SPOTTING
“_Spotting_,” as distinct from mapping or from the photography of
continuous strips, is the photography of a definite individual
objective. In military work spotting or “pin pointing” includes the
photography of particular trenches or pivotal points in a trench system
before an attack (Fig. 123), of roads or bridges along which an advance
must pass (Fig. 124), of batteries or big guns which are the subject of
artillery fire (Fig. 125), both before and after their bombardment (Fig.
126), of gun puffs or exploding bombs (Fig. 131).
The technique of spotting consists largely in getting properly over the
target and then securing the exposure at just the right moment. This is
chiefly a question of proper piloting; but the aid which can be offered
to the pilot by camera auxiliaries designed particularly for spotting
needs is very large.
Discussion of the task of the pilot who must steer a photographic plane
accurately over a previously selected point of interest cannot be
undertaken without raising the question of who should take the picture,
pilot or observer? In the English service the most general practice was
for the pilot to be charged with the responsibility both of covering the
objective and of exposing. If a propeller drive was used on the camera,
this left to the observer only the task of changing magazines. If the
camera was hand operated the plates were changed either by the observer,
or else, as was frequently the case, distance operating devices were
attached, so that the pilot even then did everything except change the
magazines, and the observer was kept free to watch the sky for enemy
aircraft. A very desirable adjunct to the camera when plates are shifted
automatically or by the observer is a distance indicator, to show the
pilot when the shutter is set. Electrical indicators for this purpose
have been devised.
[Illustration:
FIG. 123.—Low view of trenches on the Yser, showing concrete
structures undamaged by bombardment.]
[Illustration:
FIG. 124.—The Piave River. To left, destroyed bridge, to right,
pontoon bridge under construction.
Italian aerial photographic service.]
[Illustration:
FIG. 125.—Showing big gun hidden in forest. (Upper left-hand corner).]
[Illustration:
FIG. 126.—Example of spotting. Battery before and after bombardment.]
If the camera is completely hand operated, as were most of those in the
French and German services, there is little choice but for the observer
to perform the entire operation. The exposing operation could have been
delegated to the pilot, but such was not the custom with the French or
with the American squadrons using French apparatus. In this method of
operation the observer depends on the pilot to get the plane over the
target, while the pilot depends on the observer to get the picture when
the target is covered. Ample opportunity is thus offered for
misunderstanding and disagreement. This can be avoided only by excellent
sights properly aligned, for both pilot and observer, and by some means
of communication between the two men concerned.
[Illustration:
FIG. 127.—Photograph, made with long focus lens to determine the
results of aerial bombing. The “Tirpitz” battery of long range naval
guns directed on Dunkirk.]
The simplest _means of communication_ is of course direct conversation.
But this is only possible in those planes, such as the DH-9, in which
pilot's and observer's cockpits are immediately together, so that, by
shouting, any desired information can be conveyed with fair ease. When
the distance is increased to four or five feet, as in the DH 4, the
loudest shouts are totally lost in the roar of the engine and the blast
of the wind. Speaking tubes and telephones are now fairly good, but are
none too comfortable or convenient to have strapped on one's head and
face. A primitive device used to some extent in the war was merely a
pair of reins attached to the pilot's arms, by which he could be
directed which way to steer. There is much to be said for a simple
semaphore system, where an indicator in the observer's cockpit actuates
a similar dial in front of the pilot, indicating “right” or “left,”
“picture obtained,” “try again,” etc. If the observer has a sight by
which he can see far enough ahead to correct the pilot's error of
pointing, the need for an accurate sight for the pilot is diminished.
[Illustration:
FIG. 128.—Diagram showing relationship between focal length and area
covered by plate.]
[Illustration:
FIG. 129.—Diagram giving data on area covered at various altitudes by
representative lens.]
[Illustration:
FIG. 130.—Burchall Slide Rule, for calculating intervals between
exposures, and for other aerial photographic data.]
In considering the question of sights, attention may again be called to
the poor “visibility” from the pilot's seat in the present prevailing
type of two-seater tractor plane. Blind directly in front, beneath, and
to either side (Figs. 7, 8 and 9), it is no unusual thing for a pilot to
entirely miss an objective, such as a railway line, which he can only
estimate to be beneath him by judging its distance from those objects to
either side which he can actually see. The English practice of leaving a
clear space of six inches to a foot between the fuselage and the
beginning of the wing fabric, allows the pilot to look down over the
side, a decided advantage. But for photographic purposes nothing can
compare with a good negative lens carrying fore and aft lines or wires,
so that the pilot can see his objective in ample time to head directly
for it. The lens should either be large enough so that its rear edge
gives the view directly downward, or supplemented by an additional lens
pointing directly down, so that the covering of the target is assured.
To locate such a lens in the front cockpit, free of all controls, is a
very hard task; even so its view is likely to be badly interrupted by
the landing gear. Nevertheless, so important is it, both in photography
and in bombing, to have a sight by which the plane can be accurately
directed that designers of planes should recognize this need and make
every effort to provide a suitable location.
[Illustration:
FIG. 131.—Aerial bombardment of Trieste. Note falling bombs in center
of picture; and exploding anti-aircraft shells over the water.
Italian official photograph.]
[Illustration:
FIG. 132.—Example of spotting requiring exposure at exact instant.
Explosion following burst of bomb in ammunition dump.
British official photograph.]
Sights for the observer have been discussed already. Here again the
negative lens is to be preferred, but while the pilot's lens needs only
directing lines in the axis of the plane (unless he takes the picture),
the observer's lens needs both an accurate center mark and an additional
upper or lower sighting point. Accurate alignment of these marks with
the camera axis must be arranged for in precise spotting.
Accurate spotting work requiring the delineation of fine detail calls
for cameras of considerable focal length. The camera of longest focal
length used in the war was the French 120 centimeter (Fig. 41). This was
employed with great success in such work as regulating the fire of heavy
railway guns brought into range only at night, to fire a few shots at
chosen angles. Photographs taken the next day would then show the exact
spot where each shell fell, and the damage it did, to serve as a guide
for the next night's operations (Fig. 127). The field of these cameras
is quite small—8 to 12 degrees—and so not only must sighting be exact
but the area covered on the ground must be accurately known. This is to
be calculated from the altitude, focal length, and plate size, by the
relation—
distance on ground altitude
—————————————————— = ————————————
plate length focal length
Data derived from such calculations may be incorporated in tables, or
graphically in diagrams such as Figs. 128 and 129.
[Illustration:
FIG. 133.—The same subject a few minutes later. Height of smoke shown
by shadow.
British official photograph.]
These calculations and others required in mapping and stereo-work are
simply and quickly made by slide-rule devices. One of these, the
Burchell Photographic Slide Rule, developed in the English service, is
shown in Fig. 130. This consists of two dials, the center one of which
is mounted—usually by a pin pushed into a cork behind—so as to turn
freely, to permit its being set for altitude, focal length, ground
speed, plate size, etc., whereupon the area covered, or the appropriate
interval between exposures may be read off.
Cameras for spotting work should be capable of exposure at the exact
moment desired. For if the camera is ever to catch the gun as it
discharges, the bomb as it falls (Fig. 131), or the shell as it explodes
(Fig. 132), the photograph must be taken within the instant. Automatic
cameras, exposing at regular intervals, while adequate for mapping, are
not fitted for many kinds of spotting.