Determination of glacier extents by monoplotting using historical terrestrial oblique images

The melting of glaciers in recent decades is well documented in the Alps through comprehensive historical aerial images available since the 1940s. Less well documented are the glacier extents in the late 1800s and in the first half of the 20th century. In addition to historical maps, historical terrestrial oblique images play an important role for this period and can be georeferenced and analysed by digital monoplotting (Bozzini, 2012; Wiesmann 2012). These images are often the only useful source for reconstructing changes in these areas and thus offer an effective way of documenting landscape dynamics. Private and public archives (such as the archives of the DAV and ÖAV) have a large number of high-resolution historical oblique images. Using three historical photos, we show the changes in glacier extents from 1890 to about 1935 using the example of the Gepatschferner in the Kaunertal with the monoplotting tool Mono3D, which was developed at the Department of Geodesy and Geoinformation at the TU Wien (Flöry et al., 2020). Ground control points (more than 10 per image) were selected via a 3D viewer and both the interior and exterior orientation of the images estimated using OrientAL (Karel et al., 2013). With the estimated camera parameters and a reference DEM (2017, 1m resolution) the object coordinates of image pixels are calculated by monoplotting. In this way, the glacier extents were digitised and extracted as spatially referenced vector data (Fig. 1). Together with the glacier extents at the end of the Little Ice Age around 1850 (Groß and Patzelt, 2015) and the glacier extent mapped by an orthophoto from 1953 (aerial images were provided by the BEV (Austrian Federal Office of Surveying and Metrology, Vienna/Austria)), this period of around 100 years can be partially closed