Fig. 54—View across the western end of Lake Erie, looking in a northeasterly direction (see Fig. 55). Oblique photograph taken from 18,000 feet above Port Clinton, Ohio, by Lieut. G. W. Goddard, showing, in the foreground at the right, Catawba “Island,” a part of the mainland, and, at the left, Put-in-Bay and the islands around it. In the distance below the white clouds are a small island (Middle Island) and a large one (Pelee Island). In the upper left-hand corner is seen Point Pelee and the Canadian shore to the northeast of it about 30 miles away. At Put-in-Bay was fought, September 10, 1813, the Battle of Lake Erie, in which Commodore Perry defeated the British. The monument commemorating this victory can be distinguished in the photograph as a white shaft.
Although most vertical airplane photographs are in the nature of large-scale maps, this view illustrates how a large area can be covered in an oblique view taken at a high altitude—an area, when transformed, of appreciable size even on a small-scale map, such as, for example, Fig. 55.
Fig. 55—Map of the western end of Lake Erie showing the area covered within the angle of vision of Fig. 54. Scale, 1:1,400,000.
Figs. 54 (upper) and 55 (lower). For explanation, see bottom of opposite page.
Scale and Horizontal Control Of Vertical Photographs
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.
