimuth difference will be zero for all
24
FIRE CONTROL AND POSITION FINDING
30
ranges when the target is in prolongation of the line AB,
which occurs at azimuths of 280* and 100°. The 280" gradua-
tion, therefore, is placed on the azimuth circle on the right
side of the chart in prolongation of the line AB, Other
graduations are placed by means of a protractor. In the
figure, graduations are placed and marked 10' apart,
Intermediate graduations may be added as desired.
^ In order to place range graduations on the rotating arm,
a particular point is assumed where the angle BAT Is 90".
Target azimuth 190° is such a point. In this case the general
parallax formula in paragraph 29 takes the form
range
since sin BAT is unity. Using this formula the following
table is prepared for use in the graduation of the rotating
arm:
Range
d/range
p in degrees
3,000
a 0333
L91
3,500
.02Sti
].G4
4.00O
.02S()
1.43
4.50()
.0222
1.27
5,000
.0200
1.15
6,000
.0107
.96
7,000
.0143
.82
8,000
.0125
.72
9,090
.oni
.63
10,000
.0100
,57
1.^000
. 00«>7
,38
20,000
,m^
,29
25,000
.0040
.23
The rotating arm is constructed to solve azimuth differ-
ence when the line AB and the line AT are perpendicular to
each other, For any other azimuth the rotating arm graphi-
cally multiplies by the sine of the angle between the line AB
and the line AT and therefore solves completely the general
parallax formula.
25
31
COAST ARTILLERY FIELD MANUAL
FIRE CONTROL AND POSITION FINDING
31
■ 31. Azimuth Difference Chart, Type 2. — a. Description. —
(1) This device (fig. 12) consists of a graphical representa-
tion of the field of fire on which are —
(a) A horizontal plot of the station A from which data are
known and station B for which relocation is desired.
(b) An azimuth circle centered about station A.
(c) A series of azimuth difference circles whose centers are
on the perpendicular bisector of the line AB, ■
<d) A range scale pivoted about station A.
(2) Figure 12 shows a typical solution. A is the observa-
tion station and B the directing point. The azimuth and
length of the line AB are 235* and 280 yards, respectively.
The field of fire extends from 270" to 50* with 340' at its
center. The range limits are from 2,000 to 10,000 yards. In
the figure, azimuth difference circles for 1% 2"*, 3% 4% and 5*
of azimuth difference are shown. In practice, azimuth differ-
ence circles for smaller differences would be constructed, the
smallest difference at the longer ranges being 0.025".
h. Construction of azimuth difference circles. — Two propo-
sitions of geometry are used: first, the exterior angle of a
triangle is equal to the sum of the two opposite Interior angles;
and second, an Inscribed angle is measured by one-half the
intercepted arc. In figure 13, let -A represent the observation
station and B the directing point. Join A and B by a straight
line and prolong It to D. Construct the perpendicular bisector
MN of the line AB. This line is called the line of centers.
With any point on MN, as C, as a center and CB (=CA) as a
i
PiGXJRE 13. — Construction of azimuth difference circles.
27
31
COAST ARTILLERY FIELD MANUAL
radius, describe a circle. Construct the diameter through
C and B and draw TA, Select any other points at random, as
7" and T*\ and join them to A and B by straight lines. Angle
TBD=BTA-\-TAB; hence angle BTA=TBD—TAB; that Is.
the angle BTA is equal to the difference In azimuth between
the lines BT and AT, The angles AT'B, AT'*B, and ATB
each Intercept the same arc AB and are therefore equal;
hence, for all points on the circle, the azimuth difference
from A and B is the same, and is equal to the angle ATB.
If any number of circles with varying radii be similarly
drawn through A and B, each will be the locus of points of
equal azimuth difference. The following formula is useful
in constructing azimuth difference circles:
MC = BM cot BCMi^
AB cot BTA
* Assuming successive values for angle BTA in this formula, the
corresponding values of MC may be computed and tabulated.
The data for the construction of the circles shown in figure
14 are as follows :
^-140 ya
rds
BTA
(:<>t HTA
MC
1.00
57.20
8,0-21
2.00
28. W
4,010
3.00
ii», m
2,071
4.00
14.;iO
2,002
5.00
11.43
J. GOO
c. COTistruction of chart. — Select the point A, lay off and
label center line and outer limits of field of fire (for refer-
ence) , and plot the point B, Construct the azimuth circle,
with A as the center^ at the outer range limit of the field of
28
riRE CONTROL AND POSITION FINDING
31-33
fire, and graduate It. Compute and tabulate the values of
MC for the values of the azimuth difference desired. I^y off
the perpendicular bisector MN, construct the azimuth differ-
ence circles, and label them to read azimuth corrections,
construct and mount the range scale with pivot at A.
d. Operation of chart. — Set the range scale at the azimuth
to the target, and at the range to the target read the azimuth
correction. In figure 12 the range scale is set for an azimuth
of 15^. The azimuth correction for a range of 5»000 yards
is +2*, making the azimuth of the target from the directing
point equal to 17*.
. Section III -^ v •
RANGE DIFFERENCE
■ 32. Formula.— In figure 14 the range difference from the
points A and B Is calculated for point T. Angle BAT can
be obtained from the known azimuths of AB and AT.
AX—d cos BAT
PiGUEE 14. — Range difference,
Actually the range difference is AM which is obtained by
swinging an arc from B with r as a center. For all prac-
tical purposes AX^AM and equation (1) may be written
Range differences^ cos BAT (2)
It can be seen from the formula that range difference is
not considered to be affected by changes in range but only
by changes in azimuth to the target.
■ 33, Range Difference Chart — a. General. — A range dif-
ference chart (fig. 15) is actually a graphical arrangement of
the solution by formula. The chart consists of an azimuth
2417010 — 40-
29
33
COAST ARTILLERY FIELD MANUAL
circle with an auxiliary scale showing the range difference
opposite the corresponding azimuth.
b. Example. — Construct a chart of range differences from
a point B to r when the range and azimuth from ^ to T are
known. The azimuth from ^ to B is 60" and the displace-
ment Is 100 yards. Show the values of range difference to
the nearest 10 yards — that is, a maximum range difference
of 100 yards will be used until the actual difference becomes
smaller than 95 yards, when a value of 90 yards will be used
until the actual value becomes less than 85 yards, when 80
yards will be used, and so on. In order to locate the points
where a change takes place, a table is constructed from the
formula in equation (2) (par. 32) rewritten as follows:
Range difference
Figure 15. — Range difference chart.
30
FIRE CONTROL AND POSITION FINDING
33-34
1
2
3
Kange tiilTiT-
ence (yards)
Oos£f>t7'
(degrees)
itxi
1,00
9i5
.95
18
S5
. 85
>^2
■
75
. 75
41
■
^^^^^^^^^H
^
fk'i
,(S6
49
^
^^^^^^^^^H
1
65
".65
57
^^^^^^^^^H
1
4r)
,45
o:^
^^^^^^^^^H
F
35
,:^o
70
^^^^^^^1
^^^
fK^
26
-^ 5
.25
.15
.05
76
81
87
!>
^>^1
mjSi
ijy
.W
90
The angles shown in column 3 are taken to the nearest
degree. The values of the angles apply to each quadrant.
The range differences, however, are positive for two quadrants
and negative for the other two. The foundation of the chart
is the azimuth circle of figure 14. The example gives the
displacement as 100 yards and the azimuth from A to B
as 60^. The maximum range differences are then at target
azimuths of QO"" and of 240°. The former range difference is
—100 yards and the latter is +100 yards. According to
the table, 100 yards is the range difference until the target
azimuth changes IS'' on either side of the 60° and 240*
graduations. Marks are, therefore, drawn at 60 ±18 and
240±18 or at target azimuths of 78, 42, 258, and 222. The
next marks are at 60 ±32 and 240 ±32 or at 92, 28, 272, and
208 for a difference of 90 yards. Other marks are located in
a similar manner. Zero range differences are at target
azimuths 150 and 330.
Section IV
ELEVATION DIPPERENCE
■ 34. General. — The solution of elevation difference requires
the use of firing tables or of a chart based on the firing
i)
31
I
f
34-35 COAST ARTILI.ERY FIELD MANUAL
tables. The general formula for range difference (see par.
32 and fig. 14) is —
Range difference =(i cos BAT
If d in the equation is changed into elevation at the range
under consideration, the resulting equation produces the
elevation difference for that particular range. While range
difference for all practical purposes is affected by changes
in azimuth only, elevation difference is, in general, affected
both by changes in range and by ch?inges in azimuth.
■ 35. Elevation Difference Chart. — a. General. — The ele-
vation difference chart, figure 16, consists of an azimuth circle
with a rotating arm, graduated in range, pivoted at the
center of the circle. To operate the device, the arm is turned
to the azimuth of the target, and the elevation difference
is read on that vertical line which is opposite the range.
b. Example. — Construct a chart of elevation differences in
mils for a 16-inch gun, M1919, using 2,100-pound A. P.
projectile and full charge (Firing Tables 16-B-l), low angle
fire only, up to a range of 44,300 yards.
Note. — Above 44,300 yards the range is approaching the maxi-
mum. At this point the change in elevation corresponding to
a change of 100 yards in range is very large and Is not shown
accurately In the firing tables.
The azimuth from the directing point to the offset gun
is 60**, and its displacement is 100 yards. The field of fire
of this gun is from 240* through 360* to 70* azimuth.
An azimuth circle of any convenient radius is constructed
first, placing 60% the azimuth to the offset gun, opposite the
horizontal radius (fig. 16) . Next, the vertical lines are drawn.
They are equally spaced and must be sufficient in number
to accommodate the maximum elevation difference. The
maximum, elevation difference (the maximum shown in the
firing tables) in this case will be for a gun difference of XOO
yards at 44,300 yards' range which Firing Tables 16-B-l
show to be 8.9 mils. By visualizing this example and referring
to paragraph 32 (including fig. 14) and to paragraph 33
(including the table), it can be seen that all values of ele-
vation difference to the right of the pivot are negative and
those to the left are positive.
32
FIRE CONTROL AND POSITION PlNBING
35
33
35
COAST ARTILLERY FIELD MANUAL
The rotating arm is graduated in range to produce the
proper elevation difference where the gun difference is a maxi-
mum (that is, where it is equal to the displacement which
is 100 yards), in this case at target azimuth 60°. The fol-
lowing table shows the data extracted from Firing Tables
16-B-l for use in graduating the rotating arm. It shows in
colimin 2 the elevation difference corresponding to a range
^H change of 100 yards at each of the ranges shown in column 1.
1
2
Mun^a (yarfls)
Change in
t'levation
(mils) for lOO
vfirds* chango
in ranpti
0,^
5.000
.R
lO.OOtl
.9
15,000
1,1
20.000
1.3
25.000
\.r^
30.000
IJJ
:i5, m)
2.2
40,00()
2,0
41. (XW
?..2
42.000
:i. 7
43.000
4.5
44.000
7.0
44, 300
S.\>
To locate the graduations on the rotating arm, the arm
is set at azimuth 60°. The graduations are placed on the
arm by reference to the table and interpolation between
the points where the vertical lines of the chart intersect the
reading edge of the arm. For example, the zero range gradu-
ation is placed on the arm at a point six-tenths of the
distance from the zero vertical line to the 1-mil vertical line;
and the 30,000-yard range graduation is placed at nine-tenths
of the distance from the 1-mil vertical line to the 2-mil ver-
tical line. Since the general formula for range difference is
Range difference =(f cos BAT
34
FIRE CONTROL AND POSITION FINDING
35-36
it follows that with the rotating arm graduated to solve the
elevation difference for the distance d, rotation of the arm
to another azimuth will multiply graphically by cos BAT
(see flg, 14), thereby giving a general solution for elevation
difference.
Note. — ^If more than one kind of ammunition (including sub-
^^^^H caliber) Is to be used, the verticar lines should be sufficient In
^^^^^f number to accommodate the ammunition with the greatest eieva-
^^^^^ tion difference so that when ammunition is changed It will be
^^H necessary to change only the rotating arm on the chart,
^^F / ar
^W Jm ^^'
m JH di
[ I^P Cl(
.«
Section V
GUN DISPLACEMENT
36. CORRECTIONS TO DIRECTION. — a. Deftcction. — When gu
are pointed by means of deflection (cases I and n) , each gun
sight with proper deflection setting applied is directed at
the target* (See par. 21.) Therefore* no correction for
displacement is made to the deflection.
ft. Azimuth. — It is desirable to point each gun in direction
with a maximum accuracy error of 0.03". If the guns are
close to the directing point it may be possible to obtain the
^ required accuracy for all service ranges by pointing the guns
I 1^^ parallel to each other without correction. Where the field of
I H^ fire is narrow* sufficient accuracy may be obtained by causing
1 1 ** ^ the guns to converge at a central point in the fleld of Are
■ l^P when all are set with the azlmutli from the directing point to
Ik ^^m ^^^^ central point. In a fixed mortar battery, a common
Hl ^" method is to adjust the two guns of a pit to fire parallel to
^KL each other and to converge the two pits on a central point.
^H The methods of adjusting guns to converge or to be fired
^H parallel to each other are discussed in chapter 17.
^H When parallax is so large that a mean correction will not
^H suflace, the usual method is to make the parallax correction
^H in the plotting room and send separate azimuths to the in-
^H dividual guns. The Ml deflection board is equipped with a
^H^ displacement corrector so that azimuths may be furnished for
^^^^ two separate points. There is also a scale on this instrument
where the value of the parallax can be read.
35
36-37 COAST ARTILLERY FIELD MANUAL
With either the plotting and relocating board Ml or the
Cloke plotting and relocating board, separate azimuths may be
read for each gun of the battery by use of the gun plate and
the method of offset plotting. (See par. 77 J
In the absence of instruments of the required type, an
azimuth difference chart of some form must be used to make
parallax corrections* The transmitter of the data transmis-
sion system M5 (par. 186) has means of applying parallax
corrections. ^Tlk^^^^K ^ f^
■ 37* Corrections to Range (or Elevation) . — a. General. —
When the displacement is small the gun difference is negli-
gible. The fixed mortar battery furnishes a good example of
small displacement. This type of battery has two pits of two
mortars each with about 30 yards between pits. In this case
the range (or elevation) usually is determined for the directing
point of the battery, midway between pits, and no corrections
are made for gun differences. Due to the terrain, the size
of the guns, or for protection, the guns of a battery are some-
times widely separated* and corrections must be made for gun
differences. Such corrections are determined by methods dis-
cussed in paragraphs 32 and 33. They usually are applied
to the firing data in one of the ways described in b below.
b. (1) Ranges in yards, — ^When ranges are set in yards by
means of range disks, the corrections may be made either in
the plotting room or at the guns. If the plotting board is of
a type permitting relocation for individual guns (par, 77), the
range is furnished for each individual gun. If the plotting
board is of any other type, the gun differences may be deter-
mined by use of a range difference chart (par. 33) and the
range furnished for each individual gun. When corrections
are made at the guns, an arrow is painted on the edge of
the rotating platform so that it can be seen from the elevating
handwheel. This arrow is used as an index to a scale painted
on the emplacement, touching and concentric with the gun
platform. The scale is a range difference chart (fig. 15) . The
correction indicated on the scale by the arrow, when the gun
is pointed In azimuth, is applied to the range before it is set
on the range disk.
I
FIRE CONTROL AND POSITION FINDING
37
<2) Ranges in terms of angular units. — ^When ranges are
set in terms of angular units, as quadrant elevations, the
corrections are determined by means of an elevation differ-
ence chart (fiff. 16) In the plotting room, and the elevation
is sent to each gun. The transmitter of the data trans-
corrections are determined by means of an elevation differ-
ences In mils, (See par, 186.)
