[Illustration: FIG. 61--A sandy beach with beach cusps forming the extreme northwestern end of Sandy Hook, New Jersey, showing, at the left, the end of the Hook surrounded by light-colored sand shading off to shoals and bars; at the right, a broad belt of sand where a new point is beginning to form; and, between them, six cusps arranged like saw teeth. Note that a wave breaks into foam at the point of each cusp. Scale, about 1:9,000.] USE IN CITY MAPPING In city mapping, even though time be taken to establish a very elaborate system of controls, the air camera can accomplish in a few hours a task of years by ordinary methods. In fact it is only by means of air photographs that maps of a growing city can be kept at all up to date. Paris was mapped with 800 plates in less than one day of actual flying. Washington was completely mapped in two and a half hours with less than 200 exposures.[10] For the mosaic of Rochester, N. Y. (Fig. 56) 82 photographs were made in one hour and twenty minutes. There is no reason why such a mosaic with an original survey or even a number of accurately located points as a basis of control should not be sufficiently accurate for all purposes. 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. [Illustration: 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.] [Illustration: 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.]