=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. 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. [Illustration: FIG. 134.—Diagram showing effect of banking on aerial photograph.] [Illustration: FIG. 135.—Diagram showing effect of climbing and diving on aerial photograph.]