CHAPTER 6
TIMING OP POSITION FINDING SYSTEM
■ 38. General.— a. Since the calculation of firing data is not
continuous, some coordination is necessary between the op^
eratlon of calculating the firing data and that of loading
and firing the guns. Arrangements must be made either to
provide the firing data for the instant the gun is to be fired
or to fire the guns at the instant for which the data have
been calculated. The operations necessary in the process
of preparation of firing data and the firing of the guns using
those data are —
(1) Observation on the target and transmission of the
^observed data to the plotting room;
(2) Plotting of the observed position of the target;
(3) Location of the set-forward point;
(4) Relocation;
(5) Calculation of corrected firing data;
(6) Transmission of those data to the guns;
(7) Restoration of the guns to the loading
g position (after J
firing of the preceding round) ;
(8) Loading of the guns;
(9) Pointing of the guns; and
(10) Firing of the guns.
t>. Some of these operations (a above) take place concur-
rently, whereas some cannot be performed until certain
others have been completed. Operations (1) to (6), In-
clusive, are performed In order, followed immediately by
operations (9) and (10) in order. Operations (7) and (8)
need not await completion of (1) to (6) but may take place
concurrently with (1) to (6). (They may, however, require
either more or less time than that required for operations
(1) to (6), Inclusive.) Furthermore, upon completion of
operation (4) for a particular set of firing data, operation (1)
of the series for the next set of firing data may be per-
formed » followed in order by the others as before. In the
determination of the lengths of the observing interval, the
38
FIRE CONTROL AND POSITION FINDING 38-39
dead time, and the firing interval, and of the best method of
coordinating them, the principles of simplicity, speed, and
accuracy must again be applied.
■ 39. Time Intervals. — a. The observing interval must be
long enough to provide time for operations (1) to (4), inclu-
sive (par. 38a). On the other hand, it must be short enough
to provide firing data with the desired frequency. There
must be kept constantly in mind the necessity for avoiding
excessively long observing (and predicting) intervals. The
longer these intervals, the greater becomes the total elapsed
time during which a target may change course or speed
(or both) without proper corrections for these changes being
incorporated In the firing data. With the higher speeds of
modern ships and the greater times of flight corresponding
to longer ranges, the necessity for keeping the observing and
predicting intervals at a minimum assumes added importance.
An observing interval of 15 to 20 seconds with the shortest
practicable predicting interval will usually fulfill all condi-
tions satisfactorily.
b. The dead time must be long enough to provide time for
performance of operations (1) to (6) , inclusive, and (9)
and (10). Usually the system selected is such that the time
required for operations (5), (6), <9), and 10) is not greater
than that required for operations (1) to (4) inclusive, and
the dead time will be not greater than twice the observing
interval. The length of the dead time is dependent on the
combination of observing and firing intervals selected. When
more than one combination of observing and firing intervals
is possible, the one selected should be the one which gives
the shortest dead time.
c. The firing interval must be long enough to provide time
for operations (7), (8), and (9). Since we must provide for
operation over long periods, the minimum length of the
firing interval is determined by the maximum sustained rate
of fire. Its maximum length is limited only by the tactical
situation but is usually some multiple of the minimum length.
Therefore, if firing data are furnished with sufficient fre-
quency for the minimum firing interval, all normal situations
are provided for. Normal rates of fire for target practice for
each type of armament are prescribed for each calendar
39
39-40
COAST ARTILLERY FIELD MANUAL
year in the annual training memorantium, 'Instructions for
Coast Artillery Target Practice," issued by the War Depart-
ment. These rates may be considered as the maximum sus-
tained rates attainable with the types of armament for which
prescribed.
■ 40. Coordination of Timing. — The simplest combination
I of observing and firing intervals is the one in which they
are of equal length. This is standard practice for 6-inch
and 155-mm gun batteries. Larger caliber batteries have
longer firing intervals. Fttr those larger caliber batteries, the
observing interval is made such that it is contained into the
firing interval a whole number of times, and firing does not
take place on every set of firing data furnished. However,
f furnishing firing data once each observing interval allows the
. battery or one or more guns of the battery to fire without
waiting a whole firing interval, if for any reason it had been
impossible to fire on a particular set of data. For 3-lnch
rapid fire batteries the observing interval is greater than the
firing Interval and data are furnished as frequently as pos-
sible. (See par. 10.) In this case the delay In firing Is of
, no consequence.
Note.— In order to provide accurate firing data under service
conditions the observing interval must be not greater than 20
h seconds In any case.
'I^CIML)(^'^\
CHAPTER 7
OBSERVATION INSTRUMENTS
Paragraphs
Section I. General 41
n. Azimuth instruments , 42^3
in. Depression position finders 44—47
IV. Self-contained base Instruments I__ 48-51
_ r Section I _ _^.^
■J FtfT^
GENERAL
■ 41. Classification. — Observation instruments used in posi-
tion finding are classed as azimuth instruments, depression
position finders, and self-contained ran&e finders.
a. An azimuth instrument Is an instrument used for the
purpose of measuring horizontal angles (usually azimuths).
Some models are equipped also for measuring small vertical
angles. Instruments of tfils class are for use with the hori-
zontal base system. They are used also with the self-con-
tained base system. (See sec. IV.)
b. A depression position finder (D. P. P.) is an instrument
used for measuring ranges by the depression angle method
and for measuring horizontal angles (usually azimuths) . In-
struments of this class are for use primarily with the vertical
' base system. They may be used also with the horizontal base
system in lieu of an azimuth instrument.
c. A self-contained range finder is an instrimient used for
measuring ranges by ciirect observation. There are two
types of instrument, the coincidence type and the stereo-
scopic type. Later models are equipped for measuring azi-
muths. The self-contained range finder is furnished for use
with rapid-fire batteries.
Section H
AZIMUTH INSTRUMENTS
■ 42. Azimuth Instrxtment, M1910A1 (fig. 17). — a. Descrip-
tion. — ^Thls instrument is furnished for use with all seacoast
artillery, except 155-mm, for the measurement of horizontal
angles. It is not equipped to measure vertical angles.
41
42
COAST ARTILLERY FIELD MANUAL
5 3= 3 5 - ^ -
^- ^^ •■!
42
riRE CONTROL AND POSITION FINDING 42
(1) The telescope contains an optical system consisting of
an objective lens, erecting prisms, and eyepiece. Two eye-
pieces are furnished, one giving lO-power and one giving 15-
power magnification. A reticle is inserted in the system
ahead of the eyepiece with provisions for moving the reticle
into the plane in which the image Is ca5t. The reticle con-
sists of a piece of glass on which are etched a vertical line
which serves as the vertical cross wire and a deflection scale
which is in position as the horizontal cross wire. The scale
Is graduated in degrees from right to left with a least gradua-
tion of 0.02" and with 3" as the normal (or zero deviation) .
(See pars. 52 and 53.) The deflection scale is for use when
the instrument is employed for spotting. (See ch. 13.) It is
provided with a movable pointer called a "splash pointer:"
If the cross wires intersect the target at the instant of splash
and if the pointer is moved independently to the center of
the splash, the scale indicates in reference numbers the
angular deviation, as viewed from that station. Older models
of this instrument, designated as M1910, have the deflection
scale on a transparent piece of celluloid in the lower part of
the field. The scale has a least graduation of 0.05*.
(2) The base provides means of holding the telescope, of
imparting to it motion in vertical and horizontal planes,
and of measuring the horizontal movement. The principal
parts of it are the yolce, the traversing mechanism, the azi-
muth circle and index dlslc subscale, and the leveling mecha-
nism. The telescope is suspended in bearings in the yoke,
allowing about 40* of movement in a vertical plane. The
instrument is traversed in slow motion by operating the
azimuth drum cranlc, which turns the azimuth drum and
yolce, through a worm gear. The worm may be disengaged
to allow fast traversing by hand and reengaged without dls-
turbing the orientation of the instrument. The azimuth cir-
cle is graduated In degrees; the index dislc subscale is grad-
uated in hundredths of a degree. Provision is made for
traversing the telescope and yOlce independently of the azi~
muth drum and circle for use in orienting. The leveling
mechanism consists of a leveling plate, four leveling screws,
and two levels.
43
42
COAST ARTILLERY FIELD MANUAL
u>
(3) The tripod consists of a tripod head and three adjust-
able legs. The base screws onto the tripod head. Pier
mounts consisting of tripod heads on concrete or steel sup-
ports are usually provided for use in permanent base end
stations for fixed sea coast artillery.
b. Adjustment and orientation. — TTie adjustments of the
instrument consist of the exact location of the instrument
over the point representing the base end station, the leveling
of the instrument, the focusing of the eyepiece, and the
focusing of the objective and removal of parallax. TTie ori-
entation of the instrument consists of making it read the
correct azimuth of a point when sighted on that point. The
complete operation of setting up, adjusting, and orienting
is as follows:
(1) Approximate location. — Set up and adjust the height
of the tripod, mount the base on the tripod head, and suspend
the plumb bob from the base. With the aid of the plumb
bob, place the tripod and base approximately over the point
representing the base end station, making the tripod head
approximately level. Mount and secure the telescope in
place on the yoke.
(2) Approximate orientation. — Set the azimuth index and
subscale to read the azimuth of a known datum point visible
from the station. Loosen the azimuth clamp screw and turn
the telescope so that the eyepiece is slightly to the left of
the reading window and the aaimuth drum crank handle.
Lift up the instrument and tripod together and set them
down so that the telescope points approximately at the
datiun point.
(3) Exact location. — Center the plumb bob over the point
representing the Station by shifting the tripod legs, keeping
the tripod head approximately level and the telescope pointed
approximately at the datum point. When using the pier
mount, the operation of locating the instrument is done by
moimting it on the place provided.
(4) Leveling. — See that all four leveling screws have a
uniforta and moderately firm bearing on the leveling plate.
Release the traversing worm by rotating the worm box crank,
and traverse the instrument until one of the levels is parallel
to two diagonally opposite leveling screws; turn those screws,
44
r*^
FIRE CONTROL AND POSITION FINDING 42
One clockwise and the other coxinterclockwise. until the
bubble of that level is centered. The bubble will follow the
direction of motion of the left thumb. Without traversing
the instrument, center the bubble of the other level by means
of the two remaining leveling screws, readjusting each bubble
for any error caused by centering the other! (Caution: In
turning the leveling screws maintain the uniformly moderate
bearing of all screws on the plate; if the screws bind, loosen
one screw and proceed with the operation. Binding of the
screws will bend the spindle and make correct leveling of
the instrument impossible in the future.) Traverse the in-
strument through 180* and check the level; if a bubble de-
parts from the center, correct one half of the variation by the
adjusting screws on the level box and the other half by
the appropriate pair of leveling screws. Repeat the com-
plete operation until the level bubbles remain centered for
any position of the telescope in azimuth.
(5) Fociising eyepiece, — This operation consists of screw-
ing the eyepiece in or out so as to bring out most distinctly
the roughness of the cross wires. It should be done with
the telescope pointed toward the sky. This adjustment will
be constant for a given observer.
(6) Fociising objective and removal of parallax. — Direct
the telescope at the datum point and move the objective lens
in or out, by means of the focusing ring, until there is no .
parallax of the cross wires, that is, no apparent movement
of the cross wires across the image of the datum point as
the eye is moved across the eyepiece. The cause of parallax
is the lack of coincidence between the focal plane of the
objective lens and the plane of the reticle. It is often im-
possible to remove parallax completely from both the vertical
and the horizontal cross wires. In azimuth instruments, the
complete parallax adjustment should be made for the verti-
cal cross wire. This adjustment will be constant for a given
instrument. If used by another observer, he should adjust
the instrument for clearness of vision by focusing the eye-
piece not the objective.
(7) Exact orientation. — ^After all adjustments have been
made, reset the instrument to the azimuth of the datum
point, loosen the azimuth clamp screw, and bring the verti-
241701 <>— 40 4 45
42-44 COAST ARTILLERY FIELD MANTTAL
cal cross wire of the teiescope approximately on the datum
point. Tighten the azimuth clamp screw and, using the
azimuth slow motion screw, bring the vertical cross wire
exactly on the datum point. Clamp the azimuth slow mo*
tion screw. Check all adjustments and reorient tf necessary.
The orientation should be checked on at least one other
known datum point if possible.
c. Operation. — The instrument is operated by two men. an
observer and a reader. The observer receives, by telephone,
the command assigning the target. He directs the vertical
cross wire of his instrument on the target and reports **On
target.** At the command track he tracks the target, keep-
ing the vertical cross wire on the designated observing point
by turning the azimuth drum crank. At the intervals indi-
cated by the TI bell he stops tracking momentarily to permit
the reader to transmit the azimuth to the plotting room.
■ 43. Azimuth Instrument, M1918. — a. At present this instru-
ment is furnished for use with 155 -mm guns. (See note,
par. 12.) It is similar to the M1910A1 instrument. Thej^j
main dlflferences are —
(1) The telescope Is smaller and lighter.
(2) Two eyepieces are furnished, one of 10-power and one'
of 20-power.
(3) The Instrument is equipped to measure vertical angles
' - from —300 to +500 mils.
(4) The azimuth circle and index disk subscale and the
interior splash scale are graduated in mlis.
I &. The adjustment, orientation, and operation are the same
as for the M1910 instrument except that there is no pro-
vision for eliminating parallax, since the telescope is of the
fixed focus type.
Section III
DEPRESSION POSITION FINDERS
■ 44. Range Finding by Depression Angle Method. — a. The
method of range finding by means of the depression angle
is used by depression position finders employed in the ver-
tical base position finding system. By this method the range
to the target Is determined by measuring the angle at the
46
FIRE CONTROL AKD POSITION IINDIKC
44
instrument between the horizontal and the line from the
instrument to the water line of the target, and by indicating
on a graduated scale the product of the cotangent of that
angle by the height of the instrument above the target. In
this method the effect of the curvature of the earth must be
considered. The problem is illustrated in figure 18, where
O represents the position of the observer at a height OM
above sea level, the arc MT represents the surface of the
sea, and T the position of the target on the sea- By sighting
on the target the angle d is measured. This angle com-
47
44
COAST ARTILLERY FIELD MANTTAL
bined with the true height OM will give a range MP, whereas
the desired range is NT (=MTO. This range could be com-
puted by using a corrected depression angle d' or by using
a corrected height of instrument ON, The latter method is
used in seacoast artillery instruments, The instruments are
designed to correct without appreciable error for all values
of the depression angle,
b. The problem is further complicated by atmospheric re-
fraction. As the rays of light pass from the target to the
observer they are bent downward so that the apparent change
in the height of instrument due to curvature of the earth is
48
FIRE CONTROL AND POSITION FINDIHC 44-45
less than the true change. The effect of refraction is illus-
trated in figure 19 which is similar to figure 18. Because of
refraction, a ray of light from the target will reach the ob-
server by the curved path TO and the target will appear to
be on the line OR. As in the case of curvature alone » the
desired range is the range NT but the proper height of instru-
ment is the height OP. The amount of refraction is ex-
tremely variable, and the variations from normal can be
detected only by checking the instrument on a datum point
of known range.
c. Corrections for curvature of the earth and for normal
refraction are made on the instruments by graduating the
range disks for the corrected height of instrument iOP, fig.
19). Provision is made on all instruments to compensate
automatically for changes in the effect of curvature and nor-
mal refraction due to changes in the depression angle. Small
changes in the height of instrument due to tide and changes
in refraction from normal may be corrected for without ap-
preciable error. Those adjustments are discussed in detail^
in the paragraphs dealing with the separate instruments,
■ 45. SwASEY Depression Position Finder (fig. 20). — a. De-
scription.^The Swasey D. P. R is an instrument equipped
to measure horizontal angles and to measure ranges by the
depression angle. It may, therefore, be used in either a hori-
zontal or a vertical base system. It is an older type Instru-
ment. (See par. 46.)
(1) The telescope contains an optical system similar to
that of the azimuth instrument but with a larger field and
more illumination. Eyepieces are furnished for 12- and 20-
power magnifications. A vertical wire and a horizontal wire
are carried in a slide in the micrometer box allowing vertical
motion of the slide. The instrument does not have an
Interior splash scale,
(2) The cradle provides means of supporting the telescope,
of imparting to it motion in vertical and horizontal planes,
and of making the necessary adjustments to permit reading
correct azimuths and ranges. The traversing mechanism is
similar to that of an azimuth instrument. The azimuth scale
is graduated in degrees; the azimuth drum (subscale) is
graduated In hundredths of a degree. The range drum Is
49
45
COAST ARTILLERY FIELD MANUAL
graduated to indicate every 10 yards of range between 1,500
and 12»000 yards. The leveling mechanism is similar to that
of an azimuth instrument.
