Quantification of the surface covered by a parameter distributed at the soil's surface is a current work in remote sensing. The numerical value obtained is always concerned with the resolution of the sensors. We analyse the usual thresholding procedures in the light of new results on spatialization of models. We give the optimal bounds of the error on quantification due to the heterogeneity of the surface distribution and the sensor's resolution. We define new procedures for quantification of covered surfaces within a pixel.