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Quality of land cover mapping, as well as result of many other Earth observation studies can be highly affected by inherent terrain complexity. In order to cope with and overcome these difficulties there is a need for additional information, which can be incorporated in model formulation or preprocessing steps. An example of such application is topographic or terrain correction in case of multispectral measurements, where correct information about surface slope enables more precise computation of irradiance received from the Sun. This is achieved by taking into account the local geometry of the scene. In addition to direct irradiance that is mostly affected by the terrain slope and Sun position, specific geometry can also produce or influence the overall amount of diffuse irradiances from the sky and surrounding ground. As expected, such effect is present in regions with high terrain variability, which is the case in central and southern parts of Serbia. Therefore, availability of appropriate digital surface/elevation models (DSM/DEM) is highly desirable. Term appropriate suggests that in some cases there can be a compromise between quality of DSM, on one side, and the spatial resolution of satellite images, roughness of the terrain, and level of co-registration on the other. There are several parameters that determine quality of elevation data, such as: acquisition and production technique, which depends on instrument type (passive or active), spatial resolution and vertical accuracy of the measurements, presence of noise and production artifacts, rate and availability of data coverage. DEMs produced using digitized contours derived from topographic maps, or produced directly by photogrammetry from airphoto campaigns, are usually considered as very accurate. However, very often these data are not available for some regions, have high cost, or they are just outdated. On the other hand, satellite products usually have regional or global coverage, and recently many of these high quality products became freely available due to new open data policies. In this paper we focus on two global datasets that were produced by optical stereoscopic observations carried out by Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on board of NASA’s Terra satellite and Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM) on board of JAXA’s Advanced Land Observing Satellite (ALOS). More exactly, we compare specificities of two freely available global-scale 30 m products: improved version of ASTER Global Digital Elevation Model (GDEMv2) and 30 m derivative of original 5 m ALOS World 3D Topographic Data (AW3D) in the case of Serbia. Two datasets are also statistically compared over the larger region between 40 and 50 deg N, and 15 and 25 deg E. Since AW3D does not have full coverage for the given region, possible combination of two dataset is considered. Applicability of combined DEM is demonstrated over Serbia. In addition, an overview of alternative DSM products is also given, including: Shuttle Radar Topography Mission (SRTM), Digital Elevation Model over Europe (EU-DEM), and commercial products like WorldDEM and NEXTMap. Possibilities of utilizing Sentinel-1 and new initiatives like 3DEP (by USGS) in the future are also considered. |
