## 4 Results and Discussion
### Shisper Glacier (Pakistan), PlanetScope DOVE
In a second step, we generated the two DEMs using the image space approach described in Section 3. We used Metashape software [36; 37] with the NIR bands and our optimized RFMs. We chose the NIR band because it contains the fewest fraction of saturated pixels, especially in snow-covered and glaciated areas. The results are summarized in Table 2.  
Two mesh-based DEMs and 3-D textured models were created with a GSD of 9 m, covering an area of \(\sim\)400 km\({}^{2}\). Figure 12 shows the complete textured 3D-model and the extracted DEMs over the Shisper Glacier site before and after the surge. The terrain height values are scaled within 2 km (purple) to 6 km (red). The movement of the glacier downstream the valley due to the recent surge is clearly visible from comparing the 3D textured models or the DEMs. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **2017-DEM** & **2019-DEM** \\ \hline
**Nb. of Images** & **16** & **14** \\ \hline Overlapping threshold & & 20\% & \\ Coverage area (km\({}^{2}\)) & 449 & 372 \\ Re-projection error (pix) & 0.385 & 0.351 \\ Tie points & 171,558 & 121,177 \\ Average Tie point multiplicity & 2.65 & 2.54 \\ Dense point cloud & 11,291,888 & 8,404,358 \\ DEM GSD (m) & & 9 \\  & faces & 2,258,343 & 1,680,413 \\ Mesh & vertices & 1,133,266 & 842,735 \\ Texture & & \(4096\times 4096\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: PlanetScope DEM results. The accuracy of the DEMs is evaluated through comparison with the SRTM DEM [42] (Figure 12e) and with a DEM generated from WV-2 and GE-1 stereo images acquired in 2019 [43] (Figure 12f). Due to the temporal difference between the various DEMs, and the rapid glacier movement, we first selected some presumably stable areas. Note that the slope motion could have altered the topography, but in the absence of clear evidence for such a motion in that area we assume that the topography has not changed between 2000 (the year of the data acquisition used to generate SRTM [44]) and 2019. An area of ~5 km\({}^{2}\) free of clear temporal changes was selected as a reference (Black box in Figure 11). The Normalized Median Absolute Deviation (NMAD) of the elevation differences in the stable area between PlanetScope DEM and SRTM is ~12 m, and ~7 m when calculated with the WV-2 and GE-1 DEM, respectively. The statistics of the elevation differences summarized in the histograms in Figure 13 show a better accuracy of the 2019-DEM than the 2017-DEM. This difference is probably due to smaller time difference between the DEMs and their closer spatial resolution. WV-2 and GE-1 data were acquired ~20 days apart from PlanetScope data and the extracted DEM has a GSD of 2 m; however the last update of the SRTM DEM dates back to 2014 and has a spatial resolution of 30 m.  
Figure 12: DEMs and textured 3D models over the Shisper area. (**a,b**) Textured 3D models from 2017 (**a**) and 2019 (**b**). (**c,d**) DEMs extracted from PlanetScope data in 2017 (**c**) and 2019 (**d**). (**e**) SRTM DEM. (**f**) DEM generated by combining WV-2 and GE-1 stereo images acquired on 5 July 2019 and on 9 September 2019, respectively. Figure 13: Histogram distribution and statistics of elevation differences between (**a**) 2019 PlanetScope DEM and Ge-1 and WV-2 DEM (**b**) 2017 DEM PlanetScope and SRTM. We used the DEMs calculated from the PlanetScope data to assess the change of the glacier topography due to the 2018 surge. The mass transfer due to the surge is clearly revealed from the comparison of the 2017 and 2019 DEMs (Figure 14). The comparison shows a clear thinning of the glacier at elevations above \(\sim\)2900 m, where the ice surface dropped by \(\sim\)50 \(\pm\) 10 m on average, and ice gains were at lower elevations. The glacier front advanced by 3.2 km downstream, reaching a thickness of \(\sim\)170 \(\pm\) 10 m just 1 km upstream of the terminus, downstream of the confluence with the Mochwar valley. A recent study [38] presents elevation differences over the Shisper glacier using ASTER and SRTM DEMs.