Photographs taken from a light aircraft flying at a height of 1 000 or 2 000 feet can reveal ancient remains that are invisible or barely visible on the ground. These remains are often in the form of crop marks or soil marks that can be seen from the air only in certain conditions, at the right time of day and year. Shadows cast in low sun at an angle pick out the lines of old mounds, banks, and ditches, or the remains of ruined walls that were grassed over centuries ago. Aerial photographs have pinpointed the sites of previously unknown Neolithic henges, Iron Age farmsteads, Roman villas and forts, Romano‐British field systems, linear earthworks, medieval planned villages and towns, deserted medieval villages, decayed monasteries, the buried remains of early industries, and many other aspects of †landscape history. An aerial photograph can act as a blueprint of, for example, the playing‐card shape and internal divisions of a Roman fort, when taken from directly above, and can thus be a great aid to excavation. In other cases, the discovery of a new site by means of an aerial photograph can lead to intensive fieldwork and documentary research, e.g. the correlation of ridge‐and‐furrow patterns with 16th‐century maps showing strips in open fields. Aerial photographs are now a major tool for landscape studies.
The first archaeological air photographs were taken between the two world wars, but it was not until after the Second World War that systematic aerial exploration and recording began. The major collections of photographs of historical sites are those of the University Committee for Aerial Photography in Cambridge (which was founded by Professor J. K. St Joseph) and of the Air Photography Unit of the National Monuments Record (English Heritage). See M. W. Beresford and J. K. St Joseph, Medieval England: An Aerial Survey (2nd edn, 1979), E. R. Norman and J. K. St Joseph, The Early Development of Irish Society: The Evidence of Aerial Photography (1969), D. R. Wilson, Air Photo Interpretation for Archaeologists (1982), and Richard Muir, History from the Air (1983). Many local record offices or public libraries have collections of aerial photographs that were taken for other purposes but which are now of historic value, e.g. in showing the construction of new housing estates, or in demonstrating the physical patterns of towns before redevelopment. See Bob Bewley, ‘Understanding England's Historic Landscapes: An Aerial Perspective’, Landscapes, 2/1 (2001), on English Heritage's National Mapping Programme.