UAV-Based Discontinuity Analyses and Rock Fall Source Mapping in Alpine Terrain (Pletzachkogel/Tyrol/Austria)

Three major Holocene rock avalanches have sculpted the morphology of Mount Pletzachkogel (Tyrol, Austria), and rock fall processes continue to show recent activity. Coalescing sets of discontinuities and block moulds exposed in the steep and rugged limestone cliffs exemplify the broad spectrum of rotational and translational block failure modes to which the mountain is prone. As personnel safety concerns strongly limit the ability to access the 200 m high rock cliffs to make traditional structural field measurements, Unmanned Aerial Vehicle (UAV) photogrammetric surveys were performed with a compact portable multicopter. The UAV survey provided a georeferenced point cloud and Digital Terrain Model of sufficient resolution and accuracy to permit efficient extraction of structural geologic measurements by using different open source software packages. This research focuses on comparing discontinuity measurements extracted from the point cloud using manual, semi-automated, and automated techniques, to field measurements made with a geologic compass. The overall workflow of digital image processing and related structural measurement extraction is described, together with data validation procedures. The workflow described herein provides an efficient means for obtaining comprehensive and accurate data sets that mitigate personnel access constraints, are fully auditable and archivable. With increased applications of UAVs for geologic mapping and documentation, such procedures are sure to see rapidly increasing deployment, particularly in alpine terrain.