July-August - Air Defense Artillery School

Improvised Depression Position
By Captain David J. Caldwell, Coast Artillery Corps
EDITOR'S NOTE: Tile method of obtaillillg apprCY.I:imate
range as describ~ ill tile folloll'illg article lIatHrally illl'olres
certaill assllmptiolls. Ullder some atmospheric mid
tide cOllditiollS, large errors ma)' be expect~.
The method herein described may be used as an emergency
range finding system to serve when the normal range
finding equipment of a battery is not being manned, or may
be used as an expedient when other range finding equipment
is not available. Frequent use may be made of this
device in equipping observation posts with a simple yet
dependable range finder, thus enhancing the value of intelligence
reported. The system is not adapted to stations
of low elevation, 150 feet above sea level being the practical
minimum. Tests bv the writer over a six months
period at a number of different installations, varying in
height from 220 feet to 850 feet, have proven its accuracy.
Use is made of either a 1910 Al or an M1918 azimuth
instrument. The 1910 A 1 instrument, being of heavier
construction and more stable, is preferable. The azimuth
instrument must have a very stable mounting so that it
will continuously remain level and be free from vibration.
Its height above sea level must be accurately determined.
The operation in brief is as follows: The instrument is
focused on the target and the target water-lined with the
horizontal cross wire. (The 1910 Al instrument has no
horizontal cross wire; however, the tops of the horizontal
scale graduations may be used as such). Care being taken
not to change the depression angle at which the target was
water-lined, the instrument is refocused on a previously
calibrated range pole located in the vicinity of instrument,
turning the instrument in azimuth if necessary, and the
range to the target read on the scale of the range pole with
the horizontal cross wire as an index.
Preparation and Calibration of the Range Pole: This
range finding instrument makes use of the law of similar
triangles. In figure 1, I represents the instrument at a
height b above sea level E. T represents the target, R the
horizontal range to the target and h the combined vertical
effect of curvature and refraction; r is the horizontal distance
(fixed) from the instrument to the range pole and h'
is the vertical distance on the range pole from a point level
b
E
h
r
---------...,
eveI
I
:h' I
R
Fig. I
n(in inches) = ( b+h )12 r
R
Where: b and h are in feel,
Rand r in yards.
T
LEVEL
HORIZON
RANGE
POLE
Fig.2
er
with the instrument when the telescope is horizontal to the
point on the range pole where the line of sight from I toj
the water-lined target cuts the range pole.
From the law of similar triangles we have the following
relations:
r R , .. (b + 11I 12 r
h' b + h or h (m mches) = R •
where r is expressed in yards, b in feet, II in feet and R in
yards.
. The value of h for anyone particular range may be OIr
tained from Table I, Appendix VI, Fi\l 4-10, or may be
obtained approximately by substituting in the formula II inI
feet equals .185 x R2 where R is expressed in thousands of]
yards.
pose.
Either method is sufficiently accurate for this pur-
Various values
formula and the
of R iue chosen for substitution in th~
corresponding value of h' determined 1'0
each range. The range pole is then calibrated using th
values of h' so determined and labeling with the correspond.
ing range. The distance to the horizon for the particula
instrument height involved (this is the case when b equal
h) should be determined by use of the table or formula f
vertical effect of curvature and refraction, and the value 0
h' determined so that a "range to horizon" mark may
placed on the range pole.
After the range pole has been calibrated it is placed a
the correct horizontal distance from the instrument (t
distance used in the calculation), and adjusted vcrticall
until, with the instrument water-lined on the horizon. t
line of sight through the instrument intersects the range po
200