Use in Revision of Existing Maps
Another immediate use of air photographs in mapping is in the correction and revision of existing maps. So far as individual features are concerned, the air photograph is an exact record of the area exposed to its lens, and natural and artificial features are easily transferred from the picture to the map. Its great value in the saving of time and money has been demonstrated in the rapidly developing territory near Los Angeles. In 1893 the Santa Monica quadrangle was surveyed, and houses, roads, etc., as they existed at that time, are shown on the map. This area was later built up and so changed that the map was practically worthless. From information derived from air photographs the map was revised in 1920 (Fig. 51). Evidence has already been given of the efficiency of the air photograph in elaborating maps where the importance of the region is not sufficient to warrant the expense of a detailed survey of minor features, and in mapping areas inaccessible from the ground.
Fig. 62—Beach cusps extended under water and showing the interference of waves off the curved beach on the bay side north of Beach Haven, N. J. Scale, about 1:3,000.
Fig. 63—First stage in the formation of an inlet through a barrier beach, where ocean waves during some storm, probably at high tide, broke over the sand barrier 2 miles north of Beach Haven, N. J., and washed some of its sand into Little Egg Harbor. (For another good example of wash-overs, see Fig. 68.) Scale, about 1:9,000.
Fig. 64—A tidal delta in Shark River Inlet, Belmar, N. J., as photographed from a height of 10,000 feet, showing shoals at the left, which appear shadowy because they are under water, and the small channels which radiate outward from the narrow inlet like the ribs of a fan. They are the distributaries of this underwater delta. Scale, about 1:11,000.