As shown in Table 1, images acquired by the two sensors have slightly different pixel size. This is not expected to influence significantly the segmentation, especially over dense urban areas. Misclassification of small agglomerates and/or isolated structures can occur using ERS-1 images. However, the impact of these areas on the calculation of the percentage of soil sealing, which is the parameter of interest for hydrological modeling, is expected to be negligible (see Table 9). This analysis was performed at the sub-catchment scale, in order to identify the areas of the basin which, in time, were most affected by the land use change.