The objective of this study was to establish optimal procedures for the calculation of watershed slope using Geographical Information Systems (GIS). The tests evaluated the geoprocessing methods according to regression analyses with a cartoghaphically hand-measured data set. Statistical and geostatistical analyses showed the high variability of slope values, turning cartographic generalization techniques inadequate. Field measured slopes showed low agreement with cartographic and GIS estimated values. It was concluded that slope angle calculations require high resolution of the Digital Terrain Model (DTM), interpolated through kriging. Smoothing the DTM decreased the slope determination accuracy, while increased correlation when applied to the slope image itself. Adjustment factors were calculated for different conditions of resolution and smoothing tested. The estimate errors in the developed method were evaluated by mapping the standardized residues.
geoprocessing; DTM/DEM; resolution; smoothing; errors
