Development of a methodological approach for the accurate measurement of slope changes due to landslides, using digital photogrammetry

This paper presents a method for studying recent landscape evolution due to mass movements. The method presented employs digital photogrammetric techniques, combined with global positioning system (GPS) measurements, to analyse landslide features depicted in aerial images taken by ad hoc and historical flights. The method was applied and validated in a 7 -km(2) area, located on a south-facing slope of a tributary of the Pas River (Cantabrian Range, Spain). In this area many landslide deposits and features are present, dated from 120,000 years BP to the present. The method starts with the design and carrying out of an ad hoc flight to take 1:5,000 photographs to be used as a reference, using different aircraft devices to control the position and geometry of the photograms. Different ground control points (GCPs) were measured using GPS techniques to support the geomorphological and photogrammetric work. Reference and historical photograms were digitised in a photogrammetric scanner and the digital images and GCPs were incorporated into a digital photogrammetric workstation to generate the reference digital stereo models by aerotriangulation. The models generated have a precision of 21 cm for the reference images and 33 cm for the historical images. The obtained landslide maps were compared with traditional geomorphological maps and an increased precision in the volume and area measurements was confirmed. Backward crown displacements show rates for the last 15 years of 15 mm year(-1). Mass involved in landslide mobility rates in recent landslides are 440 tons.