Aerial Phototopography

a scientific discipline which studies topographic map-making methods based on aerial photographic survey materials. A distinction is made between the combined method of aerial phototopography, in which the contour elements of a map are compiled on the basis of aerial photographs and the ground relief is shown by contour lines made by plane-tabling, and the stereotopo-graphic method, in which both the contours and the contour lines are plotted by photogrammetric analysis of aerial photographs; the latter is the chief method for making topographic maps at scales of 1:2,000 and smaller.

The basic procedures of stereotopographic surveying are the obtaining of aerial photographs, the preparing of photographs, topographical geodetic field surveys, and photogrammetric analysis. Topographical geodetic field surveying consists of determining the geodetic coordinates of a limited number of points which appear clearly on aerial photographs. For this purpose it is customary to mark out a number of points before the aerial photographic surveying by positioning in the area geometric figures, such as circles or horizontal crosses, which contrast sharply with the surrounding background. The number of points whose coordinates are determined for an area varies according to the scale of the map to be compiled, the physical and geographical nature of the terrain, and the quality of the aerial photographs. For example, the geodetic coordinates are determined of two points at the beginning and two points at the end of each flight, of which six to eight consecutive aerial photographs are taken. For the subsequent photogrammetric analysis it is necessary to know the geodetic coordinates of the four points located at the corners of the overlapping parts of each two adjacent photographs. These coordinates are determined by the construction of spatial photogrammetric networks relying on points whose geodetic coordinates are known from geodetic field surveys. The construction of the spatial photogrammetric networks is accomplished by means of universal stereophotogrammetric instruments or by the analytical method, in which the coordinates of the points on the photographs are measured on a stereocomparator; for the calculations, however, electronic computers are employed which are programmed for the relations between coordinates of the points on two photographs and the geodetic coordinates of the points on the terrain. After the construction of spatial photogrammetric networks, a working original of the map is compiled, using for this purpose stereophotogrammetric instruments which permit the representation of the contour lines of the terrain relief from the aerial photographs and the construction of the contour part of the map. The results of field and office decipherment of aerial photographs are used for the construction of the contour part of the map.