Historical photogrammetry for DoDs in deglaciating environments: challenges and opportunities

Climate change is resulting in rapidly increasing temperatures in the European Alps, rising twice as fast compared to the global average, and leading to unprecedented glacier retreat. Deglaciating alpine landscapes are considered extremely dynamic, evolving rapidly over space and time. The use of DEMs (Digital Elevation Models) of Difference (DoDs) to study changes occurring in these environments has significantly increased in the last years and has been used for a wide range of disciplines. This approach builds on the growing availability of datasets (e.g. historical imagery), accessibility of drones and their sensors (e.g. LiDAR) and facilitated use of digital photogrammetry through commercial and open-source Structure-from-Motion software. However, DoDs of deglaciating landscapes tend to disregard the diversity and complexity of processes in these environments. 

In this research, DEMs were obtained using aerial archival photogrammetry (1977) for the Turtmann basin, a rapidly deglaciating Alpine valley in the Canton of Valais (southwestern Switzerland. A 2021 DEM was used as a reference to create a DoD of the basin (28km²), in order to determine net sediment erosion and deposition during this 44-year time period. 

Most changes identified in the DoD could not be attributed to sediment displacement, but rather to various ecological (e.g. tree growth), glacial (e.g. glacier ice melt) and periglacial (e.g. rock glacier and buried ice melt) processes, as well as error in the photogrammetry. The latter is amplified by the inherently steep topography of alpine basins, which means that small georeferencing errors can cause significant apparent vertical change. A series of post-processing steps were required to obtain precise sediment volumes from the DoD. 

DoDs are extremely valuable for assessing changes in rapidly deglaciating environments. However, challenges exist when applying them to such topographically complex and dynamic landscapes. These challenges must be identified and thoroughly dealt with through DoD post-processing in order to exploit DoDs to their full potential and obtain precise volumes of change. The specific post-processing steps will depend on (1) the research objective, which determines the desired precision as compared to the limits of detection, and (2) the spatial and temporal scales of the DoD, which influence the detectability of changes. In this research, the large temporal (decades) and spatial (basin-wide) scales exposed the challenges and opportunities of using DoDs in rapidly deglaciating environments. The workflow developed to overcome these challenges can be applied to other alpine basins for more precise change detection and thus allow for a better quantitative understanding of processes in deglaciating environments.