Evaluation of open Digital Elevation Models: estimation of topographic indices relevant to erosion risk in the Wadi M’Goun watershed, Morocco

Various Global Digital Elevation Models (DEM) are available freely on the Web. The main objective of this work is to evaluate the latest digital elevation models towards the estimation of morphological and topographic erosion parameters in the Wadi M’Goun watershed. We have evaluated multiple DEMs: SRTM (3-arcsec resolution, 90 m), ASTER GDEM (1-arcsec resolution, 30 m), SRTMGL1 V003 (30 m), and ALOS-PALSAR (12.5 m). We have applied for this purpose open source GIS software. To compare and evaluate each DEM, different processing methods have been applied to estimate the Wadi M’Goun watershed characteristics, namely Hypsometry, topographic slope extraction, retrieval of Slope Length and Steepness factor (LS-factor) and topographic wetness index. The accuracy of the ALOS-PALSAR and SRTMGL1 V003 (30 m) DEMs met the requirements applying to the required morphometric parameters. DEMs vertical accuracy has been evaluated by applying the root mean square error (RMSE) metric to DEM elevations vs. actual heights of 353 sample points extracted from an accurate survey-based map (toposheet). The RMSE was 1718 mm for ALOS-PALSAR, 1736 for SRTM 1-arcsec, 1958 for ASTER GDEM 1-arcsec and, 3189 for SRTM 3-arcsec. These results indicate that best accuracy is achieved with the high-resolution of the ALOS PALSAR DEM. This study suggests potential uncertainties in the open-source DEMs, which should be taken into account when estimating topographical and morphometric parameters related to erosion risk in the Wadi M’Goun watershed.