Low cost Structure-from-Motion-based fast surveying of a rock cliff: precision and reliability assessment

Abstract

Structure-from-Motion (SfM) performance was studied in terms of point clouds repeatability from a fast, low cost survey of an inaccessible slope carried out by means of a prosumer camera in emergency conditions and without ground control points. A rock cliff was observed from a distance of ~340 m and the resulting point clouds were scaled with each other with the method of homologous polylines. A terrestrial laser scanning survey was carried out in order to scale the SfM data and evaluate the results. Tests of repeatability showed standard deviations (SDs) of 11 mm and 18 mm for point clouds obtained from images taken with focal lengths of 98 mm and 55 mm respectively. In order to evaluate the performance in cases where no reference models are available, data scaling was repeated by using Google Earth data, obtaining SDs  ~8-9 % larger. Moreover, three times the SD turned out to be  ~2.5 times the ground sample distance, leading to a suggestion for a completely independent scaling if no other sources for metric correction are available.