TM 9-1220-238-14
scale. Readings to one-half mil can be set in a similar
(2) In the grid system the azimuth of a line such as
manner, using the Intermediate graduations on the
OT would be the angle d; that is, the horizontal angle
vernier scale.
measured in a clockwise direction from grid north. In
(3) When operating the slow motion vernier knob
for vernier scale readings, the brake lever must always
all drawn parallel.
be released, leaving the brake engaged, which is the
(3) Assuming that the scale straightedge could be
normal position induced by action of the spring against
swung in an arc of 1800 and could be moved back and
the lever. However, when the disk and frame assembly
forth across the plotting surface the azimuth of OT and
are rotated rapidly, for approximate setting of azimuth,
O'T could be easily laid off on the plotting surface.
the lever must be pressed in and held firmly to release
The azimuth of OT would be angle d' and that of O'T
the brake and allow freedom of rotation.
would be angle f.
d. Plotting and Scaling.
(4) The same results may be secured by using a
(1) It is first necessary to number the grids, but
plotting surface that can be turned through 360 or 6,400
before this can be accomplished, the set-up must be
mils, and a straightedge that is set in a fixed direction
visualized so that the points representing the
and can be moved back and forth across the plotting
observation posts can be plotted in proper relation to the
surface, remaining parallel to its original position.
expected target area The observation posts (OP) are
By keeping the straightedge fixed and rotating the
then plotted by means of the X and Y coordinates using
plotting surface, a more precise setting of the azimuth
a plotting scale. These posts are plotted so that they lie
can be made and the plotting can be done much more
between the vernier scale and the target area when the
quickly.
azimuth to the target area is set on the azimuth scale.
(5) The azimuth and range could be computed as
(2) The board is then ready for the azimuth from
was done in the test problem (g below), but the
the observation posts. As these are announced from
determination by means of the plotting board offers a
each OP for a single burst or flash, they are set off on
more rapid and an equally satisfactory solution
the azimuth scale and a group of intersecting lines
b. Orientation Press the brake lever to release the
drawn with the scale assembly through the respective
brake Rotate the grid disk until the graduated edge of
OP's in the direction of the target
the plotting scale coincides with the north grid line and
(3) The intersection of these plotted lines will mark
the zero on the azimuth scale coincides with the zero on
the position of the target as observed The X and Y
the vernier scale In order to get exact coincidence of the
coordinates of this point can be determined by referring
zeros, clamp the frame assembly by releasing the brake
to the grid origin and scaling the distances to each axis
lever and rotate the slow motion vernier knob until this
by means of a plotting scale The horizontal range from
condition Is effected. If this is not possible, the vernier
the target to any OP or gun position can be determined
scale or disk must be adjusted as described in d below
by scaling off the distance, using the scale provided The
c. Setting off Azimuth
azimuth of the target with respect of the gun position or
(1) When the azimuth is announced, it is set off by
directing OP can be determined by means of the
pressing the brake lever and rotating the frame
azimuth scale and vernier scale.
assembly until the zero graduation of the vernier is
(4) In actual practice, six sets of observations are
between the nearest 10-mil graduation and the next
plotted, if possible, and a mean point determined as
higher 10-mil reading on the azimuth scale; then the
explained in f and g below. Note Always use a 2H or
lever is released and the slow motion vernier knob
softer pencil for plotting. Erase with a soft rubber eraser
rotated until the numbered graduation on the vernier
containing no grit or abrasives. When possible, place a
scale corresponding to the last digit of the announced
sheet of vellum, secured by masking tape, over the
azimuth coincides with the nearest graduation on the
plotting disk and plot on vellum to preserve the surface
azimuth scale.
of the plotting disk.
(2) As an example, assume that the announced
e. Location of Hostile Batteries.
Location of
azimuth is 2,204 mils. Set the azimuth scale so that the
enemy batteries Is determined by plotting the azimuth of
zero on the vernier scale is opposite the space between
flashes from the enemy guns as observed from two or
the 2,200 and 2,210 graduations (in this case almost in
more observation posts Observed azimuths for a single
the center as the last digit is 4), then release the brake
flash are transmitted from the observation posts to the
lever and rotate the slow-motion knob until the
plotting center They are plotted as explained in d above,
graduation indicated by 4 on the vernier scale falls
and the X and Y coordinates of
opposite the nearest graduation on the azimuth is 2,208
it will be found necessary to rotate the knob until the 8
graduation on the vernier scale is opposite the 2,360
graduation on the azimuth
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