Drone, aerial and satellite photogrammetry for 4D glacier survey and monitoring

Mountainous and glacial environments are among the most sensitive to the impact of climate change. The current trend of constantly rising GreenHouse Gas GHG emission and the consequent temperatures growth have caused an increase in the equilibrium line (i.e. the line that separates the accumulation and ablation areas of a glacier) altitude and significant and constant reduction of glacial masses. The projection of the Intergovernmental Panel on Climate Change (IPCC) highlights how the largest snow mass loss is taking place in the regions with small glaciers like the Alps, with significant social and economic impacts. Acting as indicators of climate change, glaciers are environments with delicate balances that should be monitored over time. The need for monitoring the glacier areas and the masses of melting snow has required the implementation of in-situ measurement campaigns and the application of different techniques for systematic and continuous monitoring of these complex environments. Monitoring of these environments mainly takes place annually to assess the extent of the glacier retreat at the end of the summer ablation period. In the present research, different Geomatics monitoring techniques were applied to the Rutor and Indren glaciers, both located in Valle d'Aosta (North-Western Italy). The two glaciers are characterised by different sizes, accessibility and characteristics. Different digital photogrammetry techniques were applied using aerial imagery, acquired both with a drone and a professional camera mounted on board a light aircraft, as well as high-resolution satellite imagery. In situ measurement campaigns were carried out during summer 2021 to collect information, measure 3D coordinates of reference points (both natural and artificial marker) and put in place the necessary ground support and orientation of the photogrammetric flight scheduled for the end of September. Glacier 3D models were generated from both the image dataset acquired by the drone and the aerial photogrammetric flight, by comparing and analysing the results. Subsequently, thanks to this 2021 model, it was possible to co-register a previous model built from aerial images acquired in 2020 in the same period, but without any ground control points (GCP). By comparing the two models it was possible to assess the glacier extent and volume retreat over one year. The difficulties related to the equipment transportation and the limited accessibility of the areas to be monitored, together with the cost and risk of the measurement campaigns, are parameters of fundamental importance in the choice of the monitoring techniques to be adopted. GNSS data from the photogrammetric flight was therefore processed to obtain the camera positions and to build a 3D model by means of a direct photogrammetry approach (i.e. without using GPCs) for cross-comparison and validation of the results obtained. The overall goal was to assess the positional accuracy of a 3D model based on direct photogrammetry to limit (or possibly skip) the field surveying phase. Lastly, satellite photogrammetry based on a very high resolution (VHR) stereopair acquired by the Pleiades constellation was evaluated, analysing areas considered invariant over time, to understand its feasibility as a monitoring tool in wide glacial environments and the related 3D positional accuracy.