# UAV-Derived Himalayan Topography: Hazard Assessments and Comparison with Global DEM Products
## 3 Results
### Applications  
#### 3.2.1 Flood Assessment  
Tal village is confined by the Marsyangdi River to the west, steep cliffs to the east, and numerous landslides have occurred in the AOI (Figure 3). The large landslide viewed in Figure 3c occurred \(<\)1000 years ago and blocked the Marsyangdi River, forming a landslide-dammed lake [44]. The village is now situated on the low-gradient valley bottom that represents the lake infill. Gabions are situated along the edge of the agricultural land and the centre of the village to protect against high water levels during the summer monsoon (Figure 3b). There were 68 buildings present in the OpenStreetMap dataset [40], which was updated to 213 buildings using the UAV-derived orthophoto (e.g., Figure 3d). The mean building elevation above the river channel was 6.9 \(\pm\) 2.0 m for the UAV DEM and 4.4 \(\pm\) 1.6 m for the AW3D30 DEM (Figure 4). Individual buildings were resolved in the 2 m UAV DEM (Figure 4c,d); therefore, the UAV-derived building elevations above the river channel represent a roof-top height. However, it is clear that the AW3D30 DEM provided a good representation of the low-relief valley bottom and that the greatest deviation from the UAV DEM was on the steep valley slopes (Figure 5).  
Figure 3: (**a–c**) Views of the UAV-derived point cloud of Tal village; (**d**) UAV orthophoto 24 April 2018. Photographs of the mass movement events close to Tal village are provided in Figure S1.  
Figure 4: Elevation above the river channel for the UAV DEM with corresponding hillshade (**a**) and the AW3D30 with corresponding hillshade (**b**). Cross-valley profiles in (**b**) are shown in Figure 5. The mean and one standard deviation of the difference between the UAV and AW3D30 DEMs are shown for each profile. (**c**,**d**) Expanded insets (black box) showing the village of Tal and spot comparisons of elevations above the river channel. It is thought that the landslide (red rectangle) occurred in 2012.  
Similar to Tal village, the mean building elevation above the river channel was highest in the UAV DEM for Lower Pisang village (mean relative elevation of 16.0 \(\pm\) 5.2 m; Table 4, Figure 6). The SRTM had the lowest mean relative elevation (6.1 \(\pm\) 4.9 m), and the HMA-Mos (14.2 \(\pm\) 5.7 m) was closest to the UAV DEM. The minimum building elevation above the river channel was 1.8 m for the UAV DEM, although it was not clear if this was an outbuilding or occupied home.  
DEM-derived river channels were compared to those that were digitised manually using the UAV orthophoto. The HMA-Mos channel was closest to the UAV channel (mean distance of 3.1 \(\pm\) 5.6 m), and the GDEM2 was the furthest (16.7 \(\pm\) 19.5 m) (Table 4). All DEM-derived river channels had maximum deviations from the UAV channel exceeding 30 m.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**DEM**} & **Distance between DEM- and UAV-Derived** & **Mean Building Elevation above** \\  & **River Channels (m)** & **the River Channel (m)** \\ \hline  & **Mean \(\pm\) SD** & **Maximum** & \\ \hline HMA-Mos & 3.1 \(\pm\) 5.6 & 33.2 & 14.2 \(\pm\) 5.7 \\ AW3D30 & 6.4 \(\pm\) 7.3 & 36.8 & 9.2 \(\pm\) 4.9 \\ GDEM2 & 16.7 \(\pm\) 19.5 & 97.9 & 13.1 \(\pm\) 6.5 \\ SRTM & 7.6 \(\pm\) 9.3 & 47.0 & 6.1 \(\pm\) 4.9 \\ UAV & - & - & 16.0 \(\pm\) 5.2 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Spatial proximity of delineated river channels and mean building elevation above the river channel for Lower Pisang.  
Figure 5: Cross-valley profiles of the UAV and AW3D30 DEMs corresponding to Figure 4b: The mean and one standard deviation of the difference between the UAV and AW3D30 DEMs are (1) \(-\)0.2 \(\pm\) 11.5 m, (2) \(-\)3.8 \(\pm\) 9.6 m, (3) \(-\)0.7 \(\pm\) 7.4 m and (4) \(-\)0.8 \(\pm\) 5.3 m.