A theoretical possibility is the plotting of contours from stereo-aerial pictures. Given two elements of a stereoscopic pair, taken from points whose separation is known, the position of any point in space shown in the stereoscopic view can be determined by the use of the stereo-comparator. This is an instrument already employed in mountain photo-surveying, which consists essentially of a compound stereoscope in whose eye-pieces are two points movable at will so that the relief image formed by their fusion can be made to coincide with any chosen part of the landscape. The chief difficulty in the application of this idea to aerial work is to fix the base line. This problem may be met in some cases by using stereo obliques, and getting the base line by simultaneously made vertical photographs of well surveyed territory beneath. Possibly also methods can be developed by which photographs from two or more known altitudes may furnish the requisite data. [Illustration: FIG. 202.—Business section of Hampton, Virginia. A survey made by a single instantaneous exposure.] [Illustration: FIG. 203.—Mosaic map of the City of Washington. White rectangle shows portion included in next figure.] [Illustration: FIG. 204.—Portion of Washington mosaic, full size.] City mapping is a field for which aerial photography is peculiarly fitted (Fig. 202). A complete map of a large city is a labor of years. In fact, a modern city is always dangerously near to growing faster than its maps. An aerial map, on the contrary, can be produced in a few hours. Paris was mapped with 800 plates in less than a day's actual flying. Washington was completely mapped in 2½ hours, with less than 200 exposures. The entire map is shown, on a greatly reduced scale, in Fig. 203, while Fig. 204 shows a small portion of it in full size, from which can be obtained an idea of the dimensions of the original. These maps, while not accurate enough for the recording of deeds and mortgages, yet serve the majority of needs. There is indeed no reason why with long focus cameras, given several accurately marked points, the photographic map of a piece of real estate should not be made with all the accuracy needed, still leaving the whole process of partial surveying helped out by photography an enormously simpler one than the usual method.