(3) The base is a heavy metal casting which supports the
cradle and telescope.
b. Adjustment and orientation, — ^The adjustments of the
instrument are the leveling, the focusing of the eyepiece, the
focusing of the objective and removal of parallax, the check
of the range drum for telescope level, and the range ad-
PrcusE 20. — Swasey depression position finder
justment (for curvature of the earth, refraction, and tide).
The orientation of the instrument consists of making it read
the correct azimuth of a point when sighted on that point.
Small adjustments In azimuth may be made by means of
the azimuth set screws. The complete operation Is gen-
erally similar to that outlined in paragraph 42 for the azimuth
instrument, M1910A1, except that the parallax adjustment
must be made for both the horizontal and the vertical cross
50
FIRE CONTROL AND POSITION FINDING 45
wires. The additional adjustments not discussed in that
paragraph are as follows;
(1) Telescope level. — After the instrument Is leveled, the
range crank should be rotated until the level on the top of
the telescope indicates that the telescope is horizontal. The
range drum reading should then be "telescope level." If it
is not, loosen the screws attaching the range drum to the
bevel gear and, holding the telescope horizontal, rotate the
range drum until it reads "telescope level." Tighten the
holding screws. The bubble of the level should then remain
stationary while the height slide is moved to any position.
(2) Range adjustment. — After the check for telescope level,
the range adjustment may be made. As the first step, set
the height slide so that the reading on the height scale is
that of the instrument corrected for the tide (if known).
Select two datum points, i>L at a range somewhat longer,
and Dq at a range somewhat shorter, than the ranges over
which it is expected to work. Using the range crank, set
the range drum at the reading of Dl and direct the tele-
scope at that point in direction: bring the horizontal cross
wire to the water line of that point by means of the microm-
eter screw. Using the range crank, set the range drum at
the reading of Ds and direct the telescope at this point in
direction; bring the horizontal cross wire halfway to the
, ^ water line of this point by means of the height slide pinion.
"^Repeat these two operations until correct readings on both
Dl and Db can be obtained by operating only the azimuth
dnim handle and the range crank. This adjustment should
be repeated at intervals, the frequency depending on the
extent of the variation in tide and refraction.
c. Operaiion.— The instrument is operated by two men, an
observer and a reader. For the vertical base system, the
observer tracks the target, keeping the vertical cross wire
on the designated observing point by turning the azimuth
drum handle, and the horizontal cross wire on the waterline
of the target by turning the range crank. At the intervals
indicated by the TI bell, he stops tracking momentarily to
permit the reader to transmit, first, the azimuth and, sec-
ond, the range, to the plotting room. Ftor the horizontal
base system the target is tracked in azimuth only.
51
\
46
COAST ARTILLERY FIELD MANUAL
■ 46. Depression Position Finder, M1907 (fig. 21). — a. De-
scription. — The D. P. F., M1907, is a later type of instrument
than the Swasey instrument. Each Swasey D. P. F. Is made
for a particular height and is not suitable for use at different
heights. The M1907 type is issued in fifteen classes^ each
class being specially designed for use at a different range of
heights (except that classes DM and DMM have the same
range of heights; see table) with the classes overlapping so
52
FIRE CONTROL AND POSITION FINDING
46
as to cover all heights from 25 to 1,140 feet. In this way,
a much greater degree of accuracy is obtained than is pos-
sible with the older type. The instruments have interchange-
able depression mechanisms, height scales, and range dials,
making it possible and convenient to convert an instrument
from one class to another.
D
. P. R, M1907 ^^m
Class
HeiRtits for
which de-
sie^iicd (feet)
Limits of grad-
uation on rango
drum (yards)—
minimum,
maxim um
A
ll-S-Sfi
l,i5OO-I2,O00
IJ
fa^-M5
1,500-12,000
TJM
r»(>-i4fl
1,500-15,000
C
12.5-;W0
1,,tOO-1 2,000
CM
125-300
1, ,-500-20, 000
i>
2SiMm
1.500-12.000
DM
DMM*
2S0-S«0
2S0-M)
1,500-30.000
1,500-20,000
T>M1
PMMl
3S0-1, 140
3SO-l,nt?5
2, 000-20, 000
2, 000-20, 000
A A
2,VH^)
1, 000-^,000
1)1)
hMi-4m
1, mM\ 000
EK
3(M>-7.V)
1,000-0,000
K
0t)-210
00(t-0, 000
F
105-1()0
(MXM>,000
* Diflers from DM in minor structural details.
53
46 COAST ARTILLERY FIELD MANUAL
(1) Telescope.— The optical system Is similar to those pre-
viously described. Eyepieces are provided for 15- and 25-
power magnifications. A counterweight (7) is furnished for
adjusting the balance of the telescope.
(2) Table assembly. — The table and body serve the same
purpoGe as the cradle of the Swasey D. P. F. Their mechani-
cal features are, however, somewhat different. The method
of indicating azimuths is unique. Two dials and a stationary
scale are furnished: the azimuth dial, concentric with the
vertical axis of the instrument, on which are read the hun-
dreds and tens of degrees; the planetary dial, geared to
rotate once for each 10 degrees, on which are read the unit
place of degrees; and the stationary scale (or vernier) next
to the planetary dial, on which are read the hundredths of
a degree. The number to be read on the azimuth dial is
the one in coincidence with or above the index; the unit
place of degrees to be read on the planetary dial is the one
that registers with some portion of the stationary scale.
The hundredths of a degree are read from the stationary
scale where the unit degree mark of the planetary dial reg-
isters on the scale. The method of indicating azimuths is
illustrated in figure 22. The azimuth indicated in the figure
is 79.75*. The range is indicated on a dial instead of a
drum. The minimum and maximum ranges indicated de-
pend upon the class of the instrument. The leveling mecha-
nism has only three leveling screws of which any two may
be used together to level in one direction. The third screw
should then be used with each of the others to level in the
other direction, taking up one-half of the adjustment with
each pair.
(3) Base.— The base or pedestal is similar to the one pre-
viously described for the Swasey D. P. F.
b. Adjustment and orientation. — (1) General. — The adjust-
ments of this Instrument are the same as those listed in
paragraph 45b for the Swasey D. P. F.. except that no check
of the range drum for telescope level is necessary and the
range adjustment (see (2) below) is different. In the orien-
tation of the Instrument, the adjustment in azimuth is made
by clamping the table to the body of the instrument and turn-
ing the pedestal cap until the vertical cross wire is on the
64
^
FIRE CONTROL AND POSITION FINDING
46
datum point. The level of the instrument should then be
rechecked. One or two trials may be necessary before the
vertical cross wire will remain on the datum point when the
instrument Is leveled.
1
a
(2) Range adjustment. — (a) Having ascertained the con-
dition of the tide at the moment, set the slide block (27)
along the tangent screw rail (25) to that point on the height
scale (42) which corresponds with the present height of the
55
46 COAST AUTILLEHY FIELD MANUAL
instrument In feet and clamp the slide block by means of the
clamping screw,
(b) Select two datum points, Dl at a range somewhat
longer, and Ite at a range somewhat shorter, than the ranges
over which it is expected to work.
(c) Point the telescope in (iirection at Dl and turn the
■ outer sleeve nut (23), which operates the depression mech-
anism, until the range pointer indicates the range of Dl.
If the horizontal cross wire of the telescope is not on the
waterline of the datum point bring it on by means of the
compensating screw (34) , and clamp the compensating screw
to the shaft of the rack gear by tightening the set screw (28) .
(d) Point the telescope in direction at D& and turn the
outer sleeve nut (23) until the range pointer indicates the
range of this datum point; bring the horizontal cross wire
in halfway to the waterline of this point by moving the slide
block by means of the slide block adjusting screw (29) . The
nut block (30) must be held fixed while making the adjust-
, mentf
I (e) Repeat these two operations until correct readings on
I both Dl and Db can be obtained by operating over the azi-
I muth hand wheel and the outer sleeve nut.
I (/) This adjustment should be repeated at intervals, the
1 frequency depending on the extent of the variation in tide
l^^and refraction.
w^ e, OperatUm. — (1) Geneml. — The Instrument is operated
as described in paragraph 45c for the Swasey D. P. P. The
H target is tracked in azimuth by means of the pinion shaft
B head on the table at the left of the telescope and in range by
means of the outer sleeve nut (23).
(2) Precautions. — (a) The depression mechanism should
not be operated until the height setting has been made.
(b) The instrument should not be forced against the stops
provided for minimum and maximum depression.
(c) The sun shade (6) should be kept in place at all times
to preserve the proper balance of the instrument
^^__ (d> When the 15-power eyepiece is used, the counterweight
(7) should be screwed all the way out; when the 25-power
eyepiece is used, it should be moved in slightly.
56
FIRE CONTROL AND POSITION FINDING
46-47
(e) The operator when tracking should be careful not to
disturb the balance by resting his head against the eye
shield.
■ 47, Depression Position Finder Ml (fig. 23). — a. Descrip-
tion. — ^The D. P. F. Ml is the standard instrument now issued
for use with sea coast artillery. It is similar to the M1907
D. P. F. It is issued in ten classes covering all heights of
instrument from 74 to 1,395 feet. The eyepiece of tills in-
strument may be set for any desired power from 10 to 30.
Amber and blue ray filters are provided for use when desired.
The method of indicating azimuths is by an azimuth circle
giving degrees and a micrometer subscale indicating hun-
dredths of a degree. The range is indicated by a pointer
moving across a moving scale, The minimum and maximum
ranges indicated depend upon the class of the instrument.
Figure 23, — Depression position finder Ml.
57
47-48
COAST ARTILLERY FIELD MANUAL
D. P. F. Ml
Class
Heights for
which de-
signed (feet)
Limits of grad-
uation on range
drum (yards)—
mini mum,
maximum
^^^^^^^^^^^^H
^^^^^^^^^H
^^^^^^^
2
100-1S2
2.000-24,000
^^1
^^^^^r
3
150-272
2. 500-30, WH)
i><:^H
^^^^^^^^^p^ jdf^lUK. "^
4
2WM75
2,500-38,000
Kd^H
^^TJ^V^r^
5
■Sdf}-l)m
2,500-45,000
^)^
^w/SwA
6
4.S0-81()
,5.000-50,000
^^
fs^M
7
575-1, (145
,5,000-55,000
W<1
fmm
18
7,W-1, 395
3,000-0^,000
iWll
\Sm
fi
750-1, 375
5,000-60,000
1^1
iSf
10
625^1, IfiO
2, 500-45. 000
Ira
I Class 8 is not fitted for use over 54,000 yards at heights below 1,000 feet,
b. Adjustment and orientation, — The adjustments of this
instrument are the same as for the M1907 D.P*F.(par. 46t>).
In the orientation of the instnunent, the coarse adjustment
in azimuth is made by loosening the three cap screws (8>
and rotating the leveling plate (5> ; the fine adjustment is
made by means of the adjusting screws (&> .
c. Operation, — The instrument is operated as described in
paragraph 45c, for the Swasey D. P. F, The target is tra<;ted
in azimuth by means of the handwheel (3) and in range by
means of the handwheel (21). The same precautions apply
to this Instrument as to the M1907 D. P. F, (par, 46c(2> ) .
Section IV
SELF-CONTAINED BASE INSTRtTMENTS
■ 48. Principles. — a. Geometric principles, — ^The modern
horizontal base self- contained range finder is designed to
58
FIRE CONTROL AND POSITION FINDING
48
determine the range to a target by the same genera] geometric
principles as those used in the depression position finder.
The base line, being contained within the Instrument, is ex-
tremely short In comparison with the range to be determined,
consequently the angles measured are small. In order to
get accuracy comparable to that of other systems, the very
small changes of the smaD angles measured must be de-
termined with precision. The triangle solved by a range
finder is shown In figure 24. AB Is the base line, C is the
target, and AC is the range i£. In any selected instnmient,
the length of the base line and the angle at A are main-
59
48
COAST ARTILLERY FIELD MANUAL
tained constant In value. The scale on which the angle a is
read may then be graduated to read directly in range.
b. Optical principles. — There are two common methods of
determining the angle a, the coincidence method and the
stereoscopic method:
(1> Coincidence method. — (a> Figure 25 is a diagrammatic
sketch of the main features of a coincidence range finder. A
and A' are two penta prisms at the ends of the base linet B
and B' are the objective lenses, C and D are measuring prisms,
60
PIBE CONTROL AND POSITION FINDING 48
E is any position of D when reading a range, F is the range
scale, G is an ocular prism, and H is the eyepiece. The penta
prisms A and A' turn the rays of light through an angle of 90%'
the distance between them determines the length of the base
line. The measuring prisms C and D are wedge-shaped: they
are identical and are placed one inverted with respect to
the other so that all rays of light passing through them will
emerge parallel to their original course. The functioning of
these prisms is as follows: The ocular prism G serves two pur-
poses; it divides the image into halves by cutting off along a
straight line the lower half of the rays that come from one
end of the instrument and the upper half of the rays that
come from the other end, and it turns the reflected rays
through 90'', toward the eyepiece H. The observer at the
eyepiece sees a composite view, one half of the image coming
fc through A, B, C, and D, and the other half coming through
A' and B'. The dividing line between the halves Is horizontal
and very sharp and distinct.
(b) If the range finder illustrated in figure 25 is directed
at a vertical target such as the mast of a ship, and if the
target is at an infinite distance, rays of light from it entering
the two ends of the range finder will be parallel, and the
parallax angle a will be zero. If the instrument is properly
adjusted with prisms C and D in contact, each ray of light
will be refiected through two right angles, and the resulting
'image will not be distorted in any way. The upper half of
the mast will be exactly over the lower half as at (2) , figure
26, and the two images will be in coincidence.
I (c) If the same range finder is now directed at a similar
■target at a finite range, the rays of light from the target
entering the instrument will no longer be parallel and, there-
fore, the resulting image will no longer be un distorted. One
half of the image will be displaced with respect to the other
half, as at (1), figure 26, and the amount of the displacement
will be a measure of the parallax angle a and, therefore, of
the range. It is the purpose of the measuring prisms C and
D to measure this displacement. When the prism D is moved
away from the fixed prism C, a ray of light will no longer
emerge from the prisms in its original course but will be re-
fracted by both, and the amount of the refraction will be
241701^^0 5 61
48
COAST ARTILLERY FIELD MANUAL
proportional to the distance through which prism D is moved.
The effect of the movement of prism D on the image, as seen
through the eyepiece, is to move the half of the image which
has passed through A, B, C, and D laterally by an amount
directly proportional to the distance that the prism D has
been displaced. By regulating the position of the movable
prism D, the observer can bring the halves of the image into
coincidence and, since the distance that the movable prism
Figure 26. — Appearance of target in coincidence range finding.
must be displaced is inversely proportional to the range to the
target, the scale i^ may be graduated in ranges and the range
to the target indicated on that scale.
id) Since the measuring prisms used are only very slightly
wedge-shaped, appreciable displacement of the movable
prism is required to obtain coincidence even for small changes
in the parallax angle a. Therefore much greater accuracy
may be obtained than by measuring the parallax angle di-
62
FIRE CONTROL AND POSITION FINDING 48
rectly or by measuring the displacement of the halves of the
image.
(2) Stereoscopic method, — (a) The stereoscopic method of
range finding is based on the principle of stereoscopic vision,
a principle which is not involved in any of the methods pre-
viously discussed. Stereoscopic vision, or depth perception,
depends upon seeing simultaneously two views of the same
object taken from slightly different viewpoints. It there-
fore depends on the use of both eyes. It is one of the means
by which we can tell which of two objects is closer to us.
