It is the observer's duty to so time the intervals between exposures that they overlap enough, but not so much as to be wasteful of plates or film. He must also change magazines or films so quickly as to miss no territory, or if some be missed, his is the task of directing the pilot back to the point of the last exposure, where they begin a new series.
Level flying is entirely a pilot's problem. Its importance will be realized when we consider the accompanying diagrams (Figs. [134] and [135]), where the effect on the resultant picture is shown of climbing, gliding, or banking to either side. Prints from negatives distorted in this way neither will be true representations of the territory photographed, nor will they match when juxtaposed. In fact, they can be utilized only if special rectifying apparatus is available for printing. Flying at a constant altitude is similarly necessary if the prints are to be utilized without enlargement or reduction in order to make them fit.
Fig. 134.—Diagram showing effect of banking on aerial photograph.
Fig. 135.—Diagram showing effect of climbing and diving on aerial photograph.
Assuming a skilled pilot who will do his part, the next step is to calculate the exposure intervals in order to insure an adequate overlap. If a negative lens is installed which has been marked with a rectangle the size of the camera field, the simplest method is to estimate the proper instant for exposure by watching the progress of objects across the lens face. This of course requires constant attention, and it is easier to do this only occasionally, in order to determine the ground speed in terms of camera fields traversed per minute. Thereafter exposures are to be made by time, as determined by a watch or clock. Any desired degree of overlap can be chosen, and either estimated, or more or less accurately fixed by lines marked on the negative lens at a shorter distance apart than the edges of the field. The most usual overlap is 20 per cent., except for stereos, which call for 50 to 75 per cent.
In the absence of a negative lens or some other sight to show the whole camera field, it is necessary to resort to calculation from the speed and altitude of the plane, the focus of the lens and the dimensions of the plate. If A is the altitude, a the focal length of the lens, d the diameter of the plate in the direction of travel (usually the short length is chosen for economy of flights to cover a given width), f the fractional part by which one negative is desired to overlap the next, and V the ground speed of the plane, then we have, by simple proportion, that the interval between exposures, t, must be—
| Ad(1 - f) | ||
| t | = | |
| aV |
If A = 2000 meters, d = 18 centimeters, f = ⅕, a = 50 centimeters, and V = 200 kilometers per hour, this relation gives—