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— 2000 × .18 × .8 × 3600 _t_ = —————————————————————— = 10.3 seconds .5 × 200,000 The principle of overlapping map exposures is shown in the accompanying diagram (Fig. 129), together with data calculated as above for a 4 × 5 inch plate.