Urban area extraction from a satellite image

In a SPOT image, urban areas generally appear as agglomerates of numerous little uniform regions. So, they have a typical feature which is a high edge density. In a single sweeping of the image, each edge pixel is tested: if all the surfaces of neighboring regions are less than a predetermined threshold, the current edge pixel is removed. At the end of sweeping, all the internal edges of urban regions are removed but the external boundary or silhouette is kept. This method has been successfully tested on SPOT XS3 images of the region of Bourges, France.