All the affluent countries are characterized by urbanization. Increase of the urbanized areas leads to change of total nitrogen and phosphorus unit discharges from the catchment areas. Therefore, there is necessity of the watersheds parameters ascertainment for usage of them at the biogenic elements outflow calculation. For that reason, we must determine the watersheds boundaries and types of surface within the watersheds. The considered methodology includes the watersheds boundaries ascertainment by means of digital terrain model processing. The models can be produced on the base of global terrain models SRTM or ASTER GDEM, or by digitalization of hypsographic curves of topographic maps. Urban areas do not allow directly usage of the global terrain models for the watersheds boundaries ascertainment because the altitude values of the models have distortions due to the radar signal reflection from roofs, structures, trees, scrubs, and so on. There is considered method of the precise altitude values selection and producing of correct digital terrain model. The other way for the terrain models making consists in interpolation of altitude values of a point layer produced by topographic map digitalization. There is presented methodology for ascertainment of various surface types within urbanized watersheds by supervised classification of composite images in the Quantum GIS module “Dzetsaka classification dock”.