The vertical photographs taken with an air camera are, of course, of the order of large-scale maps.[7] For a lens of 6-inch focus the scale at an elevation of 2,500 feet will be 1:5,000; at 5,000 feet, 1:10,000; and at 10,000 feet, 1:20,000.[8] Air mapping, therefore, lends itself best to the production of such maps as engineering maps, city plans, topographic maps, and coast charts. In all of these maps a degree of accuracy is demanded that will give the exact location of all the features included on the map and permit the precise measurement of distances between them. To obtain such accuracy necessitates an elaborate system of control stations as a basis on which the surveyor works out his triangulations and traverses. In the United States these controls have been established principally by the United States Coast and Geodetic Survey.[9] To construct a map from air photographs, varying in scale and distorted as they often are because of the impossibility of holding the plane at an absolute level and because of the stretching or shrinkage of the photographic paper, would require a great amount of triangulation and traverse in order that the control might be sufficiently detailed to permit the accurate mounting of the photographic prints. But, given these controls, the air camera can, without further adaptation, supply details that heretofore required the laborious processes of plane-table mapping. The topographer can place the two-dimensional details from photographs and then go into the field with only the contouring to be done. [Illustration: FIG. 56] [Illustration: FIG. 57] [Illustration: FIG. 58--Index map showing the location of the areas shown on airplane photographs in this book within the Atlantic seaboard of the northeastern United States, except those whose exact location is unknown (Figs. 21, 23, 28, 29, and 77). Scale, 1:2,800,000.] [Illustration: FIG. 59--An area where charting of the coast line is difficult: Marshlands on Chesapeake Bay which are exposed at low tide and submerged at high tide. Location: South of the mouth of the York River, Virginia, between the estuaries of Poquoson River and Back River. Scale, about 1:2,500.] [Illustration: FIG. 60--Beach and bluff: Left shore of the York River north-northwest of Gloucester Point, Va., showing a tied island (one of the Mumfort Islands) and a narrow band of beach between the water and the bluff, which is 20 to 50 feet above the water. The plain back of the bluff is recognized by the checkered pattern made by the cultivated fields. Scale, about 1:9,000.]