The paper presents elaborated methodology for Saint Petersburg and its outskirts watersheds parameters ascertainment. The methodology is based on usage of the digital terrain model, which was created as result of topographic maps digitalization. Usage of the most popular global digital terrain models such as SRTM, ASTER GDEM makes difficulties under the conditions of big cities. The models were processed on the base of Earth surface radar mapping. Therefore, the altitude values have distortions due to the radar signal reflection from roofs, structures, trees, scrubs, and so on. Thus, the global models include noisy data. The distortions can lead to mistakes at determination of the watersheds boundaries. Topographic maps digitalization allows elaborating of digital terrain models without distortions of the altitude values. The watersheds boundaries ascertainment was carried out by means of the ArcGIS tool Watershed. In addition, we use for the ascertainment and
determination of the catchments areas structure some tools of Quantum
GIS (QGIS) and SAGA.