The stereoscopic sense alone does not permit of great ac-
curacy in the direct estimation of distances but, using this
sense, an observer can tell with considerable accuracy when
two objects are at the same distance from him. It is this lat-
ter ability which is used in stereoscopic range finders,
k Cb) Figure 27 is a scliematic diagram showing the elements
"of a simple stereoscopic range finder. The essential features
are the pent a prisms Bb and Bl. which refiect through right
angles the light rays that enter each end of the instrument;
the objectives Ob and Ol; the mirrors Mr and Ml, acting to
refiect the light rays through right angles once again; the
reticles Rn and Rh, with their symbols (indicated by the small
circles) engraved thereon; the eyepieces En and £l; and the
two adjusting prisms or wedges, one of which, Wf, is fixed, and
the other, Wm, movable. These wedges are identical in shape
but one is inverted when mounted in position. When the
movable wedge Wm is moved to the left so that the surfaces
of the two wedges touch, the exterior faces are parallel and
hence light rays will pass through without appreciable re-
fraction. When the wedges are separated, each refracts a
light ray passing through by an equal amount but in opposite
directions. The path of a ray emerging from the left of Wp
is parallel to its direction when it enters from the right of
Wm. The action of the two wedges is to displace the ray by
an amount proportional to the distance between the wedges
without changing the resulting direction of the ray. In the
latest instruments, the movable wedge Wm is not moved
laterally as shown in the diagram, but both wedges are ro-
tated in surface contact. The result is the same.
63
48
COAST ARTILLERY FIELD MANUAL
(c) Suppose the range finder is directed at an object at an
infinite range, for example, a star. Light rays entering Bb
and Bl will be parallel and will be refiected along the axis
of the instrument to Mr and Ml and then to Rr and Rl,
where the two images of the star will coincide with the
reticle symbols, provided, of course, that the wedge Wm is
in its infinity position, that is, against Wf, When the ob-
server looks through the eyepieces, he will merge the two
images of the star and the two images of the reticle symbol
Into one» and both objects will appear to be at point 1. (To
make the figure clear, the distance between the eyepieces
has been greatly exaggerated.) Now suppose the range
finder is pointed at some target at a finite distance. The
rays of light entering Bl and Br will no longer be parallel.
The instrument will be traversed so that the ray entering
Bh will be perpendicular to the axis of the instrument. The
left image of the target will be reflected, as before, to the
left reticle at the same place as the symbol. The ray enter-
ing Br will not be perpendicular to the instrument's axis, and
hence if Wm is touching Wf, the right image of the target
will be refiected to some point on the reticle as that marked
a. To the observer, the target will seem to be at point 2,
64
FIRE CONTROL AND POSITION FINDING 48-49
short of point 1, where the reticle symbols still appear to be.
H Wm is displaced to the right to the position marked X,
the ray of light will, through the double refraction of the two
wedges, be displaced so that It is reflected on the reticle at
the point marked &. The target will now appear to be at
point 3, at a greater range than that to the apparent posi-
tion of the object formed by the reticle symbols. At some
Intermediate position, such as that marked V, the displace-
ment of the light ray caused by the two wedges will be suffi-
cient to cause the ray to be reflected on the reticle where
the symbol is engraved. The target and reticle symbols will
both appear to be at the same range, both appearing to be
at point 1, and stereoscopic contact will have been estab-
lished. The movement of the wedge Wm, which is propor-
tional to the distance to the target, is measured on a scale
which is graduated in terms of ranges. For a comprehensive
description of this method of range finding and of instru^
ments of this type see TM 4-250.
■ 49. Description. — a. Self-contained range finders are de-
signed for determining both ranges and azimuths at the same
time, although in practice they are not always so used, (See
par. 16.) They are usually provided with an auxiliary sight
at which a tracker is stationed to assist the observer in track-
ing the target. There are three principal parts to a self-
contained instrument, the range finder containing the optical
system and the measuring mechanism, the base containing
the tracking and leveling mechanism, and the tripod or
mount. The methods of indicating azimuth and range are
similar to those previously described for other instruments.
b. Instruments of different characteristics, some of the coin-
cidence type and some of the stereoscopic type, are in service.
The principal characteristics which differ are the base line
lengths (from 9 to 30 feet), the power (from 15 to 30), and
the maximum range which the instrument is capable of de-
termining. The stereoscopic height finder Ml originally
designed for antiaircraft aTtillery has been adopted as the
standard self-contained range finder for seacoast artillery.
65
49-50
COAST ARTILLERY FIELD MANTJAL
PiGUBE 28. — 9-foot range finder < cotncidence type).
■ 50. Adjustment. — The exact method to be used to adjust
each particular instrument is described in a pamphlet or
handbook furnished with the instrument. The following de-
scriptions of methods apply generally to instruments of the
type mentioned or to the particular instrument mentioned
in each case. The adjustments required are the leveling of
the instrument; the setting of the interpupillary distance for
the observer's eyepieces (stereoscopic type only) : the focusing
of the eyepieces; the collimation of the tracker's telescope
(if provided) with the observer's line of vision; the halving
adjustment for the coincidence type or the height of image
adjustment for the stereoscopic type; and the range
adjustment.
a. Leveling. — This operation is performed as previously
described for other Instruments.
h. inierpTipillary distance. — The interpupillary distance is
the distance between the pupils of the observer's eyes. The
eyepiece should be set at this distance. It is particularly
important that this setting be made accurately or stereo-
scopic observation will be Inaccurate. Each observer should
determine his proper setting and should habitually make this
setting as soon as he arrives at the instrument.
c. Focusing of eyepieces. — ^This operation is performed as
previously described for other Instnmients.
d. ColUmaiion of tracker's telescope.— This operation con-
sists of making the line of sight of the tracker's telescope
converge at the desired range with that of the observer.
It is done by pointing the instnmient, by means of the ob-
server's telescope, at a point of suitable range and adjusting
the tracker's telescope to intersect that point.
e. (1) Halving adjustment. — This adjustment is required
when the two Images formed by the rays of light entering the
two ends of the coincidence range finder from the target are
not refiected along the same horizontal line. When this is
66
FIRE CONTHOL AND POSITION FINDING
50
''j^rA/mt/jTm/wm
/v/? AO/i/sr/m
-^AA^ /jmCArm
■j^fA5i/R/mmo3
A5r/6/^mf/?
,M/^mf /CAT/ON if MS
5CAlf
fv^Ao/t/jr/m
nm/oAf m^fciAAfPs
PfAflA AM/5r/m /^/S7
^AfTAodjfcr/yim^
cou/mm/? f^/s/^
Figure 29. — 15-foot range finder (coincidence type).
67
50 COAST ARTILLERY FIELD MANUAL
the case a portion of the target may not be visible (deficiency
halving error, fig, 26 ® ) , or the same portion of the target
may be seen in both the upper and lower half of the field of
view (duplication halving error, flg. 26 ®), Except for the
distorted view of the target, these errors are not serious if
the portion of the target upon which coincidence is being
obtained is perpendicular to the dividing line of the field of
view. H it is not perpendicular an appreciable error will be
introduced. In ® and ® » figure 26, if coincidence had been
obtained on the right edge of the fiag instead of on the staff
the results would have been quite different. The adjustment
is made by means of a prism in one end of the instrument.
By moving this prism, the observer moves the rays of light
passing through It vertically until that half of the image
formed by these rays is refiected along the same horizontal
line as the rays forming the other half of the image.
(2) Height of image adjustment. — The height of image ad-
justment of a stereoscopic range finder corresponds to the
halving adjustment of the coincidence range finder. H in a
stereoscopic instrument one image is higher or lower than
the other, the observer will undergo considerable strain in
fusing them into a single object, and stereoscopic observation
will be hampered. If the images are considerably separated
he will be unable to fuse them at all, and stereoscopic observa-
tion will be Impossible. The adjustment is made in the same
way as for the halving adjustment.
/. Range adjustment. — (1) For a coincidence type instru-
ment (fig. 29) , this adjustment is made preferably by use of
a datum point at known range. The instrument is set to reg-
ister the range to the datum point. If the partial images
do not coincide, the correction wedge dial is carefully moved
until coincidence of the partial Images is properly defined.
The coincidence adjustment should be checked at two or more
known ranges, one of which should be at the longest range
at which good definition can be obtained. This adjustment
should be made with great care preferably under favorable
weather conditions. When there are no objects at known
ranges from the instrument or when the adjustment must be
made at night and there are no fixed lights at known ranges,
the moon or a bright star may be used as an infinite point for
68
i
FIRE CONTROL AND POSITION FINDING 50
obtaining coincidence. The astlgmatizer is employed in
making this adjustment, causing the star or moon to appear
as a streak of light. Proceed as for an object at known range,
revolve the measuring knob until the measuring scale registers
infinity, and then rotate the correction wedge until the lower
and upper halves of the streak of light are in coincidence.
(2) For a stereoscopic height finder Ml (par, 49), the ad-
justment is made preferably by an internal adjuster. This
adjuster consists of a special optical system which refiects
rays of light from a single light source within tlie instrument
into the main penta prisms of the range finder. These rays
of light are parallel and therefore the source of light appears
to be at infinity. The range scale is set to read infinity, and
the observer makes stereoscopic contact with the internal ad-
juster target and reticle symbol using the correction knob
instead of the measuring knob. The range scale does not
move but remains set at infinity. The range corrector setting
is read from Its scale. At least ten readings are taken and
the average determined for the correct setting. The range
corrector setting may be determined in the same manner by
making streoscopic contact when sighting on a celestial body
at night or on an adjusting lath furnished with the instru-
ment. The lath consists of a simple bar on the ends of which
are mounted two identical targets separated from each other
by exactly the same distance as the length between the win-
dows of the range finder. When the lath is set up parallel
to the tube of the range finder the lines of sight from the ends
of the tube to the corresponding targets are parallel, and the
observer sees a single target at an apparent range of Infinity.
In using the lath it is necessary to introduoe an additional
lens in each telescope to permit focus on a near object since
the instrument is normally focused at infinity. Another
means of making the adjustment Is by use of a datum point
at known range. The known range is set and a series of
stereoscopic contacts is made as previously described,
(3) Pbr a self-contained instrument, the range adjustment
varies with the observer, the visibility, the temperature, and
the condition of the instrument. If any of these factors
changes a new range adjustment should be made* There-
sa
4
i
50-61 COAST ARTILLERY FIELD MANUAL
fare, the adjustment should he checked at frequent inter-
vals--^t least once each hour during operation.
■ 51, Operation. — a. A self-contained range finder is oper-
ated by from one to four men. The 9-foot coincidence range
finder (fig. 28) Is operated normally by one man» an observer,
^^ who observes with the right eye in one eyepiece and reads
^P ranges with the left eye In another eyepiece. An instrument
of the type shown In figure 29 is operated by two men, an
observer and a trainer. The trainer identifies the target by
using the finder, bringing the intersection of the finder cross
wires on the portion of the target most suitable for observa-
tion. Ranges are read from the inside range scale or from
the outside range indicator. The operation of a stereoscopic
range finder requires four men: a stereoscopic observer, a H
^■p ^^ lateral tracker, a vertical tracker, and a range reader.
^P J^ b. With any Instrument the observer receives the command
W assigning the target and tracks the target in range. When j^ ^
W , using a coincidence range finder he should select some part 1
I I of the target which will allow coincidence to be made on a \
I well-defined line perpendicular to the halving line; when ^
[ using a stereoscopic range finder he should place the reticule
I symbols near but not In contact with the Image of the target. ■
il At intervals indicated by the TI bell or buzzer (par. 16). J
I he takes an observation and stops tracking momentarily to J
^ read or permit the reader to read the range and transmit Ir j
It to the plotting room. The trackers on a stereoscopic ^ i
■ instrument assist the observer by tracking the target in
direction and in elevation,
c. The range-finder should be located where there will be
minimum interference with observation due to the smoke and
blast of firing. Since the accuracy of a stereoscopic Instru-
ment Is affected by rapid changes in temperature and by un-
equal changes in the temperature of different parts of the
tube, the Instrument should be so located that the tube is
not subjected to changes of sunshine and shade. If possible,
^^^ it should be located so that the observer will not be forced to
observe through heat waves reflected from such surfaces as
sand and concrete. The observer should not be required to
70
FIRE CONTROL AND POSITION FINDING
51
observe over long periods of time. Fifteen minutes should
be the maximum duration of any one observing period; the
accuracy of observations extended over longer periods is likely
to be greatly reduced due to eyestrain and fatigue.
d. The target Is tracked in azimuth by a separate observa-
tion instrument placed near the range finder. The operation
of tracking in azimuth is described in paragraph 42.
CHAPTER 8
REFERENCE NUMBERS
■ 52. General. — Reference numbers are used extensively in
all position finding systems to expedite the transmission of
data and minimize the possibility of mistakes. They are
arbitrary numbers employed to represent actual value3 of
units of measure used in seacoast artillery firing. The ref-
erence number representing zero units is called the **normal/'
and is so selected in the series as to avoid the use of plus
or minus, up or down, and right or left. There are several
systems of reference numbers that are used in seacoast artil-
^lery firing. Examples are given In paragraphs 53 to 56,
inclusive.
■ 53. Deflection. — Deflection for aU seacoast artillery ex- k '
ri cept those 155-mm guns which have not yet been converted ft
I is measured in degrees and hundredths. (See note, par. 12.) l
i The original system was from approximately l.OO to 5.00
ft with 3.00 as the normal. In this system a defiection of .
If left 1.20" is represented by a reference number of 1.80, and I
1 a defiection of right 120^ by a reference number of 4.20. T
1 Due to the increased range of modern cannon and to the Jr
higher speeds of targets, with the consequent possibility of f J
larger defiectlons, this system was found to be Inadequate.
■ There are two new systems in use, one with a normal of
6.00 and the other with a normal of 10.00. Defiection for
155-mm guns not yet converted to degrees is measured in
mils; the normal of the scale is 300. All instruments on
which defiections are set are provided with a scale of ref-
erence numbers. The interior scales of azimuth instruments
(par. 42) are graduated in reference numbers for defiection
but with the scale reversed so that deviations to the left
read greater than normal. With this arrangement the ref-
^" erence number of the deviation is the same as that of the
defiection to be set on the gun sight in order to correct for
that deviation.
72
FIRE CONTROL AND POSITION FINDING
54r-56
i
■ 54. Angular Travel. — On the plotting board, M1904 (par.
67) , angular travel is measured in degrees and hundredths.
The reference numbers have a normal of 15.00. In this sys-
tem a rate of angiUar travel of left 1.35 "* is represented by a
reference number of 13.65, and a rate of angular travel of
right 1.35^ by a reference number of 16,35. These reference
numbers are used also on the travel computing mechanism
of the deflection board, M1905. The reference niimbers on
the travel computing mechanisms of the deflection board Ml
are in degrees with a normal of 6.00.
■ 55. Wind. — The speed of the wind is measured in miles
per hour. The reference numbers have a normal of 50.
A tail wind (tending to assist the projectile in flight) and
a wind blowing the projectile to the left are represented by a |
reference number greater than 50; a head wind (tending
to retard the flight of the projectile) and a wind blowing
the projectile to the right, by a reference number less than
50. This system is used on the wind component indicator,
on the range correction board, and on the deflection board.
■ 56. Range Deviations and Corrections. — a. Range deviations
and corrections usually are determined in terms of percent-
age of the range with a unit of one- tenth of one percent.
The reference numbers for this element of data have a normal
of 300. In this system, a deviation of over 1.5 percent or
a correction of plus 1.5 percent is represented by a reference
number of 315 (300+15) , and a deviation of short 1.5 percent
or a correction of minus 1.5 percent by a reference number
of 285 (300—15).
l>. On the correction slides (for range) on the M1904,
M1906, M1906M1, and M1911 plotting boards (pars. 67, 70,
and 71) the normal of the scales is 2,000, corrections being
set in yardSt
73
CHAPTER 9
PLOTTING BOARD ACCESSORIES
Paragraphs
Sectiou X, Prediction scale 57-58
It. Set-forward rule 59-61
in. Set-forward chart 62-63
IV. Set-forward scales 64-65
V. Targ 66
Section I
1
^ PREDICTION SCALE
■ 57. Description.— The prediction scale (fig. 30) is an in-
strument Issued for use In measuring the rate of linear travel
of the target and, with a set-forward rule or chart (see pars.
59 to 63, Incl.), in locating the set-forward point on a
plotting board. The instrument consists of a straight piece
of metal with both edges beveled. Each beveled edge is
graduated uniformly from the center outward In both direc-
tions with zero at the center. The graduations are so ar-
ranged that the scale may be read from either side. Two
small knobs are provided lor handling. A prediction scale
is graduated to correspond with the scale of the plotting
board with which it is to be used.
ai
■ 68. Operation.— In locating the set-forward point —
a. The zero graduation is placed at the last plotted point
and the distance between the last two plotted points is
measured.
