Resolution and Precision of Fast Long-Range Terrestrial Photogrammetric Surveying Aimed at Detecting Slope Changes

Structure-from-motion (SfM) is currently used for geological-geomorphological purposes under the condition that the modeling is based either on several ground control points (GCPs) well distributed in the scene or on direct georeferencing (DG). In emergency conditions and in presence of active morphodynamic processes, it could be unfeasible to use GCPs or DG. A study aimed at evaluating the quality of the results achievable by means of completely free SfM modeling of images taken from a distance of some hundred meters is shown in this paper. It is based on an experiment with an artificial target and some surveys of a bedrock scarp, where resolution and precision are evaluated as empirical functions of distance and focal length, taking into account the issues related to the scale factor. The problems related to the recognition of localized surface changes by means of multitemporal surveys are also studied. The primary result is that the free approach can really be used in geomorphological and seismotectonical surveying carried out in emergency conditions.