The struggle of choosing the “best” data basis for remote geomorphological analyses – Quality assessment of open-source digital elevation models in Northwest Namibia

Today, digital elevation models (DEM) form the data basis for most geomorphological analyses. With the wide range of freely available, global DEMs, the question arises which option is the best choice for the desired analysis. The quality of DEMs of equal spatial resolution can be characterized by their vertical accuracy and precision. While the vertical accuracy describes the one-dimensional difference between the DEM and the Earth’s surface, the precision describes the spatial error distribution within the dataset.

To study the geomorphologic evolution at the dry limit, the sparsely vegetated Northwest of Namibia provides a promising landscape due many ephemeral rivers incised differently far into the geologically complex Kunene Highlands and draining towards the Skeleton Coast, which is covered to a large extent by dune fields. This versatility, however, provides challenging condition for the assessment of DEMs, strongly dependent on the respective underlying remote-sensing technology. For this reason, we conducted a quality assessment of the commonly used SRTM, ASTER, ALOS and GLO-30 DEMs – all featuring an equal spatial resolution of one arc-second (~30 m). The absolute vertical error has been assessed by validation using geodetic ICESat-2 data. To quantify the precision of the DEMs in relation to the morphometric complexity of the terrain, surface roughness measures were applied. In addition, it was investigated whether the lithology has an impact on the accuracy and precision. In conclusion, the GLO-30 turned out to be as yet the most accurate (RMSE = 1.1 m) and precise open-source DEM for Northwest Namibia.