:ing J
ittTT" i' :'4':?'*a;''i"T'Ti^T'nn"'T'T^'T m ' i ' i-^joo
1
O P^tPicnoN ecAL.£ Ci 1
^.j::x4X^i.AU.u:i^i^^^^
1
Figure 30. — Prediction scale.
b. The distance plotted is the linear travel of the target dur-
ing the dead time plus the time of flight of the projectile.
74
FIRE CONTROL AND POSITION FINDING
59
Section II
SET-PORWARD RULE
■ 59. Description. — a. The set -forward rule (hg. 31) is for
use with the prediction scale to determine the location of the
set 'forward point. It solves the equation
y=^it+D)
where
y = travel during time of fiight plus dead time
jTf—r&tG of travel (yards/seconds)
f = time of fiight
D = dead time
b. the mechanical construction of the rule is the same aa
that of an engineer's slide rule. The body is divided by an
undercut slot in which a slide travels. Mathematically, the
rule is a logarithmic slide rule. Values of X are represented
on the lower scale, which Is a logarithmic scale of values
from 50 yards to 700 yards with log 50 as the origin. Values
of the factor ■ „ ^ - are represented on the scale on the
M
slide. This is a logarithmic scale for values of the factor
when D=60 seconds, M— 60 seconds, and t Is varied from 20
to 75 seconds. The scale, then, covers the values of the factor
from 1.333 to 2.250 with log 1.333 as the origin. The scale Is
marked in terms of time of flight for ease in use. Values of
Y are represented on the upper scale, which is a logarithmic
scale for values from 100 to 1,300 yards with log 100 as the
origin. All scales are graduated to the scale of 1 inch=0.1216
logarithmic units (approximately). This rule is satisfactory
for a dead time of 60 seconds and for an observing interval
which is any submultlple of 60 seconds. A new scale for the
factor — j^ must be constructed for any other combination
of observing Interval and dead time. The rule shown pro-
vides for a maximum target speed of slightly over 20 knots
(travel =700 yards per minute) . Fbr use when firing at mod-
ern high-speed targets a rule should be constructed to pro-
vide for speeds up to 45 knots (travel =1,500 yards per minute)
or greater.
75
59
COAST ARTILLERY FIELD MANUAL
76
FIHE CONTROL AND POSITION FINDING
60-61
■ 60. Operation. — In operating the set -forward rule, the time
of flight and the travel during 1 minute must be furnished
the operator. Since the time of flight depends on the re-
sult of the operation in which it is used, its true value can
be obtained only after a series of approximations. In actual
practice, the time of flight to the last (preceding) set-forward
point is usually sufficiently accurate for this purpose. The
operator of the rule notes the range to the last set-forward
point as called out by the plotter and uses the time of flight
corresponding to that range. The travel during 1 minute
is called out by the plotter. The operator of the rule sets
the index (arrow) opposite the correct value of the yards
per minute travel, on the lower scale. Opposite the proper
time of flight he reads the yards travel during the time of
flight plus 1 minute, from the upper scale. The plotter then
locates the set-forward point at that distance ahead of the
last plotted point and on the expected path of the target.
Note. — The use of this rule requires that the time of flight for
the particular range be known. This conversion of range to time
of flight may be accomplished directly by graduating the time of
flight scale on the rule tn terms of range for the particular gun
being used.
■ 61. Notes on Constritction. — a. Although only a new scale
for the factor
it^D)
M
Is necessary for a new combination
of observing interval and dead time, some advantage will
be gained by constructing a complete rule. First, the scale
of the rule may be enlarged to give more space for grad-
uating the rule. Secondly, the X scale and the — — —
M
scale may be interchanged to speed up the operation of the
rule. This will allow the operator to keep the index on the
slide set at the time of flight to the set-forward point. The
slide is then in position so that all X values are under the
corresponding Y values for that particular time of flight,
and the operator may read the travel to the set-forward
point as soon as the plotter calls out the rate of travel. The
following notes will be of assistance in the construction of
a set-forward rule:
(1) Use material that is insensitive to temperature
changes.
241701^-
-e
77
61-62 COAST ARTILLERY FIELD MANUAL
(2> Use a scale of about 1 Inch =0.05 of a logarithmic unit.
(3) Use five-place logarithms in calculations for construct-
ing the scales and calculate the scaled distances to two
decimal places.
(4> Plot graduations for every 10 yards on the X and V
scales.
(5> Plot graduations for each second of time of flight
(within the selected limits) on the — j^ — scale.
(6) Locate all scales centrally on the rule. The position
of the setting index on the slide may be determined as follows;
Set the X scale under the V scale for any ratio, such as
y:X=2:l. Place the index opposite the graduation on the
— j^ — scale representing that value of the time of flight that
makes the factor i^4^ equal to 2.0.
M
(7) Label the rule clearly with the values of D and M for
which it is constructed.
(8) For details on the construction of logarithmic scales
see appendix IT.
b. In selecting the value of M for use in measuring the rate
of travel, it is necessary that it be equal to or some multiple
of the observing interval. The usual practice is to take M
equal to two observing intervals^ This procedure will gen-
erally give smoother readings of the travel than when only
one observing interval is used.
Section III
SET-FORWARD CHART
■ 62, Description. — The set -forward chart is a device for
solving the equation ^^^^
in chart form. It is designed to replace the set -forward rule.
The chart consists of a tabulation of the values of Y for all
values of X and t within the selected limits, with D and M
as constants. In each row the value of i is held constant
and X is increased by 10-yard increments from left to right.
In each column the value of X is held constant and t is in-
78
79
62-64
COAST ARTILLERY FIELD MANITAL
ci'eased by 1-second increments from top to bottom. The
values of y are shown to the nearest 5 yards. The values of
t are shown in the left-hand column. The values of X are
shown on a T-square which rides on the board. The ranges
corresponding to the times of flight for a particular combina-
tion of gun» projectile, and powder charge may be shown in
another column on the board. Figure 32 shows a portion of
a set- forward chart. The values used in the formula for con-
structing the chart shown in the figure are —
,.j X=m to 260 yards
' i=3 to 54 seconds
"" £)=40 seconds
M=40 seconds
Note. — ^The ch&rt shown provides for a maximum target speed of
about 12 V^ knots (400 yards per minute). For use when firing at
modern high-speed targets the chart should be extended to provide
for speeds up to 45 knots (1,600 yards per minute) or higher.
■ 63. Operations.— a. In operating the set-forward chart the
time of flight (or range) and the rate of travel must be fur-
nished the operator. The operator sets his T-square at the
time of flight (or range) of the last set -forward point. Oppo-
site the rate of travel as called out by the plotter, he reads
the yards' travel to the set-forward point to the plotter.
The plotter then locates the set- forward point as before.
5. There is little choice between the rule and the chart.
A rule may be constructed that can be read as easily as a
charts making the possibility of errors of operation equal.
The principal source of error in the use of either instrument is
the possibility of error in transmisson between the plotter
and the operator of the chart or rule.
Section IV
SET-FORWARD SCALES
■ 64. Description. — To obviate the necessity for a prediction
scale and a set-forward rule (or chart) and to enable the
plotting of the set-forward" point by a single operation, a
series of scales is sometimes constructed. These are called
80
FIRE CONTROL AND POSITION FINDING
64
*' set -forward scales." (See fig. 33.) Each scale of the series
has on it two sets of graduations with a common zero near
the center. The set of graduations in one direction is drawn
20-SECOND DEAD TIME
^"Travel setfobward^i^t
TIME OP RANde
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42
NOTE: ON EACH SCALE, TABULATE RANGE
CORRESPONDING TO TIME OF FLIGHT.
Figure SS.'—^et'forward scales.
81
64-65 COAST ARTILLERY FIELD MANUAL
in reference numbers to any convenient scale and represents
travel of the target (X). The set in the other direction is to
a scale
M
times as large and represents the values of Y for correspond-
ing values of X. A separate scale must be constructed for
each value of t. Usually each scale is designed to be used
for a range zone about 2,000 yards deep, that is, for a par-
ticular series of ranges covering about 2,000 yards — from
5,000 to 7,000 yards, for example. The time of flight for
the middle of the range zone — for 6,000 yards if we are con-
sidering the range zone extending from 5,000 to 7,000 yards —
is assumed to be sufficiently accurate for all ranges in the
zone. One set of scales is equally suitable for all scales of
the plotting board but is accurate only for the particular
values of D and M used in their construction. A series Is
therefore required for each particular combination of D and
M used. Set-forward scales cannot be conveniently or accu-
rately used for mortars or for guns when firing at high angles
of fire. Such scales are not articles of issue and when desired
must be constructed locally. If made of paper they should,
for convenient use, be cut up and pasted on metal strips and
placed in a portable rack so that they are readily accessible
to the plotter. ^-^ pr J^
■ 65. Operation. — a. The plotter selects the proper scale for
the current range. He places the zero of the scale at the last
plotted point and measures the distance to the preceding
plott^ point on the X scale. He marks the set-forward
point the same number of graduations ahead of the zero
mark on the Y scale.
b. Since this method of locating the set-forward point
requires the service of only one man, no transmission of data
is necessary. This removes the possibility of errors in trans-
mission, speeds up the operation, and lessens the noise in
the plotting room. On the other hand, it introduces a new
source of error because of the possibility that the plotter
may select the wrong scale for the current range.
82
FIRE CONTROL AND POSITION FlNPlNG
ee
Section V
TARG
■ 66, Description and Operation, — The targ is an intrument
used to mark the successive positions of the target and to
assist in the reading of azimuths and ranges on the board.
It Is wedge-shaped with a pushpin arrangement at the apex
for Indenting the paper covering the plotting board.
83
CHAPTER 10
PLOTTING BOARDS
Paragraphs
SEcnoN- I. Plotting board. Ml 904 (Whistler-Hearn) 67-69
n. Mortar plotting boards 70-71
m. 110* plotting boards, M1915. Ml 918. and M3 73^74
IV- Plotting and relocating boards. Ml 923 (Cloke),
and Ml 75^77
Section I
PLOTTING BOARD, M1904 C WHISTLER-HE ARN)
■ 67. Description^ — The plotting board » M1904 (fig. 34) , is an
older type board for use with fixed seacoast artillery guns and
howitzers. It provides means for locating the target by
either the two-station (horizontal base) or the single-station
(vertical or self-contained base) method, for locating the set-
forward points for relocating the set- forward point in range
i and azimuth from the directing point when using case in
^^ pointing, and for determining the rate of angular travel of
"-^ the target from the directing point when using case H
I ^^K pointing.
I 1^^ "^^ board proper is a wooden drawing board, slightly more
than a semicircle, mounted on two trestles to place it at
a convenient height for use. A piece of drawing paper may
be mounted on the board for use in tracking a target.
A base line arm is placed along the rear edge of the board
parallel to the diameter of the circle. The positions of the
observation stations are represented on that diameter by
station blocks placed on the base line arm. The primary
station is at the center of the circle. "Hie secondary station
block may be moved along the base line arm. It is placed
on that arm at its proper distance to the right or left of the
primary station by means of scales on the base line arm, A
vernier on the station block permits setting the length of the
base line to the nearest yard.
The position of the directing point is represented by the
point about which the gun arm pivots in the gun arm cen-
84
^
FIRE CONTROL AND POSITION FINDING
67
ter. It is located with respect to the primary station by
offsets, one along the base line and the other perpendicular
to the base line. Those offsets are set off to scale by means
of the lateral and the longitudinal adjusting slides attached
to the base line arm.
85
67 COAST ARTILLERY FIELD MANUAL
Metal arms are pivoted at the positions of the two ob-
servation stations and the directing point to represent the
lines of sight from those points to the target. These arms
are referred to as the primary, secondary, and gun arms.
Their reading edges are graduated in yards of range to the
scale of the board.
The normal scale of the board is 300 yards to the inch,
but gun arms graduated to a scale of 450 yards to the Inch
have been issued to the majority of major caliber batteries
to Increase the field of fire for which the board may be used.
The scale of the station arms must conform to that of the
gun arm used.
The outer edge of the board forms an arc of a circle about
the primary station as a center. This circle is called the
"main azimuth, circle." Notches are cut at 1* intervals
along the circumference of this arc. An azimuth scale is
inserted in the main azimuth circle with graduations to
cover the azimuths in the semicircle to the seaward of the
base line for the selected set-up. The limiting azimuths are
the azimuth and back azimuth from the primary to the
secondary station to the nearest full degree. Verniers grad-
uated in hundredths of a degree are provided at each end
of the base line arm by which that arm may be set on one
end to the exact azimuth and on the other end to the exact
back azimuth of the base line from B^ to B^.
The primary arm is set to the azimuths sent in from the
primary station by means of the main azimuth circle and
an index box attached to its outer end. The index box pro-
vides means for locking the arm in the notches of the main
azimuth circle and for setting off hundredths of a degree.
Since the secondary arm is pivoted at some point other
than the center of the main azimuth circle, an auxiliary arm
and a coupler are provided in order that the same azimuth
circle may be used for setting azimuths from the secondary
station. The auxiliary arm is pivoted above the primary
arm at the primary station. The coupler is a piece of metal
equal in length to the base line, that is, to the exact distance
between the centers of the primary and secondary station
block pivots. It connects the outer ends of the secondary
and auxiliary arms by pivots about which the two arms
86
FIRE CONTROL AND POSITION FINDING 67-68
turn as they are moved In azimuth across the board. The
coupler remains parallel to the base line, and the secondary
arm remains parallel to the auxiliary arm. An Index box
is placed on the auxiliary arm, and that arm is set to the
azimuths sent In from the secondary stations. Since the
secondary arm is held parallel to the auxiliary arm it will
also be set at the azimuths sent in from the secondary
station. ~
To allow intersection of the primary and secondary armis
at extreme ranges on the board without interference of
the primary arm index box, the primary arm Is offset at its
outer end. (See fig. 34.) The azimuth scale is therefore
a double scale with an outer row of graduations for the
primary arm and an inner row for the secondary arm.
The degrees of azimuth of the gun arm are read from
the gun arm azimuth circle on the gun arm center. The
hundredths of a degree are read from the gun arm azimuth
subdial which is geared to the gun arm center.
Angular travel is read in reference numbers on the tally
dial and the tally subdial, degrees on the former and hun-
dredths of a degree on the latter. The dial and subdial are
actuated by movement of the gun arm.
The gun arm center is provided with a correction slide
and an azimuth correction scale on which flat corrections
In range and azimuth may be applied to the uncorrected range
and azimuth of the set-forward point. These devices are
graduated in reference numbers, with the normal of the
former 2,000 and of the latter 15.
The center line of the board is the radius that divides the
board into halves. It passes through the center notch in
the main azimuth circle and the center of the primary arm
pivot, and is perpendicular to the base line arm when the
base line verniers are set at zero,
■ 68. Orientation'. — a. Base line arm and main azimtUh
circle, — (1) The first problem is to determine the proper
degree readings that should be inserted along the circle. As
stated before^ those readings are determined by the azimuth
and back azimuth of the base line from the primary station,
the nearest full degree being taken. For example, assume
87
'k
68 COAST ARTILLERY FIELD MANUAL
that the board is to be oriented for a left-handed base line
the azimuth of which is 212.14* (zero south) from B^ to B^
The azimuth circle should then read azimuths from 212''
to 32% with 212" on the left end of the base line arm. If
the base line were right-handed and the azimuUi 212.80*,
the azimuth circle should read azimuths from 33° on the
left to 213" on the right end of the base line arm* If the
main azimuth circle is properly oriented, the center line of
the board will coincide with the perpendicular to the base
line to the nearest full degree. This azimuth scale is usually
marked by Ordnance Department before the board is issued
to the using battery.
