## 3 Results I: DEM Quality
### Accuracy of Khumbu Himal Pleiades DEM based on RPC  
At the moment, no other high-accuracy DEM of the Khumbu Himal region is publicly available to assess both the accuracy of the heights and their horizontal position in the derived Pleiades DEM. We therefore used differential GNSS solutions for the x, y, and z positions of seven GCPs that were measured in April 2016. The corners of four roofs in the villages of Gokyo and Dragnag, as well as at a ruin south of Gokyo, and two massive boulders were measured with differential GNSS using a Trimble Geo7X with Zephyr antenna and a local base station. Point positions were collected at 1-s intervals over a period of at least 10 min and post-processed differentially with the base station to give a high precision of the location. Figure 8: Profile 1, located west of the tongue of Ngozumpa glacier. The profile shows the surface differences in off-glacier terrain between the spring 2015 and autumn 2015 acquisitions from south to north. Figure 9: Profile 2, located east of the tongue of Khumbu glacier. The profile shows the surface differences in off-glacier terrain between the spring 2015 and autumn 2015 acquisitions from south to north. \begin{table}
\begin{tabular}{c c} \hline \hline
**Acquisition** & **Standard Deviation** \\ \hline Spring 2015–Spring 2016 & 1.18 \\ Spring 2015–Autumn 2015 & 0.42 \\ Autumn 2015–Spring 2016 & 0.43 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Standard deviation of surface elevation difference over stable terrain. Due to the uneven distribution of GCPs within the study area, these points were not incorporated into the DEM generation, but are instead used as a measure to check the accuracy of the spring 2016 DEM used as the master DEM in the Khumbu Himal study area, which was based on imagery acquired three weeks earlier than the GCPs. The horizontal accuracy was estimated from offsets between the x and y coordinates of the GCPs and their clearly visible positions in a pan-sharpened image that was formed using the near-nadir PA/XS images and orthorectified using the DEM processed solely with the use of RPC information (Table 4). It should be noted that the distances were measured from the corrected dGNSS position to the center of the cell defined as the respective corner. Therefore, the offset is not necessarily the true distance, but it still gives an overview of the accuracy of the DEM at its calculated resolution. The combined horizontal offset reaches about 2.5 m in some cases, which is in line with the results from previous studies [44] and significantly lower than the 8-10 m offsets reported in other studies [41], which can be considered a satisfying result from the RPC, TP, and co-registration workflow. The comparison also shows there to be no systematic offset between the DEM and the GCPs, as even the values for neighbouring GCP locations (e.g., Gokyo and Gokyo hospital or Dragnag 1 and Dragnag 2) show offsets in different directions. Therefore, as was also noted by [PERSON] et al. [33], an improvement \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**ID** & **Name** & **Offset x [m]** & **Offset y [m]** & **Combined Horizontal Offset [m]** & **Offset [z]** \\ \hline
1 & Boulder (N of Gokyo) & \(-1.2\) & \(-0.4\) & \(1.3\) & \(-6.2\) \\
2 & Gokyo Ri & \(0.1\) & \(-0.5\) & \(0.5\) & \(-5.8\) \\
3 & Dragnag 1 & \(-1.9\) & \(0.8\) & \(2.1\) & \(4.2\) \\
4 & Dragnag 2 & \(1.7\) & \(-0.6\) & \(1.8\) & \(-2.3\) \\
5 & Gokyo & \(-1.2\) & \(0.3\) & \(1.2\) & \(-4.1\) \\
6 & Gokyo hospital & \(0.6\) & \(-2.2\) & \(2.3\) & \(-4.4\) \\
7 & Ruin (S of Gokyo) & \(-2.2\) & \(1.1\) & \(2.5\) & \(-1.3\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: X and y offset of the ground control point (GCP) in relation to the 2016 DEM. Figure 10: Post co-registration surface difference and location of profile 1 and profile 2 in the area of overlapping scenes west of the tongue of Nøozumpa glacier and east of the tongue of Khumbu glacier.