Automatic determination of imperviousness in urban areas from digital orthophotos

Determination of impervious areas in urban regions is the most labour-inten sive part of data acquisition for rainfall-runoff modelling in urban hydrol ogy. This paper presents an automatic determination method of imperviousnes s from aerial photographs. The colour, CIR (colour infrared) aerial photogr aphs and orthophotos used have a ground resolution of 25 to 75 centimetres. A maximum likelihood classification algorithm was applied to assign each p ixel to a surface category. Classification results were then overlaid with the subcatchments to determine the imperviousness of each subcatchment. Cla ssification and overlay were carried out with the raster-based GIS IDRISI. The method was tested on various catchment areas, and the results compared with data obtained from manually digitised surfaces. Accuracy of the estima ted imperviousness for the entire catchment areas was within 10 %. The devi ations for individual subcatchments were much higher. Equivalent results we re obtained for colour and CIR photographs. A combination of both spectral ranges resulted only in a slight improvement. Consequently, this does not j ustify the additional costs of the second image. The developed method is an interesting alternative for use on large catchment areas where manual digi tisation is very rime-consuming and, thus, expensive. (C) 1999 Published by Elsevier Science Ltd on behalf of the IAWQ. All rights reserved.