(2) The next operation is to set the base line arm to the
exact hundredth of a degree of the azimuth. In the first ex-
ample, the base line arm should be rotated clockwise through
0.14°. In the second example it should be rotated counter-
clockwise through 0.20*. To prevent errors, both verniers
should be used and both ends of the base line arm should be
set accurately.
b. Secondary station, — Set the station block of the sec-
ondary station at the distance <to scale) from the primary
station equal to the length of the base line, to the right for
a right-handed base line and to the left for a left-handed
base line. The approximate position of the station block is
marked by a countersunk recess In the board.
c. Gun arm center. — The gun arm azimuth circle on the
gun arm center must be oriented to read the aaimuth of the
gun arm, and the gun arm center must be moved to the posi-
tion of the directing point. To orient the gun arm azimuth
circle, bring the gun arm center over the primary station by
setting the zeros of the longitudinal and lateral adjusting
slide verniers to the zeros of their respective scales. Set the
azimuth correction scale to normal by bringing the zero of the
worm gear to 15 on the scale and setting the scale of hun-
dredths on the micrometer head to zero. Set the primary
arm to the azimuth of the center line of the board (with the
Index disk on the primary arm at zero). Place the targ
against the reading edge of the primary arm and bring the
gun arm against the targ. The gun arm is now pointing at
the same azimuth as the primary arm. By means of the
sa
I
FIRE CONTROt AND POSITION FINDING 68-69
adjusting screw, set the azimuth pointer at the gun arm
window to the whole degree of azimuth of the center line of
the board. If necessary move the markings of the gun arm
azimuth circle until this can be done. Next set the gun arm
azimuth subdial indicator to zero. The indicator may be
adjusted to the nearest ^4° by loosening the screw holding
it in place. Finer adjustment must be made by loosening the
azimuth subdial retaining screw and moving the subdial until
the pointer is at zero.
Note, — The tally subdial must be removed to allow access to the
azimuth subdial retaining screw.
The final setting should be verified by bringing the gun
arm against the targ several times and checking the readings.
The gun arm center should now be moved to the position on
the board corresponding to the position of the directing point.
This is done by setting the proper values of the offsets from
the primary station on the longitudinal and lateral adjusting
scales. i J^ \
H 69. Operation. — The number of men necessary for the
operation of the plotting board, Ml 904, and the operations
performed on the board depend upon the method of locat-
ing the target (two-station or single-station), the method of
pointing (case II or case III) , and the method of predicting
(by prediction scale and set- forward device or by set-forward
scale) . The operation is most complete when using the two-
station method with case II pointing and the prediction scale
and set-forward device. Therefore, operation under those
conditions will be discussed first, followed by a discussion of
the differences introduced by the other methods mentioned
above.
a. Horizontal base, case //. — Five men are necessary for the
operation of the plotting board, M1904, when using the hori-
zontal base system with case II pointing and the prediction
scale and set-forward rule or chart — the plotter, the primary
arm setter, the secondary arm setter, the angular travel device
(tally dial and tally subdial) operator, and the set-forward
rule (or chart) operator.
(1) Tracking, — Each arm setter wears a telephone head set
connected to the reader at the corresponding observation
station. He receives the azimuth of the target as called out
89
69
COAST ARTILLERY FIELD MANUAL
by the reader at the sounding of the TI signal and sets his
station arm to that azimuth^ repeating the azimuth back to
the reader. When his arm is properly set he calls set, (To
reduce confusion in the plotting room» It is often preferable
not to have the arm setter repeat the azimuth but to have
the reader repeat the hxmdredths of a degree of azimuth.)
When both arm setters have called set, the plotter places
the targ accurately at the intersection of the station arms
and marks on the plotting board the position of the plotted
point. He then calls or signals clear, upon which the arm set-
ters move their arms away from that part of the board to
give the plotter space in which to work. This operation is
repeated for each plotted point.
(2) Determination of rate of anffular travel of target. —
When the first plotted point is located, an arm setter swings
the gxm arm up to the edge of the targ at that point and
holds It in this position while the angular travel device oper-
a tor sets the tally dial and subdial at normal, (At this time
the plotter announces the approximate range and the angular
travel device operator announces the approximate azimuth
to the target from the directing point for use in adjusting aU
instruments and guns to the approximate position for oper-
ation.) When the next plotted point is located and the gun
arm swung against the targ held at this point, the angular
travel device operator calls out the angular travel reference
numbers from the tally dial and subdial for use on the deflec-
tion board and again sets the two dials at normal. This op-
eration is repeated for each plotted point and follows immedi-
ately after the plotting. Angular travel may be taken be-
tween set- forward points, but the results are less accurate
due to possible errors in prediction.
<3) Location of $et- forward point.— After at least two and
preferably three plotted points are located, a set -forward
point may be located. The plotter estimates the expected
course of the target and places the edge of the prediction
scale along that line with the zero at the last plotted point.
He calls out to the set-forward rule operator the travel of
the target during 1 minute (that is, travel during 1 minute
when yards travel in 1 minute is the basis used to constructs
the rule; see pars, 59 to 61, incL) as is indicated by the plotted
1
90
FIRE CONTROL AND POSITION FINDING
points. The set -forward rule operator calls out the travel
diH-ing time of flight plus 1 minute. The plotter then marks
with the targ the position of the set -forward point along
the edge of the prediction scale at the proper distance ahead
of the last plotted point. This operation is repeated for
each set-forward point and follows immediately after the
determination of the rate of angular travel.
(4) Relocation.—'Wlien the first set-forward point is lo-
cated, the gun arm is swung against the targ held at that
point and the plotter reads the uncorrected range to the
set-forward point from the range scale on the gun arm.
He then places a small triangle (A) around the point as a
distinguishing mark to avoid confusion with later plotted
points. ITiis operation is repeated for each set-forward
point and follows immediately after the location of the
set-forward point.
&. Vertical and self-contained bases, case II. — ^A change to
vertical or self-contained base makes a change in the opera-
J tion of tracking only. All other operations and their se-
quence are unchanged. The number of men may be reduced
■ by one since only one arm setter is necessary. ITie variations
j in procedure are as follows:
The arm setter receives both azimuth and range fl-om the
reader. He sets his station arm in azimuth, calls set, and
repeats the range to the plotter. The plotter places his targ
at that range along the station arm, marks the position of
the plotted point, and calls clear. ITie arm setter then
moves his station arm away as before.
i c. Case III pointing. — When using case IH pointing the
azimuth is used in place of defiection, therefore the rate of
angular travel is not necessary. ITie operation of determin-
ing the uncorrected azimuth to the set-forward point is per-
formed immediately after the determination of the uncor-
rected range. The number of men is unchanged,
<1) Determination of uncorrected azimuth. — At the same
time that the plotter is determining the uncorrected range
to the set-forward point, the angular travel device operator
notes the uncorrected azimuth as indicated on the gun arm
azimuth scale and subdial. He calls out the azimuth imme-
diately after the plotter calls out the range.
1
91
6&^71
COAST ARTILLERY FIELD MANUAL
(2) Location of predicted point. — ^If when using case III
pointing it is desired to locate the predicted point in addition
to the set -forward point, the plotter marks the predicted
point with the targ as he calls out the travel during the
dead time to the set -forward rule operator. He then pro-
ceeds with the location of the set-forward point as before.
After the uncorrected firing data for the Set-forward point
have been determined, the plotter moves the targ to the
predicted point and swings the gun arm against the targ.
The angular travel device operator then reads the azimuth
of the predicted point from the gun arm azimuth scale and
subdiaL
d. Predicting by set-forvxird scales. — When predicting by
set -forward scales, the set- forward rule Cor chart) and its
operator are no longer necessary. In this case the plotter
locates the set- forward point as described In paragraph 65
and proceeds to the determination of the uncorrected firing
data. The predicted point, if desired, may then be located
by sliding the set- forward scale along the expected path of
the target until the graduation that was opposite the next
to last plotted point Is opposite the last plotted point, and
marking the position of the predicted point opposite the
zero of the scale.
Section H
MORTAR PLOTmSTG BOARDS
m
■ 70. Mortar Plotting Boards, Ml 906 and Ml 906 ML^These
boards are similar to the plotting board. Ml 904, for guns:
They are designed particularly for case in pointing which
is the method of pointing mortars. The principal change
consists of installing a more accurate azimuth indicating
device on the mortar arm center. The adjustment, orienta-
tion, and operation of these boards are the same as for the
gun plotting board described in paragraphs 67 to 69, inclusive,
■ 71. Mortar Plotting Board, M1911 C36D<*). — As its name
implies, the form of this board is a full circle. It is made
in two semicircular sections. The center of the board is
taken to represent that base end station which is farther
from the directing point of the battery. In former models of
92
FIRE CONTROL AND POSITION FINDING 71-72
plotting boards the primary station was represented at the
center of the board. In the Ml 911 board, under ordinary
conditions, the secondary station will be the center of the
board. The board is designed to be used with any azimuth
of base line; any location of the outer station between 800
(1,000 for boards Nos. 1 and 2) and 8,000 yards to the right
or left of the center station; and any location of gun between ■
800 (1,000 for boards Nos. 1 and 2) and 8,000 yards to the
right or left of the center station and less than 2,100 yards
to the front or rear of the base line. The mortar arm is
provided with a sliding range scale by means of which flat
range corrections of plus or minus 500 yards may be made
from the normal position which is marked 2,000. The center,
outer, and mortar arms are graduated for ranges between 300
and 16,000 yards. A detailed description of the board and
its operation may be found in Ordnance Department publica- ^ ^
tions, copies of which are issued to batteries equipped withj
boards of this type. m
Section in
~i 110^ PLOTTING BOARDS, M1915, M1918, AND M3
■ 72. Description. — These boards are for use with all types
of fixed seacoast artillery cannon. They provide means for \
locating the target by either the two-station or the single -
station method, for locating and relocating the set-forward
point in range and azimuth, and for determining the rate j
of angular travel when case II pointing is being used. The ^
llO"* plotting board. Ml 9 18, is practically identical with \
the M1915 board except that the former is slightly larger
and covers longer ranges. The M3 board is similar to the
other two, but is a board made for a particular battery and
has an azimuth circle and a plotting surface covering the
whole seaward field of fire of that battery. Only the Ml 9 15
board (fig. 35) will be discussed in detail.
The board Is similar In principle to the Whistler-Hearn
type of plotting board. It Is designed to accommodate the
increased ranges of modern cannon and to Increase the
accuracy of mechanical operation in plotting and relocation.
The board proper is a wooden board supported on a frame
on four wrought-iron pipe legs with provisions for approxl-
241701«--40 7 93
72
COAST ARTILLERY FIELD MANUAL
94
FIRE CONTROL AND POSITION FINDING 72
mate leveling. An azimuth circle of about 150'' of arc is
provided on the periphery of the frame.
On this board the directing point is at the center of the
azimuth circle and the gun arm is pivoted in the gun center
bracket above this point. Vertical pivoting is also provided
to facilitate handling the gun arm when operating the board.
A circular bronze plate called the "station plate" is placed
on the board with its center at the center of the azimuth
circle. The primary and secondary stations are represented
by sleeves placed in the station plate and located In azimuth
and distance from the directing point. On this board, the
azimuth covered by the azimuth circle are not limited by the
azimuth of the base line. Therefore, the azimuth circle may
be marked to cover any segment of the circle about the direct-
ing point. The azimuth represented by the center line of the
board may be used as a reference line in locating the station
positions. Positions for auxiliary and emergency stations of
the battery may also be included on the board.
The station arms are pivoted in sockets in the sleeves and
are furnished with couplers equal in length to the displace-
ment of the stations from the directing point. The couplers
are attached to Index boxes riding along the azimuth circle
and keep the station arms at all times parallel to imaginary
auxiliary arms pivoted at the directing point. This arrange-
ment permits the use of the same azimuth scale for all three
arms and eliminates the use of a gun arm center, a source of
mechanical error on the Whistler-Hearn board.
In order to cover 360^ of azimuth, four positions of each
observation station are provided in the station plate, and four
rows of azimuth scales differing by 90 *" are marked on the azi-
muth circle. The quadrants are lettered A, B, C, and D; all
sf;ation positions and the azimuth scale for the same quad-
rant are marked with the same letter. The principles in-
volved in this arrangement are illustrated in figure 36. In
this figure the normal set-up of the board is shown at the left.
This is the set-up for quadrant A in which the center line
of the board represents 180" azimuth (from south). The
positions of the station sleeves for this quadrant are shown
at B' (A) and B' (A).' The set-up for which the center line
of the board represents 90* azimuth is shown at the right.
95
72
COAST ARTILLERY FIELD MANUAL
If now the figure at the right is revolved clockwise through
90° and placed on the figure at the left, the station sleeves
will fall at the positions B' (B) and B' (B). By pivoting the
station arms in the latter positions and by decreasing the
readings of the azimuth circle by 90°, the board will duplicate
the set-up for quadrant B. The same provisions may be made
for quadrants C and D. i
When one or more observation stations are located so close
to the directing point that the station sleeves cannot be pro-
vided without mechanical interference, all four positions for
each station so located are put on a center station plug which
fits into a square bushing In the station plate. An arrow
on the plug pointing to the quadrant letter on the station
plate Indicates the quadrant position being used- To change
quadrants, remove the plug and replace it In the desired
position.
FiQU^g 36. — Arrangement of quadrant positions on 110** plotting
board.
Azimuths of the station arms are read by means of the
index boxes attached to the couplers. Degrees of azimuth
are Indicated on the azimuth scale; hundredths of a degree
are indicated on a subscale on the index box. Two subscales
are provided on each index box, one engraved and filled in
in black, the other in red. The black subscale is the normal
one for use and indicates the azimuth of the imaginary aux-
iliary arm and, therefore, of the station arm. The red sub-
scale is for use when the target is in such a position that the
station arm or coupler covers the black subscale. It indicates
96
FIRE CONTROL AND POSITION FINDING 72-74
an azimuth 3" greater or less than that of the arm, according
to whether it is to the right or to the left of the black sub-
scale looking from the oenter of the board. The gun arm
is provided with a single subscale. The least reading on
the subscales is 0.05*'. There are four holes in the gun arm
Index immediately over the four azimuth scales on the
azimuth circle. The azimuth scale being used is indicated
by placing the index pin in the proper hole.
An auxiliary azimuth circle, graduated in degrees, is riveted
to each coupler in such position that when oriented the bev-
eled edge of the station arm will indicate on the auxiliary
circle the azimuth of that arm to the nearest degree.
■ 73. Orientation. — The station sleeves are placed and the
azimuth scales oriented by Ordnance Department personnel
before the board Is issued to the using battery. The only
orientation necessary at the battery is selection of the proper
station arms with their couplers, determination of the quad-
rant most suitable for the position of the target, placing of
the station arms in the proper sleeves for that quadrant, and
insertion of the index pin in its proper hole In the gun arm
index.
■ 74. Operation. — This board has no special device for the
determination of angular travel, other plotting room devices
having been adapted to the use of azimuths of successive
plotted (or set-forward) points in detenninlng the angular
travel of the target for case n pointing. Therefore, the op-
eration of the board for case II pointing and the operation
for case in pointing are identical and both are similar to
that described for the Whistler-Hearn board when using cas^
ni pointing. (See par. 69.) For horizontal base tracking
with prediction scale and set-forward device, the personnel
consists of the plotter, two arm setters, and a set-forward
device operator. The azimuth of the set-forward point is
read by one of the arm setters. For vertical or for self-con-
tained base tracking or for prediction by set-forward scales,
the variations in procedure are the same as for the Whistler-
Hearn board. (See par. 69& and d.)
h
75 COAST ARTILLERY FIELD MANUAL
Section IV
PLOTTING AND RELOCATING BOARDS, M1923 (CLOKE)
AND Ml
■ 75. Description. — a. The plotting and relocating board,
M1923 (see fig. 37) » is for use with all types of mobile seacoast
artillery. It provides means for performing the same oper-
ations as does the 110 '^ board.
b. The 110° board is entirely satisfactory for cannon on
fixed emplacements but has no provisions for readily changing
the position finding set-up. Since the set-up of a mobile
battery is different for each position occupied, a suitable
plotting board for mobile seacoast artillery must provide means
for readily setting up and as readily changing any selected
arrangement of observation stations, directing point, and field
of fire within wide limits. Thie Cloke board provides these
means together with increased accuracy and range over the
Whistler -Hear n board. The Cloke board also provides means
for relocation of the set-forward point from any point in the
vicinity of the directing point.
c. The Cloke board is wooden with an azimuth circle of
188'' of arc along the periphery. The azimuth scale is marked
on metal strips that fit in a slot in the azimuth circle and
may be readily removed and replaced. Markings for both mils
and degrees are provided.
