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].
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.
CHAPTER XXVI
MAP MAKING
Technique of Negative Making.—Stated in its simplest terms, the whole problem of making a photographic map from the air consists in taking a large number of slightly overlapping negatives, all from the same altitude, with the plane flying uniformly level. When trimmed and mounted in juxtaposition, or pasted together so as to overlap in their common portions, the prints from these negatives constitute a complete pictorial map. There is thus furnished by a few hours' labor topographic information which would be the work of months to obtain by other means.
The making of map photographs involves all the special technique of spotting, with much in addition. The pilot's task is not merely to go over one object; he must navigate a narrow path, at a constant altitude, on an even keel. If he is to make not merely a ribbon, but a map of considerable width, he must take successive trips parallel to the first, each displaced just far enough from the previous course to insure that no portion is missed—a difficult task indeed.