A photograph of a portion of the Earth's surface taken from an aircraft or from a satellite. Most often, photographs are taken sequentially and overlap each other, to give complete coverage of the area of interest. Thus they may be viewed stereoscopically to give a three-dimensional view of the Earth's surface. Although the camera in the aircraft may be pointed obliquely to the side of the line of flight, the most common type of aerial photograph is taken with the camera pointed vertically downward beneath the plane.
Aerial photographs have two main uses, the making of planimetric and topographic maps, and interpretation or data-gathering in a variety of specialized fields. All modern topographic (contour) maps are made from stereoscopic aerial photographs, usually black-and-white panchromatic vertical photographs. The science of making accurate measurements and maps from aerial photographs is called photogrammetry. The geometric discrepancy between the distribution of features seen on the photograph and their actual distribution on the Earth's surface depends on several variables, the most important of which is the relief of the surface. The error becomes greater as relief increases. Photogrammetry deals with these errors and allows highly accurate and complete map production. Topographic maps are made from one or more stereoscopic pairs of aerial photographs by means of optical-mechanical plotting machines or mathematical methods (analytical photogrammetry). See also Cartography; Geophysical exploration; Topographic surveying and mapping.
The other major use of aerial photographs is photo interpretation, utilizing all types of film. Photo interpretation is the attempt to extract information about the Earth's surface (and sometimes, subsurface) from aerial photographs. The systematic study of aerial photographs, particularly when they are viewed stereoscopically, gives specialists the ability to gather rapidly, record, map, and interpret a great deal of information. In addition, an aerial photograph is a record at a moment in time; photographs taken at intervals give an excellent record of the change with time in surface features of the Earth.
