## 4 Practical Experiments
### Shisper Glacier
#### 4.1.2 Evaluation--Shisper Glacier  
To assess the accuracy of the generated DEMs, DoD analyses were conducted in specific regions, comparing the PSS DEMs with the GLO-30 DEM used as a reference. These analyses focused on stable areas such as Ali Abad village, the broader Ali Abad region encompassing steep northern mountainous terrain, and a section of mountainous terrain partially covered by snow in the central study area. These selections aimed to investigate the impact of snow cover and steep terrain on DEM accuracy. Additionally, intersection errors were examined to assess triangulation precision. The GLO-30 DEM demonstrates global absolute and relative vertical accuracies of <4 m in steep terrain within an empirical 90% quantile [6]. However, an analysis according to individual region types shows systematically higher deviations in mountainous regions. This is particularly true for regions of the \"Rock and Ice\" type, into which the Muchowar and Shisper glacier areas are categorized. Here, the empirical 90% quantile is given as <14 m, which must be taken into account when interpreting the following DoDs [40]. The DoD analysis of the PSS DEM from 2017 in Ali Abad/Karimabad, as shown in Figure 6, revealed a mean deviation of 0.99 m (median of 0.43 m), a StdDev of 9.44 m, and an NMAD of 7.94 m in the anticipated stable region. These findings align closely with those of [16], who reported a mean deviation of 0.3 m, a StdDev of 11.0 m, and an NMAD of 12 m in comparisons to SRTM data for the same year. A slight negative systematic deviation along the Hunza River is noticeable, which will be discussed in more detail below. Analysis of the Aliabad/Karimabad patch (see Figure 7) highlights significant DoD deviations in rock slope regions north of the settlements, likely due to low texture, shadowing, and steep ray intersection angles impacting 3D reconstruction accuracy. In addition, the lower quality of the reference DEM in this area must be considered. The error map, however, shows no anomalies in the northern slopes but reveals a systematic pattern in the  
Figure 6: PSS DEM from 2017 over the stable area of Ali Abad/Karimabad village. The DoD illustrates the elevation differences between the PSS-derived DEM and the GLO-30 DEM. The accompanying histogram represents the DoD’s raster statistics, including mean, median, and StdDev. Additionally, the NMAD was computed for the outlined area, resulting in 7.94 m.  
south, where ray intersection errors are higher. This may result from southern areas being reconstructed using only two DEMs from stereo pairs captured with similar configurations (see _Count_, Figures 3 and 7). The ray intersection angles in this area were likely less favorable than in the northern area. Regions with multiple paired DEMs benefit from greater redundancy, improving accuracy. In the southern region near the Hunza River, a slight systematic negative deviation is observed, with the river lying several meters below the village, forming a steep edge in the DEM. The exact cause of this deviation remains unclear. Possible contributing factors include limited textures, as seen in the orthophoto, and suboptimal ray intersection angles. Terrain changes, such as landslides, between the GLO-30 DEM and the PSS 2017 DEM cannot be ruled out, nor can uncertainties in the GLO-30 DEM itself. High-resolution reference data collected concurrently with the PSS images could provide clarity. The analysis of the Muchowar glacier patch (see Figure 8) reveals significant DoD deviations in steep terrain areas with pronounced shadowing or snow coverage in the PSS (see orthophoto), potentially linked to snow thickness variations. Snow-covered regions generally pose fewer issues for 3D reconstruction itself than expected, as indicated by the error map, apart from some higher ray intersection errors in the eastern section, typically at steep edges. This may be due to the use of the NIR band as recommended for icy/snow-covered terrain.