Factors affecting the detection of slope instability with air photographs in an area near Sevenoaks, Kent

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Abstract
Summary: Twenty-six sets of air photographs were used to test the best photographic conditions for detecting landslips, mainly old periglacial solifluction movement, along a ten mile stretch of sloping ground. A total of 141 old landslip features were detected and many varied in clarity on the different photographs, some being hard or impossible to detect on foot without the aid of air photographs. The repeated images of the same landslips on different sets of photographs produced a total of 2 715 photo-features for statistical studies. Analysis of the results showed that the best technical photographic conditions for detecting such features are the use of infra red colour film, stereoscopic photography with a short focal length lens, a nominal photographic scale of about 1:5 000, and a sun angle that casts local but not very extensive shadows. It is considered that automatic dodging can both enhance and suppress vital information on images, and that the best seasons for photography are likely to be spring and late summer to early autumn.

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