Automatic conversion of colour distribution in overlapped aerial photographs

Aerial photographs used in photogrammetry come in overlapping pairs. Frequently the photographs differ in colour distribution, although they show largely the same land site and they are taken by the same air-camera. Almost always, this difference does not affect the accuracy of photogrammetric measures. However, if the photographs are used to make an orthophotomosaic, the difference in colour produces artificial boundaries in the resulting maps, and certainly it does not look good in the eye. This paper proposes an automatic conversion of the colours of one aerial photograph to the colours of the other. The method is based on the overlapping of the aerial photographs, typically 65%. It is assumed that, in the overlapped region, the difference is an artifact of the camera, the scanner or other reasons, which systematically affects all the colours. Thus it is possible to transform the histogram of each band of one aerial photograph to the corresponding histogram of the other aerial photograph. This is done by fitting a parabola to each histogram and then matching the parabolas, converting the colours on the second aerial photograph. The method has been tested with numerous aerial photographs and it gives satisfactory results.