Historical glacier outlines from digitized topographic maps of the Swiss Alps

Abstract

Abstract. Since the end of the Little Ice Age around 1850, the total glacier area of the Central European Alps has considerably decreased. In order to understand the changes in glacier coverage at various scales and to model past and future streamflow accurately, long-term and large-scale datasets of glacier outlines are needed. To fill the gap between the morphologically reconstructed glacier outlines from the moraine extent corresponding to the time period around 1850 and 10 the first complete dataset of glacier areas in the Swiss Alps from aerial photographs in 1973, glacier area from 80 sheets of a historical topographic map, so-called Siegfried map, were manually digitized for the publication years 1878-1918 (further called first period, with most sheets being published around 1900) and 1917-1944 (further called second period, with most sheets being published around 1935). The accuracy of the digitized glacier areas was then assessed through a two-step validation process: the data was (1) visually and (2) quantitatively compared to glacier area datasets of the years 1850, 1973, 15 2003, and 2010, which were derived from different sources. The validation showed that at least 70 % of the digitized glaciers were comparable to the outlines from the other datasets and were therefore plausible. Furthermore, the accuracy of the manual digitization was found to be lower than 5 %. The presented datasets of glacier outlines for the first and second periods were found to be valuable source of information for long-term glacier mass balance or hydrological modelling in glacierized basins if the uncertainty of the historical topographic maps is considered in the interpretation of the results. The 20 datasets can be downloaded from the FreiDok plus data repository (https://freidok.uni-freiburg.

Combination of historical and modern data to decipher the geomorphic evolution of the Innere Ölgruben rock glacier, Kaunertal, Austria, over almost a century (1922–2021)

“…After co‐registration of the FW1922, the contour lines were manually vectorized and converted to a 5 × 5 m DEM using the Topo to Raster tool implemented in Esri ArcMap (v.10.6.1). This tool is based on the ANUDEM program developed by Hutchinson et al 61 To minimize the uncertainty during digitization of historical maps, both the interpretation of the signatures and the vectorization of the features were performed by one interpreter 62 . In a further step, the derived DEM was co‐registered to the ALS2017 DEM using the python tool pybob, 63 which is based on the algorithm for iterative co‐registration of DEMs proposed by Nuth and Kääb 64 …”

“…However, only the position of the maximum glacier extent at the glacier front can be estimated, as clear features have been erased by the moving rock glacier. As historical maps provide the opportunity to conduct area‐wide glacier reconstructions (e.g., Freudiger et al, 62 Rastner et al, 70 Salerno et al 71 ), the mapping of the 1922 glacier margin was carried out based on the FW1922 map (see Section 2.3.2).…”

Inventory and evolution of glacial lakes since the Little Ice Age: Lessons from the case of Switzerland

“…Glacier boundaries for the end of the LIA (~1850) were generated by Paul (2003) and Maisch et al (2000) and are available in vector format at the Global Land Ice Measurement from Space (GLIMS) data browser (GLIMS, 2020; Raup et al, 2007). Freudiger et al (2018) extracted and provided glacier boundaries from the georeferenced Siegfried maps, dating from roughly around 1900 (with up to several years earlier or later; for details, see Freudiger et al, 2018). Additionally, we used the vectorized (Maisch et al, 2000) glacier boundaries for the 1973 Swiss Glacier Inventory (Müller et al, 1976), also available from GLIMS.…”

Halving of Swiss glacier volume since 1931 observed from terrestrial image photogrammetry

“…To exclude areas of possible elevation change over the 85-year period, we mask glaciers, perennial snowfields, and lakes. To do so, we use (i) the glacier and snow mask that Freudiger et al (2018) digitised from the "Siegfried maps" made in 1917-1944 and (ii) the swissTLM3D product to mask natural and artificially dammed lakes (swisstopo, 2020).…”

“…We quantify the uncertainty comprising these errors by comparing terrain assumed to be stable in our elevation change estimates. Stable terrain excludes ice, perennial snow patches (Freudiger et al, 2018), and lakes. Dams created between the 1920s and 1980s are especially important to exclude as they otherwise introduce unwanted positive changes of up to > 50 m. While other factors such as landslides, vegetation change, and buildings may also affect stable terrain, visual inspection showed no obvious signs of their presence.…”