Terrestrial laser scanning
Picture: NGUs long-range terrestrial laser scanner in front of an unstable rock slope (Photograph: T. Oppikofer, NGU).
Terrestrial laser scanning (TLS, also called ground-based LiDAR) is based on the acquisition of a point cloud of the topography using the time-of-flight distance measurement of an infrared laser pulse, and requires no installation of reflectors. The instrument sends out a laser pulse in a precisely known direction. The laser pulse is scattered by an object (ground surface, buildings, vegetation…) and a portion returns to the scanner, which measures the time-of-flight of the laser pulse and so computes the distance to the object. Sweeping the laser pulse in different directions leads finally to the acquisition of a point cloud – a sort of 3D image of the topography. A spatial resolution of one point every 5 to 15 cm is generally chosen for common applications. NGU possess an Optech ILRIS-3D LR with a range in practice of up to 3500 m and the capability to scan even in wet conditions.
NGU uses terrestrial laser scanning mainly for the investigation and periodic displacement measurements of unstable rock slopes and rockfall areas. However, the technique is also suitable for other landslide types. On rock slopes, the high-resolution point clouds are used for the structural characterization of the rock mass by measuring the orientation of major discontinuity sets (joints, fractures, faults etc.) using for example the software Coltop3D . Using multi-temporal point clouds (multiple acquisitions with a certain time interval of several months to years), we can detect and quantify changes in the topography, due for example to rockfalls or landslide displacements. This change detection is possible over the entire area affected by a landslide and not only selected single monitoring points. This is one of the advantages of the terrestrial laser scanning technique compared to other classical monitoring instruments.