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There can be problems in obtaining suitable aerial photographs for fieldwork purposes and these include copyright issues and cost. There are many companies who either specialise in selling photographs or will take photographs of a chosen site for you, however, these services can be expensive. Technological trends seem to indicate that more and more inherently digital recording systems will be used in many applications where aerial photographs have been traditionally employed. Geometric errors in representation of location prevent direct use of aerial photography as the basis for measurement of distance or areas. These errors are known and understood and it is possible for photogrammetrists to use photographs as the basis for reconstruction of correct positional relationships and the derivation of accurate measurements. Aerial photographs unlike maps have a range of scales that vary in proportion to the elevation of the terrain involved. Once the scale of photograph is known at a particular elevation, ground distances at that elevation can be estimated from corresponding photographic distance measurements. The next section deals with the reliability of aerial photographs
and the various uses for the information provided by them.
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