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Abstract

The alpine landscape is rugged, dominated by glacial morphogenesis, comprising specific landforms of different sizes and shapes with a marked vertical relief and hierarchical-ordering of the forms. Digital geomorphometric analyses of such a land surface in a high level of detail can exploit mainly data acquired by airborne laser scanning or photogrammetry. However, the level of detail captured is limited to several meters or decimetres in these datasets for the relatively high above ground flying heights. Terrestrial laser scanning (TLS) and close range photogrammetry generate 3-D point clouds of ultra-high spatial resolution but their application is limited by extreme environmental conditions. TLS data often contain data shadows in alpine landscape resulting in inhomogeneous spatial distribution of the acquired 3-D point cloud. Such data properties cause creation of overly smoothed surfaces or artefacts in digital elevation models (DEMs) interpolated into a grid (raster). The sub-horizontal field of view in TLS can be compensated by sub-vertical field of view of digital cameras installed on unpiloted aerial platforms (UAVs). Such a photogrammetric 3-D reconstruction of terrain with UAVs is based on structure-from-motion (SfM) image matching techniques. The measurement precision and accuracy of UAV-SfM is lower than with TLS but the UAV-SfM data can be used in filling the TLS data voids. In this paper, we present the results of combined use of TLS and UAV-SfM for high-resolution modelling of a glacial cirque in the Tatry Mountains, the Carpathians, Europe. The achieved accuracy (1 standard deviation) of mutual co-registration of 18 TLS positions was 4.2 mm. The accuracy of georeferencing the final TLS data in the national cartographic system was 33.2 mm based on 15 ground control points. The UAV-SfM dataset was spatially coregistered on the TLS dataset with the accuracy of 137 mm and point filling the TLS voids were used to generate the final DEM of 0.5 m cell size from the combined point clouds.