Photogrammetry of Unstable Rock Slopes

Abstract

Providing temporal data of potential rock slope instabilities is important for assessing their dynamics and improving projections. Acquiring slope movement data prior to the advent of satellite radar can often only be found through the analysis of historical aerial photography. This project examines slope displacements over extended periods using photogrammetry techniques, improving orthorectified images with higher resolution digital elevation models (DEM) and comparing to orthoimages generated using lower-resolution DEMs. The orthoimages based on lower resolution DEMs are collected from Norge I Bilder (NIB), Norway’s national archive of orthorectified images and readily available. The areas studied are Tussafoten and Skutshorn, with the present study showing that there has been movement dating back to 1969 and 1987 respectively. Raw aerial images are acquired and orthorectified through structure-from-motion for the Tussafoten slide, while NIB images are used for both Tussafoten and Skutshorn. By applying image matching to the different types of images, being single orthoimages, orthomosaic and NIB-images their displacement results are compared. Orthomosaics lead to a large range of displacements, including over stable ground, indicating uncertainty. The NIB-images use a coarse DEM with a 10 m resolution and have relatively large distortions in the steeper areas where the rockslides are occurring. Although the NIB-images are cheap and easily accessible, their distorted displacement values do suggest that they are not entirely correct. While the orthomosaics are mosaiced together from the single orthoimages, some of the distortion can be avoided by using the single orthoimages. The displacements from single orthoimages result in values that have a degree of certainty of 1 m, with results close to that which is expected from previous studies.