Long and close-range terrestrial photogrammetry for rocky landscape deformation monitoring

The paper describes the processing and validation of a series of terrestrial photogrammetric surveys carried out from 2017 to 2020 for monitoring the stability of a cliff in Cortes de Pallás (Spain). The complexity of the target area, which has a strong orography, a water reservoir, and many obstacles such as electrical power lines or vegetation, makes difficult the use of any measurement technique. After considering solutions such as long-range laser scanning or close-range mobile mapping, which were unsatisfactorily tested and therefore rejected for future campaigns, the use of combined short and long terrestrial photogrammetry proved an efficient method for quick and massive monitoring of the entire cliff with an overall accuracy of several centimetres. All the steps undertaken for the centimetre level accuracy deliverables, which include camera calibration, bundle-adjustment, dense point cloud generation, 3D modelling, and validation of the 3D models by using external geodetic information, will be presented. For the sake of conciseness, only results for the last two campaigns (5th and 6th), as well as the comparison between the last (6th) and the first (1st) campaigns, will be discussed. In addition, photogrammetric results will be validated by analyzing the metrics on four target-based micro-geodetic check points, located on key critical areas of the cliff selected by civil and geotechnical engineers from the Department of Roads and Infrastructures.