UAV-supported mapping of glacier forefield
In summer 2019, we were back in the Italian part of the European Alps. We visited two glaciers overlying different glacier bed lithologies. These are the Marmolada glacier in the Dolomites resting on permeable carbonate rocks and the Vedretta Occidentale di Ries / Westlicher Rieserferner glacier in the Vedrette di Ries mountain group overlying generally impermeable non-carbonate rocks. This time we focused on the mapping of recently exposed glacier forefields, which can provide evidence for subglacial processes that are otherwise difficult to observe directly. Mapping was performed at two different resolutions: a medium resolution field mapping of the entire glacier forefield and a high resolution on-screen mapping of small-scale erosional features by means of Unmanned Aerial Vehicle (UAV) derived orthomosaics and digital elevation models, restricted only to a smaller part of the glacier forefield.
Study areas
Vedretta Occidentale di Ries glacier
Vedretta Occidentale di Ries / Westlicher Rieserferner glacier (46.9069° N, 12.0965° E; glacier ID: G012950E46902N) is a small plateau glacier located on the north-facing slopes of the Vedrette di Ries mountain group (3358 m asl – Monte Nevoso) in Italy. The glacier is flowing from a ~1.6 km wide basin near the mountain crest into two tongues separated by arête. The glacier front has retreated by up to 1.5 km since the Little Ice Age, while its area has reduced by ~53% since then. In 2015, the glacier covered an area of 1.7 km2 at altitudes between 2770 m and 3230 m. The mean and maximum ice thickness in 2011 was 27 m and 69 m, respectively [1]. The underlying bedrock is prevalently composed of tonalite and granodiorite, whereas paragneiss and pegmatitic gneiss build the westernmost part of the glacier bed.
Vedretta Occidentale di Ries glacier forefield.
Marmolada glacier
Marmolada glacier (46.438° N, 11.871° E; glacier ID: G011871E46438N) is a small glacier located on the north-facing slopes of the Marmolada massif (3343 m asl), the highest massif of the Dolomites (Eastern Alps) in Italy. The glacier front has retreated by up to 1.2 km since the Little Ice Age, while its area has reduced by ~66% from the first documented observations in 1888 [2]. In 2015, the glacier covered 1.48 km2 [3] and the mean and maximum ice thickness was 12.5 m and 40 m, respectively [4]. At present, the glacier is divided into several smaller sectors. The glacier overlies massive Ladinian limestone, called Marmolada limestone.
Marmolada glacier forefield.
UAV survey
UAV survey was carried out by means of autonomous mission flying. Aerial photographs of the bedrock surface were collected using an Unmanned Aerial Vehicle (UAV) DJI Mavic Pro. DJI Flight Planner and Litchi software were used to plan the multi-battery mission and automate the flights. The flying height was pre-programmed to 8 m above ground and the flying altitude has been adjusted every 10-13 m of flying distance, using a LiDAR derived DEM with a resolution of 1 m for a terrain altitude. Images were taken with 65% side overlap and 80% forward overlap with pre-programmed time intervals of one frame every two seconds and flying velocity of 2.7 km h-1. In total, 600-800 images per area were captured.
Automated flights over the forefield of the Marmolada glacier. Basemap source: GoogleEarth.
Image post-processing
UAV-generated images were processed in the Agisoft Metashape Professional Edition software, following the standard photogrammetric workflow: 1) photo alignment, 2) generation of a dense point cloud, 3) building of a mesh and textures, 4) generation of a digital elevation model, and 5) generation of an orthomosaic. The dense point cloud was generated at ultra high-resolution and aggressive depth filtering mode, and the digital elevation model and orthomosaic with 4-8 mm spatial resolution were obtained.
Post-processing of images in Agisoft Metashape software.
Mapping – work in progress
The UAV-derived digital elevation models and orthomosaics will be used in the ArcMap 10.5 software to manually map the distribution of small-scale features, such as glacial grooves and striations, subglacial carbonate crusts, Nye channels, lee-side cavities, karst depressions, quarried blocks, bedrock structures, etc.
Solutional furrows and subglacial calcite percipitates on the deglaciated limestone bedrock surface in front of the Marmolada glacier. The arrow indicates the former ice flow. Photo: Renato R. Colucci.
[1] Fischer, A., 2011. Die eisdicke des Westlichen Rieserferners – Vedretta Occidentale di Ries. [2] IGM, 1888. Tavoletta F. 11 II N.E. La Marmolada, 1:25.000, edizione 1888, in: L’Universo. [3] Carton, A., Bondesan, A., Benetton, S., 2017. Marmolada, la regina della Dolomiti, in: Itinerari Glaciologici Sulle Montagne Italiane 3. Itinerari 19A e 19B. pp. 189–212. [4] Santin, I., Colucci, R.R., Žebre, M., Pavan, M., Cagnati, A., Forte, E., 2019. Recent evolution of Marmolada glacier (Dolomites, Italy) by means of ground and airborne GPR surveys. Remote Sens. Environ. 235, 111442.