Abstract. We use high-resolution aerial photogrammetry to investigate glacier retreat in great spatial and temporal detail in the Ötztal Alps, a heavily glacierized area in Austria. Long-term in situ glaciological observations are available for this region as well as a multitemporal time series of digital aerial images with a spatial resolution of 0.2 m acquired over a period of 9 years. Digital surface models (DSMs) are generated for the years 2009, 2015, and 2018. Using these, glacier retreat, extent, and surface elevation changes of all 23 glaciers in the region, including the Vernagtferner, are analyzed. Due to different acquisition dates of the large-scale photogrammetric surveys and the glaciological data, a correction is successfully applied using a designated unmanned aerial vehicle (UAV) survey across a major part of the Vernagtferner. The correction allows a comparison of the mass balances from geodetic and glaciological techniques – both quantitatively and spatially. The results show a clear increase in glacier mass loss for all glaciers in the region, including the Vernagtferner, over the last decade. Local deviations and processes, such as the influence of debris cover, crevasses, and ice dynamics on the mass balance of the Vernagtferner, are quantified. Since those local processes are not captured with the glaciological method, they underline the benefits of complementary geodetic surveying. The availability of high-resolution multi-temporal digital aerial imagery for most of the glaciers in the Alps provides opportunities for a more comprehensive and detailed analysis of climate-change-induced glacier retreat and mass loss.

Abstract. Glaciers all over the world experience an increasing mass loss during recent decades due to change in the global climate. This leads to considerable environmental consequences in the densely populated Alps and many other mountain ranges in the world. We used high-resolution aerial photogrammetry within the AlpSenseBench project to investigate glacier retreat in great spatial and temporal detail in the Ötztaler Alps, a significant glacier area in Austria. Long-term in situ glaciological observations are available for this region, and a multitemporal time series of digital aerial images with a spatial resolution of 20 cm acquired over a period of 10 years exists. Glacier retreat of all 25 glaciers in the region, including the Vernagtferner, was analyzed by investigating glacier extent and surface elevation changes, derived from the aerial images by digital surface model (DSM) generation. Due to different acquisition dates of the large scale photogrammetric surveys and the glaciological data, a correction was established using a dedicated unmanned aerial vehicle (UAV) survey across the major part of the Vernagtferner. This allowed us to compare the mass balances from geodetic and glaciological techniques, which reveals the potentials of the combination of these two techniques for gaining a better insight into glacier changes and its spatial distribution. The results show a clear increase of glacier mass loss for all glaciers in the region, including the Vernagtferner over the last decade. Additionally, the influence of debris-cover on mass balance, as well as the magnitude of dynamic processes, could be quantified. The comparison of geodetic elevation differences and the interpolated glaciological data reveals that there exists a high potential in detecting local peculiarities of mass balance distribution and for correcting small scale deviations, not revealed in the interpolated glaciological information. The availability of high resolution multi-temporal digital aerial imagery for most of the glaciers in the Alps will provide a more comprehensive and detailed analysis of climate change-induced glacier retreat.