Comparison of TLS and photogrammetric workflows for tracking Alpine rock slope failures

Abstract. This paper presents a comparative analysis of photogrammetric and Terrestrial Laser Scanning (TLS) workflows for geo-monitoring alpine rock slope failures, focusing on the quality of the resulting point clouds and 3D displacement vectors. We compare two tracking workflows, a patch-based approach for TLS point clouds and a 2D feature-based method for photogrammetry. We evaluate both methods using data from the Hochvogel Mountain test site, collected across two measurement epochs. The results demonstrate that both TLS and photogrammetry effectively detect ongoing movement patterns down to a few millimeters, with similar accuracy despite slight differences in noise levels. Both methods offer high spatial resolution and sensitivity to small-scale displacements, showing strong agreement with reference measurements from tachymetry. Differences mainly arise in the spatial distribution: the TLS method is sparse in planar areas but very regular, while the photogrammetric method yields a significantly higher number of vectors, concentrated in regions with strong texture variation, but also includes more outliers.