Use of UAV/Structure-from-Motion (SfM) for engineering geological landslide mapping

Abstract

Accurate, high-resolution topographic models are fundamentally important for detailed engineering geological mapping. Nevertheless, high economic costs of topographic data collection are often exacerbated by remoteness of field sites, rendering cheaper, more portable surveying platforms (i.e. terrestrial laser scanning or GPS) impractical. An alternative is Structure-from-Motion (SfM) photogrammetry, which allows for rapid, high-resolution modelling of terrain, comparable in resolution to LiDAR (~0.1 m), but at a fraction of the cost. Utilising SfM in conjunction with Unmanned Aerial Vehicles (UAVs) as means of a low-altitude survey platform, this allows for rapid collection of terrain imagery, with a minimum of ground-control-points. The imagery is then used as a basis for construction of high-resolution engineering geological maps in a 3D space. SfM generates high-resolution topography and co-registered texture (colour) from an unstructured set of overlapping photos taken from varying viewpoints and known camera orientations. The techniques were applied to the coastal landslide at Ohuka (Port Waikato), and the Kepa Road landslide, Auckland. Both of these slope failures are complex, with head and lateral scarps delineating the extent of failure, and irregular surface topography indicative of more recent slope failures in the form of flows, slips and compressional features. The SfM imagery captured the wide range of engineering geological features in a high degree of detail as 3D digital elevation models. In addition, sequential studies can be used for 4D landslide assessments. Hence, the SfM application represents an effective, financially viable alternative to traditional topographic surveying and LiDAR, particularly for practical application in remote, inaccessible regions.