Abstract
The Neva Bay and eastern part of the Finnish Gulf are affected by toxic contamination and eutrophication due to superfluous anthropogenic income of total nitrogen and phosphorus. Monitoring of the total nitrogen and phosphorus content within Saint Petersburg is carried out only in the Neva River and its spill-streams. Therefore, it is necessary to evaluate modules of the total nitrogen and phosphorus outflow from watersheds of the watercourses without monitoring river stations. For this reason, it is necessary to definite boundaries of the watersheds. The paper presents proprietary technology for the watershed boundary definition within Saint Petersburg and its outskirts. 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 developed on the basis of remote sensing of the Earth’s surface. Therefore, the values have distortions due to the signal reflection from roofs, structures, trees, scrubs, and so on. Thus, the global models include noisy data. The distortions can lead to mistakes at definition of watershed boundary. Topographic maps digitalization allows elaborating of digital terrain models without distortions of altitude values. We have made the new digital terrain model of Saint Petersburg and the outskirts by means of Saint Petersburg and Leningrad Region topographic map digitalization. The map was published in 2001. Its scale is equal to 1:200 000. The feature point layer of the altitudes in the Baltic system was created on the base of the map. The digital terrain model was created in ArcGIS by universal kriging interpolation. Dimension of the model cell side is equal to 100 meters. Definition of the watersheds boundaries was carried out by means of the ArcGIS tool Watershed. In addition, we use for the definition some tools of QGIS and SAGA.
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