Verification of UAS photogrammetry approach for mass movement monitoring in alpine terrain: a case study in High Tatras, Slovakia

Abstract

The alpine terrain with its exposed georelief and climatic conditions supports a wide range of natural processes with various morphodynamic phenomenons. The National Park of High Tatras is probably the most visited park in Slovakia and the instability of talus cones during times of of torrential rains thus may present a significant danger to the tourists in their vicinity. In this paper we present a procedure for documenting and monitoring the talus cone stability, which is a typical geological phenomenon in alpine areas, using the SfM (Structure from Motion) method based on UAS imaging. A significant problem of documentation in the alpine environment is posed by the character of the terrain, which is inclined with a slope usually between 20° and 60°, often very rugged, consisting essentially of larger or smaller boulders ranging in sizes from 10 cm to 1 m (often with the occurrence of isolated stone blocks of several meters). In our study, we have shown that in difficult mountain conditions, UAS photogrammetry is capable of providing models of comparable quality with those acquired by TLS for monitoring movement of the talus cones. Mean absolute deviation of 0.028 m and a standard deviation at an absolute distance of 0.029 m. A mean difference of 0.008 m is practically negligible in the altitude component, and the standard deviation is 0.032 m we can conclude that with the exception of vegetation-covered areas, the terrain model obtained from low-cost UAS photogrammetry achieves qualitative (precision) parameters comparable to those obtained by terrestrial laser scanning and is thus suitable as a basis for systematic monitoring that will form a basis for identification of surface changes at the centimeter level.