The use of new elevation data (SRTM/ASTER) for the detection and morphometric quantification of Pleistocene megadunes (draa) in the eastern Sahara and the southern Namib
New freely available digital elevation data derived from the Shuttle Radar Topography Mission (SRTM-3) and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) allow, for the first time, the use of area-wide high-resolution elevation models. They are useful for various investigations, especially in regions that are devoid of detailed topographic maps. In this contribution, the geomorphological, sedimentological, geochronological, and archaeological results of the Cologne Collaborative Research Centre ACACIA are presented, showing that the southern Great Sand Sea of Egypt (eastern Sahara) is composed of a complex dune pattern of Pleistocene megadunes (draa) and Holocene dunes. A conceptual model is presented suggesting draa, formed by a strong unimodal trade wind regime that acted, most probably, as helical roll-vortices. The draa differ conspicuously in their dimensions and in their evolution from the smaller longitudinal dunes, which arise from the recent bimodal wind regime. First results of comparative investigations in the southern Namib Erg (Namibia) yielded similar conditions and a synchronous landscape development. Based on these results, the second part of the paper evaluates the elevation data for both regions with respect to their availability for the determination of draa and dunes and the morphometric quantification of the draa. Due to their different resolution (SRTM-3: 90 m, ASTER: 30 m) and their different sensor technology (SRTM-3: radar, ASTER: stereo data), both models have assets and drawbacks but can complement each other. With the calculation of geomorphometric parameters (elevation, slope), the ASTER digital elevation model allows the location of the draa of the Great Sand Sea of Egypt, the quantification of their spatial distribution, size, volume, and consequently the Holocene dune movement. In conclusion, the resolution of the ASTER elevation model is applicable for quantitative analyses of well-defined morphological elements with altitudinal variations > 40-50 m. The volume estimation presented provides a tool for a realistic assessment of the sand mass balance of sand seas. This is not possible with the SRTM-3 model and generally impossible with both models if the altitudinal variation is < 20-30 m (e.g. the draa in the southern Namib Erg). As most of the world's ergs contain draa and dunes of respectable heights, the demonstrated methodology could be useful for further investigations concerning dune movement, desertification and climate change.