## 3 Results I: DEM Quality  
### Comparison of Pleiades DEM with ALS-DEM in the European Alps  
To investigate the absolute accuracy of the DEM calculated from Pleiades tri-stereo data in high mountain areas, the data from the Eastern Alps was compared with the ALS-DEM, which was taken to be 'correct'. Co-registration of the two Pleiades DEMs to the ALS-DEM revealed significant horizontal offsets and more minor vertical offsets (Table 2). Results can be substantially improved (Figure 7) by shifting the respective DEM according to these offsets, as suggested by [PERSON] [42].  
To assess the errors remaining after co-registration, the surface difference between each Pleiades-DEM and the ALS-DEM in the areas that were identified as likely to be stable was investigated. Although stable areas are scarce within the mountain environment, three areas with a total size of ca. 15,000 m\({}^{2}\) were defined, with a mean absolute surface difference to the ALS-DEM of 0.78 m and 0.60 m, for the 2014 and 2015 Pleiades DEM, respectively. These submeter residual differences demonstrate that through using the processing methods applied here, it is possible generate DEMs from the tri-stereo Pleiades images that offer a sufficient accuracy for many applications, even in mountainous areas. Since the errors remaining after the co-registration are non-linear, they are not easily corrected. Especially in the northwestern part of the study area, a lower number of TP due to the snow-covered surface in all of the acquisitions can also lead to a decreased fit between the different DEM.3.2 relative orientation of Pleiades DEM in the Khumbu Himal study area.  
To cope with the horizontal inaccuracies in the Khumbu Himal data, the DEM of spring and autumn 2015 in the Khumbu Himal were co-registered to the 2016 data (see Section 2.2.3 and Table 2). To better understand the remaining vertical error after the co-registration, 12 stable areas with inclination <40\({}^{\circ}\) and a total size of 6.5 km\({}^{2}\) were selected throughout the study area. The mean absolute difference of the two 2015 DEM to the 2016 DEM was calculated, and shows an average of 0.60 m for spring 2015 and 0.40 m for autumn 2015, and the standard deviation of the surface elevation difference in the stable areas for all three differences is shown in Table 3. This suggests that while the co-registration to the spring 2016 DEM does not necessarily increase the absolute accuracy, as we do not know which of the DEMs is most accurate, the relative accuracy within our dataset is sufficient  
Figure 7: Statistical function for 2014 Pleiades raw data and co-registered data of the Hochjochferner area.  
to permit DEM differencing analysis for processes resulting in surface elevation changes >1 m after performing co-registration.  
The surface difference in off-glacier terrain has been investigated to give an estimation of the error within the surface elevation differences. The surface differences along two profiles close to the glacier tongues of the Ngozumpa and Khumbu glacier show the errors in the dDEM (Figures 8 and 9), while their locations are shown in Figure 10. Both profiles show differences of up to \(\pm\)1 m in stable areas, with very few outliers of up to \(\pm\)7 m in profile 2.