; d. Two arms, one referred to as the plotting arm and the
other the relocating arm, are pivoted at the center of the
azimuth circle. The arms are set in azimuths by means of
subscales at their outer ends similar to those on the 110^
board. The least readings of the subscales are 1 mil and
0.05\ Each arm Is provided with range graduations on Its
reading edge and with four sets of removable range scales so
marked that the scale of the board may be made any one
of the following: 300, 600 ,750, or 1,500 yards to the inch.
e. The base line is represented on the platen, a movable
plate pivoted to a slide which fits over the plotting arm. The
platen pivot remains coincident with the reading edge of
the plotting arm and coincides with the center of the azimuth
circle when the slide is pulled against its stop at the inner
end of the plotting arm. A clamp on the platen, when
98
^
FIRE CONTROL AND POSITION FINDING
75
99
75 CXJAST ARTILLERY FIELD MANUAL
tightened, prevents rotation of the platen about its pivot.
One observation station is represented by a push button at
the platen pivot. The other station is represented by a
push button that is placed in the master key. The master
key is carried on a slide on the platen and may be moved
toward or away from the platen pivot. There Is no mechani-
cal connection between the platen and the relocating arm.
/. The directing point is represented by the gun push but-
ton which is fastened to the platen by a double slide, allow-
ing movement of the gun push button either parallel to or
perpendicular to the platen. When it is desired to relocate
the set-forward point from more than one point — for in-
stance, from each gun of the battery — a gun plate may be
attached to the platen and the positions of the guns may be
located on this plate. Figures 37 and 39 show both the gun
push button and the gUn plate in position. Ordinarily only
one of these parts is attached when the board is used.
g. The Cloke board solves mechanically the same mathe-
matical problem that the Whistler-Hearn or the 110* board
does and uses the same observation data in its solution, but
its method of solution differs considerably. On the Whistler-
r Hearn board the azimuths are set from the center of the
circle outward and the position of the target moves about the
fixed base line; on the Cloke board the azimuths are set
from the circle Inward to the center, and the base line, mov-
ing parallel to itself, moves about the fixed position of the
target at the center of the circle. Figure 38 Shows the rela-
tion between the two boards for a typical set-up. The arrows
Indicate the direction in which azimuths are set on the two
boards. It may be seen from the figure that the arms of
the Cloke board correspond to extensions of the arms of
I the Whistler -Hearn board. With these extensions the arms
could be set at the azimuths received from the observation
stations as readily by the lower half of the azimuth circle.
if properly marked, as by the upper half. When the target is
tracked on the Whistler -Hearn board the angles at B' and B'
between the station arms and the base line change, but the
position of the base line with respect to the azimuth circle
remains fixed. The same condition applies on the Cloke
board; provision is made for changing the angles at B' and
100
^
FIRE CONTROL AND POSITION FINDING
75
B' while the base line is maintained parallel to Its original
position. When orienting the board an orienting position
for the platen is established. Before each TI bell, the platen
is brought to this orienting position and the platen clamp is
loosened to allow movement of the plotting arm and the
platen slide without rotation of the platen. After the plot-
ting arm is set to the new azimuth, the platen clamp is
tightened and the platen is slid along the arm to its position
for plotting.
~ fOO*
^ 7i. On both boards the position of the target is indicated
by the intersection of the arms. On the Whistler-Heam
board that position is plotted, and its relative motion about
the fixed directing point is used to <ietermine the firing data.
On the Cloke board the movement of the directing point
is plotted, and its relative motion about the target Is used
to determine the firing data. Since the same target and
the same directing point are involved, the relative motion
is the same on both boards; if the target is moving clockwise
on the Whistler-Hearn board, the directing point on the
Cloke board will also move clockwise about the target.
Therefore, the two sets of firing data will be identical.
101
75 COAST ARTIIIERY FIELD MANUAL
i. The method of determining the proper position along
the plotting arm at which to stop the platen, when plotting*
depends on the method of target location used, two-station
or single-station. In the two- station method, the arms are
set to the azimuths received from their respective stations,
and the platen is moved out until the master key touches
the relocating ami; in the single -station method, the plot-
ting arm is set to the azimuth received from the observation
102
FIRE CONTROL AND POSITION FINDING 75—76
station, and the platen is moved out until the range is
Indicated on that arm by the index on the platen slide.
j\ The plotting and relocating board Ml (fig, 39) is simi-
lar in construction and operation to the Cloke board. The
principal differences are in the construction of the azimuth
scale and the base line stop. The azimuth scale is in de-
grees only, and the readings are marked on an endless chain
which can be adjusted by turning a handwheel to put any
desired azimuth reading, from 0" to 360% at the center of
the arc; the arc subtended by the azimuth circle is about
120^. The base line stop has been redesigned to facilitate
orientation. There are four sets of scales for the plotting
and relocating arms as follows;
Maximum range
Scale (yards per inch ) (yards )
200 12,800
400 25, 600
800 51,200 ^
1,000 64.000 ^j
■ 76. Orientation, — a. Base line and azimuth circle. — ^Prom"
figure 38 it may be seen that in order to duplicate on the
Cloke board the triangle shown on the Whls tier-Hear n board
the two base lines must be parallel. The same relation may
be shown when the base line does not pass through the
center of the azimuth circle, for example, on the 110° board.
It follows from this that the operation of orienting consists
of the same problem on the Cloke board as on all other
plotting boards, namely, placing the base line in its proper
angular relation with respect to any selected radius of the
azimuth circle. There is, however, an additional step re-
quired in orienting the Cloke board; that is. the establish-
ment of an orienting position at which the platen may be
readily reoriented during plotting.
The first consideration is the selection of the azimuths to
be covered by the board. The usual procedure is to put the
azimuth of the center of the field of fire at or near the cen-
ter line of the board. This may be done by sliding the metal
strips of the a^muth scale around the circle until the desired
azimuth marking is at the center of the arc. In case it is
desired to set up the board for the simultaneous use of
103
76 COAST ARTILLERY FIELD MANUAL
mils and degrees there is one precaution to be observed. At
the reference line (zero) of any azimuth circle and at each
multiple of 9^ of azimuth the full mil and degree grad-
uations coincide. On the Cloke board those points are in-
dicated on the ■ azimuth circle by longer lines that are at
other points. Azimuths that are multiples of 9^ should be
set at the longer lines.
The next step is the selection of positions to represent the
observation stations* On the Cloke board the positions of
the observation stations, such as B^ and B^, and of G are
represented on the platen as previously explained. The
board may be set up for operation with the platen and the
relocating arm on either side of the plotting arm; either
station may be located at the platen pivot. As a general
rule the more convenient arrangement is to put at the platen
pivot the station more distant from the directing point.
There are, however, practical considerations which occa-
sionally dictate the choice of positions. Sometimes the po-
sition finding set-up is such that the mechanical construc-
tion of the board limits the choice of positions in orienting.
If the orientation is prevented by mechanical interference,
reverse the platen and station positions and reorient. A
little experience in the use of the board will be of assistance
in selecting the arrangement most convenient for orienting
and plotting.
(1) Ordinary orientation (by azimuth and length of base
line), — ^Because of its flexibility the Cloke board may be
oriented in several ways. The ordinary method of orienta-
tion ts by use of the azimuth and length of the base line.
This method should be used whenever possible because it
is more accurate than either of the other methods. The
method is illustrated in figure 40. In that set-up, the platen
pivot has been selected to represent B^ and the master key
to represent B\ The azimuth of the base line is 107^. and
its length is 6,000 yards. The procedure is as follows:
(a) Release the platen clamp and slide the platen along
the plotting arm until the slide touches its stop at the inner
end of the plotting arm. This brings the platen pivot over
the center of the aximuth circle.
Note. — The azimuth at which the plotting^ arm is set is
Immaterial.
104
FIRE CONTROL AND POSITION FINDING
76
(b) Set the relocating arm at the azimuth of the base
line and clamp it. According to the general rule, the azi-
muth set should be that from the outside to the center of
the circle, or from the station represented by the master
key to the station represented by the platen pivot — in this
case from B^ to B".
(c) By means of the relocating arm, set the master key
at a distance from the platen pivot equal to the length of
the base line.
Note. — This may be done either before or after the relocating
arm is set at the azimuth of the base line.
(d) Holding the platen slide against its stop, swing the
platen about its pivot until the edge of the master key is
against the reading edge of the relocating arm and tighten
the platen clamp. The platen is now oriented.
(e) With the platen slide still against its stop, bring the
base line stop against the edge of the platen and clamp the
base line stop. This establishes the orienting position for
the platen for use in plotting.
(2) Orientation by datum point — This method is of par-
p ticular value in orienting a board for an emergency set-up
in Which the azimuth and length of the emergency base line
are not known. By this method the platen may be oriented
for any two observation stations from each of which the
azimuth and range to a point in the field of fire are known.
This method also is illustrated in figure 40. The procedure
is as follows:
(a) Set and damp each arm at the azimuth from the
station it represents to the datiun point.
(b) Release the platen clamp and the slide holding the
master key and slide the platen along the plotting arm until
the index on the slide is set at the range from the station
represented by the platen pivot (in this case B") to the datum
point.
(c) Holding the platen slide at that range, swing the platen
about its pivot and move the master key along the platen
until the master key touches the relocating arm at the ranee
from the station it represents (in this case B^) to the datum
point. Clamp the platen to the slide and the master key
to the platen. The platen is now oriented.
\
I
105
76
COAST ARTILLERY FIELD MANUAL
(d) Slide the platen along the plotting arm until the slide
touches its stop at the center of the circle; bring the base
line stop against the edge of the platen and clamp the base
line stop. This establishes the orienting postion for use in
plotting.
Figure: 40.^ — Orientation of Cloke board, ordinary and datum point
methods.
106
FIRE CONTROL AND POSITION FINDING 76
(3) Orientation by eQUilateral triangle. — This method may
be used when, for ease in plotting, it is desired to represent a
particular station at the platen pivot, and the azimuth from
the station represented by the master key to the station
represented by the platen pivot cannot be set on the board.
For instance, if in the situation shown in figure 40, B ' were
at the platen pivot and B " at the master key, the azimuth for
use in the ordinary method of orientation would be 287.00^.
This azimuth is not included in the semicircle on the board.
This method is illustrated in figure 41. This is the original
.set-up with the platen and the relocating arm on the opposite
side of the plotting arm» which places B ' at the platen pivot
and B ^ at the master key. The azimuth of the perpendicular
to the base line Is 107.00°— 90.00° =17.00*. The procedure
is as follows: *^^_
(a) Set and clamp the arms as follows: l^^l
f ^^^ i- The arm on the right to the azimuth of the perpen-
^ J^H dicular ^30
jC^B 2. The arm on the left to the azimuth of the perpen^jp-
C^* dicular -i-30^
r (b) Release the platen clamp and the slide holding the
I master key and slide the platen along the plotting arm \mtil
1 th
1 «
the index on the slide is set at the range equal to the length
of the base line.
(c) Holding the platen slide at that range, swing the platen
about its pivot and move the master key along the platen
until the master key touches the relocating arm at the range
equal to the length of the base line. Clamp the platen to
the slide and the master key to the platen. The platen is
now oriented.
(d) Slide the platen along the plotting arm until the slide
touches its stop at the center of the circle; bring the base line
stop against the edge of the platen and clamp the base line
stop.
&. Directing point. — After the base line has been oriented,
the position of the directing point must be located. The ori-
entation of the directing point consists^of placing it in its
proper relation to the observation stations; that Is, placing
107
76
COAST ARTILLERY FIELD MAITOAL
the gun push button on the platen in its proper relation to
the platen pivot and the master key. If the azimuth and
distance from the directing point to the station represented
by the platen pivot are known and if that azimuth can be
set on the azimuth circle, the position of the gun push button
may be found in the same manner as that of the master
key in orciinary orientation. (See ail) above.) When this
4700'
{NORNWL PLUS 30*)
REUCKXUUQ
ARM
34700*
(NORfML MrNUS 30*1
O DEGREES (SOUTH)
1700-
(NORMAL TO BASE UNE)
PiGUBE 41. — Orientation ot Cloke board, equilateral triangle method.
108
FIRE CONTROL AND POSITION FINDING 76-77
method is lised, the platen must be held in its orienting posi-
tion against the slide stop and the base line stop. This
method may be used regardless of the method used in orient-
ing the base line. An alternative method of orientation is
the datum point method described in a(2) above, using the
azimuth and range from the directing point to the datum
point. When the gun plate is used and the location of all
guns of the battery is desired, their positions on the gun
plate may be located in the same manner as that described
for the location of the gun push button.
c. Orientation for single-station position finding. — ^When
the single-station method of position finding is used, put that
station at the platen pivot and G at the gun push button and
proceed as for orientation of a base line. (See a above.)
■ 77. Operation. — Since on this board the rate of angular
travel is determined by azimuths as on the 110* board,
the operation of the board is the same for case H and for
case in pointing. It is similar to that described for the
Whistler-Hearn board when using case IH pointing <see par.
69) except in the operation of tracking. For horizontal base
tracking with prediction scale and set-forward device, the
personnel consists of a plotter, two arm setters, a platen oper-
ator, and a set-forward device operator. The azimuth and
range to the set-forward point are indicated by use of the re-
locating arm. For prediction by set-forward scales, the vari-
ations in procedure are the same as for the Whistler-Hearn
board (par. 69d).
a. Horizontal base tracking. — ^Each arm setter sets his sta-
tion arm to the azimuth received from the reader and calls
SET. While the plotting arm is being set, the platen operator
keeps the platen at the orienting position against the slide
stop and the base line stop with the platers clamp loosened.
When both arm setters have called set, the platen operator
tightens the platen clamp and slides the platen along the
plotting arm until the master key touches the relocating
arm. He may start this operation as soon as the plotting
arm is set, but he may not complete it until the relocating
arm is set. The plotter marks the position of the plotted
point on the board by pressing the gun push button and calls
241701^^0 8 109
77 COAST ARTILLERY FIELD MANUAL
CLEAR. The platen operator releases the platen clamp and
withdraws the platen to the orienting position. This pro-
cedure is repeated for each plotted point.
b. Vertical and self-contained bases. — ^A change to vertical
or self-contained base makes a change in tracking only. All
other operations and their sequence are unchanged. The
number of men may be reduced by one since only one arm
setter is necessary. The variations in procedure are as
follows :
The arm setter sets the plotting arm to the azimuth re-
ceived from the reader, calls set, and repeats the range to the
platen operator. The platen operator slides the oriented
platen out until that range is indicated on the plotting arm
by the index on the platen slide and calls set. The plotter
then marks the position of the plotted point as before.
c. Relocation for more than one jxAnt^ — When using the
Whistler -Hearn or the 110^ board, the set-forward point may
be relocated with respect to the directing point only. If the
guns are so widely separated that displacement corrections are
necessary, difference charts (see pars. 26 to 37, incl.) must be
used. The Cloke board, however, permits the relocation of
the set-forward point from all guns of the battery by the use
of the gun plate and a special method called "offset plotting."
The gun plate Is attached to the platen and the positions of
the guns marked on the gun plate as explained in paragraph
16b. To avoid predicting for each separate gun position the
procedure is altered slightly. The plotted points are marked
by the push button at the platen pivot instead of by the
gun push button, and predictions are made for that station
instead of for the directing point. The oriented platen is
then moved out until the platen pivot is over its set-forward
point, and the set-forward point for each gun is Indicated by
the gun positions* on the gun plate. The variations in the pro-
cedure are as follows:
When the platen operator calls set, the plotter marks the
position of the platen pivot on the board. After he locates
the set-forward point he places the targ at that point and
the arm setter brings the plotting arm against the targ. The
plotter then slides the rider (shown in fig. 37) along the
plotting arm until the finder on the rider touches the targ,
110
[
I.
FIRE CONTROL AND POSITION FINDING
77
and clamps the rider to the plotting arm. The platen op-
erator, having withdrawn the platen to its orienting position,
tightens the platen clamp and brings the oriented platen
against the rider. (This places the platen pivot over its set-
forward point) The plotter places the targ at each gun posi-
tion in turn and the arm setter brings the relocating arm
against the targ. The plotter then reads the range and the
arm setter reads the azimuth to each set -forward point.
