Figure 7. (a) Comparison of the deep learning mapped rock glacier boundary (in yellow) with the manually delineated polygon (in red) in the training dataset. The IoU between the two is 0.871. The black arrow indicates the flow direction. (b) Similar visual comparison between the automatically outlined boundary (in yellow) and the manually mapped one (in red) in the validation dataset, with an IoU of 0.804. (c) Example of a rock glacier newly discovered by deep learning with good delineation accuracy. (d) Examples of two automatically identified and outlined rock glaciers (in yellow) that need manual modifications (in blue). The landform IDs of these examples are labelled on the figures. The background is a Sentinel-2 image acquired on July 12th, 2018.
4.2 Geomorphic characteristics of the mapped rock glaciers
Table 2 presents the overall geomorphic information of the mapped rock glaciers. Among the 413 rock glaciers (RGs), almost half of them (202 in total) are spatially connected to glaciers or debris-covered glaciers (G-RGs), and the debris-mantled slope-connected rock glaciers (DMS-RGs) are the second largest category, accounting for ~35% (143 in total) of the mapped landforms. There are 41 rock glaciers occurring at the glacier forefield (GF-RGs) and 27 developing at the terminus of talus (T-RGs), taking up ~10% and ~7% of the total amount, respectively.
All RGs are located at altitudes between 3,389 m and 5,541 m, with an average of 4,623 m. The G-RGs have a similar mean altitude of 4,546 m. Both groups (namely all RGs and the G-RGs) of landforms show a norm distribution in altitude (Figure 8a, c). The DMS-RGs generally occur at a higher altitude (Figure 8b), the average of which is up to 4,889 m, whereas the GF-RGs and T-RGs are distributed at a lower elevation band (Figure 8d, e), whose average altitudes are 4,265 m and 4,332 m, respectively.
The G-RGs are the largest with an average area of 0.40 km2 for individual landforms, followed by GF-RGs with a mean area of 0.38 km2. Both are much (~50%) larger than the mean area (0.26 km2) of all RGs. The DMS-RGs are the smallest (0.05 km2), covering ~7% of the total area occupied by all RGs in the study region. The mean surface slope of all RGs is 17º, which is similar to the mean slope (18º) of the T-RGs. The G-RGs and GF-RGs have relatively flat surfaces with mean slope angles of 14º and 15º, respectively, whereas the DMS-RGs develop a steeper average slope angle of 23º. Most (64%) of the mapped RGs occur on east-facing (0º–180º) slopes (Figure 9a) as the movement towards eastern direction is sensitive to the InSAR detection, though the AI-based sub-dataset does not suffer from this problem. Among different categories, the G-RGs and GF-RGs are more frequently located on northeastern-facing (0º–90º) slopes (Figure 9c, d), whereas the DMS-RGs and T-RGs mostly move towards southeastern directions (90º–180º) (Figure 9b, e).