Thresholding constitutes a rudimentary processing in the case of small structures. The case of water areas seems however simpler to treat by this method, in particular when multispectral data are available, because channels R and XS3 present a good contrast [N. Haala, 1992] and a good homogeneity.

Contribution of out-image data, such as the data bases, makes it possible to force the zone of detection and thus to fix locally the thresholding parameters.