I
CHAPTER 11
RANGE CORRECTION DEVICES
Paragraphs
Section I. General 10-19
II, Wind component indicator 80-82
m. Range correction board Ml 83^6
IV, Other models of range correction boards 87-90
V, Percentage corrector Ml 91-92
Section I
* GENERAL ^ _
■ 78. F'UNCTiONs, — The functions of range correction devices
are to provide means for determining the range corrections
due to nonstandard ballistic conditions, to apply those cor-
rections to the range to the set-forward point, apply range
adjustment corrections as a result of observation of fire, and
to transform the corrected range into suitable data for point-
ing the guns in elevation.
The necessity for ballistic corrections was discussed in para-
graph 23, The corrections are determined by a range cor-
rection board supplemented by the wind component indicator.
This board presents data ftom the firing tables in graphical
form and in such manner that the algebraic sum of the cor-
rections may be determined readily and as readily changed
to meet the changing conditions which arise.
The adjustment corrections are determined after observa-
tion of fire and after operation of the devices comprising the
spotting system of the battery. The necessity for these cor-
rections and an explanation of the spotting devices are dis-
cussed in chapter 13. The adjustment correction is deter-
mined in the same units as the ballistic correction, that is,
percentage of the range.
The ballistic and adjustment corrections are applied to
the range to the set-forward point on a percentage cor-
rector, which also transforms the corrected range into
suitable firing data when necessary. There are two occa-
sions when transformation is necessary. The more frequent
112
FIRE CONTROL AND POSITION FINDING 78-79
occasion is when the pointing device on the gun is marked
In angiUar units (either mils or degrees) instead of range.
The other occasion is when the pointing device is marked
in units of range but the range elevation relation used in
marking is different from the range elevation relation that
should be used for the particular combination of gun, powder
charge, and projectile. » ^^^^^^^^^
■ 79- Meteorological Message. — a. Description. — (1) In-
formation as to variations from standard of atmospheric
conditions are contained in the meteorological message.
Data contained in this message are determined by the per-
sonnel of the meteorological station and supplied to the using
battery hourly during any period when firing is expected.
The meteorological message consists of groups of symbols
arranged in codified form. The message starts with the code
designation of the sending station, repeated, consisting of
three letters the first of which is always the letter M; the
other two are the identifying letters of the station. This is
followed by several number groups. The first number group
has five digits and the remaining groups have seven digits
each. The five-digit group has the following significance:
The first digit is either the figure 2 or the figure 3 denoting
the type of the message. The figure 2 denotes that the
message is of the type suitable for high angle fire; the figure
3, that it is of the type suitable for low angle fire. The sec-
ond and third digits of the group give, in hundreds of feet,
the altitude of the meteorological datum plane (m. d. p.)
above sea level. The fourth and fifth digits give the tem-
perature at the m. d. p. in degrees Fahrenheit. The seven-
digit groups are similar in type and significance except that
each refers to a particular altitude above m. d. p. The first
digit of each group designates that altitude zone. The sec-
ond and third digits indicate the direction from which the
ballistic wind is blowing in hundreds of mils clockwise from
north. The fourth and fifth digits give the speed of the
ballistic wind in miles per hour. The last two digits give
the ballistic density in percent of normal.
113
79
COAST ARTILLERY FIELD MANUAL
(2) The following Ls a typical meteorological message:
MFMMFM
30278
0241699
1231799
2221899
3211800
4211900
5202001
6202001
7192102
8192102
9192102
0182103
1182203
The message may be translated as follows:
Meteorological message from station PM for low angle
'fire.
Altitude of m. d. p. — 200 feet above sea level.
Temperature at m. d- p- — 78" F. i
AUitudo 7.0 nti
(surface)
l.._
2 -
:i
4
5
n
7^
S
9
10
11
Upper limil
of attEtndtt
zone in ft'el
COO
1,500
:i.ooo
a 000
9. mi
12.0*30
J 5, 000
18,000
;jo,ooo
Direction from
whieb balJistie
wind is blowing
in mils elock-
\visfi from north
2,400
2.300
2,200
2.100
2,100
2.000
2,000
1,900
1,900
1,900
1,800
1.800
Speftd of
ballistic
wind in
ra. p. h.
-11
Density in /
percent of
normal
09
I>9
90
100
100
101
10]
J02
102
102
103
114
FIRE CONTROL AND POSITION FINIUNC 79
b. Application. — -(1) The data taken from a meteorological
message for a selected firing will be that contained in the
five-digit group and one of the seven-digit groups. The
seven-digit groups contain data as to the ballistic wind and
the ballistic density. The ballistic wind is a fictitious wind,
constant in magnitude and direction, which would have the
same total effect on the projectile during its fiight as the
true winds of varying magnitude and direction that are
actually encountered. It is computed from observations
taken on the true winds at different altitudes above the
m. d. p. Likewise, the ballistic density is a fictitious constant
density which would have the same total effect as the various
true densities and is computed from observations and
formulas. Each seven-digit group contains the data for the
ballistic wind and ballistic density for one altitude zone only.
The particular seven- dig it group appropriate for use is that
group of which the altitude above the m. d. p. is nearest to
and not less than the maximum ordinate of the trajectory,
for the range to the target, as measured from the battery level.
When the battery and the m. d. p. are not at the same altitude
above sea level, the temperature and the ballistic density
must be corrected for the difference in altitude. The data
concerning the ballistic wind are taken without change.
Formulas for the correction of temperature and ballistic
density may be found in all firing tables. The data from
the meteorological message are used, part on the wind com-
ponent indicator and part on the range correction board as
will be discussed later.
(2) The following example shows the application of a
meteorological message to a selected firing:
A battery of 12- inch guns. Ml 895. on barbette carriage,
M1917, using 975-pound projectile (Firing Tables 12-F-3) . is
firing at a target at a range of 18,400 yards. The altitude of
the battery is 20 feet above sea level. What data from the
meteorological message given in a above should be used?
What is the corrected data?
Solution: From part 2, table A, Firing Tables 12-F-3, the
maximum ordinate for a range of 18,400 yards is found to be
4,405 feet. Therefore, data for the fourth altitude zone
115
79-81 qOAST ARTILLERY FIELD MANUAL
< upper limit 4,600 feet) should be used. The complete data
from the message are —
Altitude m. d. p, — 200 feet.
Temperature at m. d. p. — 78 degrees F.
Ballistic wind — 2,100 mils from north, at 19 m. p. h.
Ballistic density — 100 percent.
The corrections for temperature and ballistic density should
be as follows:
Temperature-— an Increase of Vs^ for each hundred feet
decrease in altitude or y5*'X1.8=+0.36^
Ballistic density — an increase of 0.3 percent for each
hundred feet decrease in altitude or 0.3X1.8=4-0.64
percent.
The complete corrected data are — 'mj ^fS
Temperature — 78^ (nearest degree) . ^^w * _
Ballistic wind — 2,100 mils from north, at 19 m, p
Ballistic density — 101 percent (nearest percent)
Section II
WIND COMPONENT INDICATOR "
■ 80. Wind Correction Problem. — In making corrections for
the effect of the wind, the ballistic wind is resolved into two
components — one in the plane of fire, affecting range; and
the other perpendicular to the plane of fire, affecting di-
rection. The problem is illustrated by the sketch at the top
of figure 42 where a wind of magnitude GW is blowing toward
W at an angle WGR with the plane of fire. GR represents
the magnitude of the range component and RW the magni-
tude of the deflection component.
■ 81. Desceiption. — The wind component indicator (flg. 42)
is a device for mechanically resolving the ballistic wind into
Its range and deflection components. It consists of a circular
plate (P) surrounded by an azimuth circle, and an arm (A),
called the target arm, pivoted at the center and riding above
both. The plate is stationary; it is engraved with cross sec-
tion lines spaced in units of miles per hour but marked In
wind reference numbers with 60 as the normal. (See par,
66.) An index (K} at the bottom of the plate Is used to set
the wind azimuth. The azimuth circle is movable; it is en-
116
FIRE CONTROL AND POSITION FINDING
81
graved with two azimuth scales, an inner scale in mils and
an outer scale in degrees. The zeros of the two scales are
180' apart. This arrangement was chosen to permit the use
of wind azimuths in mils from north and target azimuths in
The target arm is en graved with a linear"
scale graduated in miles per hour. By means of this scale
tne setting end (H) of the pointer carried by the arm may be
set so that its reading end (H*) is at a distance from the
117
81-82 COAST ARTILLERY FIELD MANUAL
center of the circle equal to the speed of the ballistic wind.
If, after the speed of the ballistic wind is set on this scale,
the azimuth circle is set by means of the index iK) on the
plate and the mil scale, to the azimuth from which the ballis-
tic wind is blowing, and the target arm is set to the azimuth
of the set-forward point by means of the index (C) on the
arm and the degree scale, the situation shown in the sketch
at the top of figure 42 will be duplicated on the indicator ex-
cept that the triangle GWR will be turned over and the lines
GW and GR will have exchanged places. This arrangement
was chosen to permit reading the values of the effects from
the fixed plate. A single instrument serves for both range
and defiection computations. The range component may be
read from the vertical scale, and the deflection component
^from the horizontal scale, on the plate. It will be noted that
readings less than 50 indicate a wind retarding the projectile
or blowing it to the right, and that readings greater than 50
indicate a wind accelerating the projectile or blowing it to
the left.
■ 82. Orientation and Operation. — The operator sets the
pointer to the wind velocity and the azimuth circle to the
azimuth from which the wind is blowing, using the mil scale.
These settings are obtained from the meteorological message.
He keeps the target arm set to the uncorrected azimuth of
the set-forward point as cailed out by the plotter, using the
degree scale and the target arm index for the setting. The
range component reference number is used on the range
correction board and the defiection component reference
number on the defiection board.
Note, — ^When the firing battery is using azimuths of the target
TSrlth zero north, the instrument if operated normally will give
wind effects In the wrong directions. If the azimuth of the set-
forward point is in mils the correct effects will be obtained by
setting the index of the target arm on the mil scale. If the azimuth
is In degrees, the correct effect will be obtained by setting the wind
azimuth at the top of the plate instead of at {K) and setting
target azimuths as usual.
The wind component indicator is not issued to batteries
equipped with the defiection board Ml. In batteries so
equipped the wind reference numbers are read from the wind
component indicator on the defiection board.
118
FIRE CONTROL AND POSITION FINDING 83
Section III
RANGE CORRECTION BOARD Ml
■ 83. Descriptiott. — a. A range correction board (fig. 43) is
a mechanical device for determining the algebraic sum of the
range corrections due to prevailing nonstandard ballistic con-
ditions. This correction is called the ballistic correction.
The Ml board is typical and since it is the present standard
range correction board will be explained in detail.
h. The board consists of four major parts: a chart bearing
curves which indicate the individual corrections; a ruler
that makes the algebraic addition (see par. 14, appendix II) ;
a scale which indicates the ballistic correction; and a mount
for the whole. «.
(1) The chart has a set of curves for each nonstandard
condition for which correction is made. Each set consists
of a series of curves, one curve for the standard condition (the
normal of the set) and one for each unit of variation from
standard that it is desired to show. The curves are plotted by
rectangular coordinates with range as ordinates and range
effects in percent of the range as abscissas. The range and
other pertinent firing table data are listed along the sides of
the chart. The data for plotting are taken from the firing
tables. A chart must be constructed for each combination of
gun, powder charge, and projectile. Further details on the
construction of the chart may be found in appendix TI. The
Ml board utilizes charts for the type of armament being used.
These charts are mounted on rollers so that the desired chart
may be placed in position for use. The charts on this board
have curves for muzzle velocity, atmospheric density, height of
site (or tide), ballistic wind, weight of projectile, elasticity,
and rotation of the earth.
Note. — The effect of rotation of the earths in both range and
direction, varies with the latitude of the firing position, the azimuth
of the plane of fire^ and the elevation (or range). Since only two
variables may be shown on one set of curves and It was considered
impracticable to furnish sufficient curves for all situations in a
readily usable form, one of those variables had to be eliminated.
The variable causing the least changes in the effect is the latitude.
It was therefore decided to construct the curves for a mean latitude
of 30* for use within the United States. Each curve is plotted for
a selected azimuth of the target.
119
"i
^
83 COAST Artillery field manual
(2) The correction ruler consists of a strip of metal with
two raised bars extending across it. The upper bar is fixed
to the ruler; the lower bar is movable and may be slid across
the ruler in either direction. A system of gears actuated
by a knob is provided for sliding the movable bar. Mounted
on the two bars is a slide for each set of curves on the chart.
Each slide has a double -action clamp by which it may be
clamped to either of the two bars. When the clamp is moved
to the position M, the slide is clamped to and moves with
the movable bar; when the clamp is moved to the position S
the slide is clamped to the fixed bar and the movable bar
may be moved independently of the slide. The ruler when
mounted for operation is above the chart and parallel to the
range lines.
(3) The correction scale is engraved on the upper edge
of a plate attached to the ruler. It is graduated In reference
nximbers of percent of the range with 300 as normal. (See
par 56.) An index attached to the movable bar indicates
the correction. On the lower edge of the plate is an auxiliary
scale similarly graduated by which the plate may be moved
to offset the normal of the correction scale if desired. A
fixed index below the plate registers on the auxiliary scale.
Arbitrary corrections in terms of percent of the range may
J be added algebraically to the ballistic correction by these
means.
(4) The mount is a metal case that contains the charts
and rollers. The ruler is fixed to the top of the case
by clamps allowing a slight movement of the ruler for
adjustment.
c. When a slide is moved from its normal curve to the
intersection of the proper range line and the curve repre-
senting the nonstandard condition that prevails, the Index
on the movable bar is displaced in the same direction and
by the same amount. By setting each slide in turn, the
algebraic sum of the corrections is indicated on the correction
scale.
d. The board is designed for continuous operation through-
out the firing. As the range to the set-forward point changes,
the chart is moved to keep the proper range line under the
mler. Each slide may then be moved in turn to bring it
120
FIHE CONTROL AND POSITION FINDING
83-85
to the intersection of the curve with the new range line.
This operation changes the correction by the amount of
change due to the change in range and has the same effect as
though the slides were all brought back to normal and reset
at the proper curves. Changes in any nonstandard condition,
such as a change in the wind reference number due to a
change in the azimuth of the target, may be made in the
same way,
■ 84. Adjustment. — a. Tlie mechanical adjustments of the
board are the adjustment of the chart and the adjustment
of the correction ruler. The chart must be adjusted on its
rollers so that the normal lines on each set of curves will not
be displaced laterally as the chart is moved past the ruler.
The correction ruler must be placed parallel to the hori-
zontal range lines on the chart and clamped in that position.
These adjustments may be tested as follows:
Set the slides at their normal correction curves and move
the chart back and forth on the rollers. The normal curves,
which are straight lines, should remain imder the indices
of the slides for all positions of the chart.
&. A further adjustment must be made prior to the opera-
tion of the board. This adjustment consists of setting the
index oh the movable bar at the correction on the correction
scale that is the algebraic sum of the corrections indicated
by the slides. The easiest way to do this is to set all slides
at their normal curves and the index on the movable bar
at normal (300) on the correction scale. This setting will
be referred to hereafter as the initial setting. If it is desired
to use the auxiliary scale it also must be set at normal.
■ 85. Operation. — a. The operator of the board turns the
roller handle until the appropriate chart for the firing ap-
pears under the correction ruler. He adjusts the ruler and
tests the adjustment of the chart making any adjustments
found necessary. He makes the initial setting of the board
and turns all slide clamps to the position S on the slide.
He ascertains the proper data as to nonstandard conditions
and records with chalk in a convenient place in the space
provided near the top of the board the necessary data to
121
85 COAST ARTILLERY FIELD MANUAL
indicate the curves representing those nonstandard condi-
tions. He obtains these data as follows:
(1) Muzzle velocity, in foot-seconds, from the range officer.
(2) Atmospheric density, in percent of normal density,
from the meteorological message (corrected for difference
in elevation).
(3) Height of site or tide, in feet, from the range officer and
the tide station.
(4) Ballistic wind, in wind reference numbers, from the wind
component indicator.
(5) Weight of projectile, in pounds, from the range officer
(who gets the average weight from the battery executive).
(6) Elasticity, In degrees Fahrenhei
…[truncated]