Photogrammetric Methods for Monitoring Cliffs with Low Retreat Rate

Sea cliff retreat is dominated by mass movements which are a source of natural hazard. Hazard assessment requires complete inventories of cliff failures, usually based on multi-temporal aerial surveys by means of simplified or photogrammetric methods. However, low retreat rate cliffs still pose problems, because parts of the cliff top contour remain unchanged during the monitoring period. The inherent difficulties of a multi-temporal processing of aerial photos (different flight directions and scales, insufficient ground control and poor radiometric quality) require a refined photogrammetric assessment in order to overcome discrepancies such as false mass movements that can not be included in the inventories. Because cliff retreat is discontinuous in space and time, better sampling of the process requires the widening of the monitoring time window, by use of older aerial photos, which involve further problems as unavailable camera calibration and photo distortions. To address these problems, the 13km long sea cliffs of Burgau-Lagos coast (Southwest Algarve, Portugal) were studied using digital photogrammetry methods, which involved aerotriangulation, stereo plotting of the cliff top, ridges and toe and automatic generation of digital terrain models. Inconsistencies in the results for the different epochs were solved by systematic stereo photo interpretation, supported by oblique aerial photos. All relevant results were organised in a Geographic Information System designed for the purpose allowing future and more elaborated analysis.