Automated photogrammetric reconstruction of Birch Glacier, Switzerland (1946–2025): A high-density time series of topographic change preceding catastrophic glacier collapse

As glaciers retreat, permafrost degrades, and mountains destabilize, modern landscape evolution is increasing the potential for catastrophic events, such as the Birch Glacier collapse on May 28, 2025. To improve our understanding of mass movements in mountainous regions and support future hazard assessment and risk mitigation efforts, we are generating time series of glacier surface elevation change from historical aerial photography provided by the Swiss Federal Office of Topography (Swisstopo). In this case study, we leveraged multi-temporal photogrammetric reconstruction and Digital Elevation Model (DEM) coregistration techniques, implemented in the Historical Structure from Motion (HSfM) pipeline, to generate an ~80-year record of self-consistent DEMs and orthoimage mosaics from analog film imagery collected over the Birch Glacier between 1946 and 2010. From 1985 until 2010 we generated nearly annual surface measurements, making this a unique and remarkably dense historical time series. The time series is augmented with modern surface measurements generated from linescan and UAV imagery collected during the period of 2010 to 2025. To quantify the uncertainty of elevation change measurements we compute residuals with respect to the swissSURFACE3D elevation over stable ground, defined by the swissTLM3D land surface classification. The reconstructed time series provides geometric constraints to precisely model the preconditioning phase leading up to the May 2025 Nesthorn-Birchglacier hazard cascade, which may help mitigate future risks in mountainous terrain (see Jacquemart et al. 2026 in GM3.1)