# Rapid-DEM: Rapid Topographic Updates through Satellite Change Detection and UAS Data Fusion
## 3 Results
### Updated DEM  
Visual analysis of the UAS survey results showed that additional development had occurred since the priority queue was generated (Figure 10). The updated site data contained multiple new structures, retaining walls, and vehicles including large cranes. Two stormwater control ponds that are required to minimize runoff and sediment transport from the construction sites were present in both the Planet imagery and the UAS derived-DEMs.  
Resampling the bare earth UAS DEM to 3 m resolution reduced the high level of detail captured in the 15 cm (Figure 11).  
The fused DEM (i.e., UAS DEM fused with the USGS NED DEM) had a mean difference in elevation of \(0.12\pm 2.52\) m with the USGS NED DEM. Large variations in elevation were due to the cutting and filling of the terrain during construction (Figure 12). The mean edge elevation difference was \(0.59\pm 2.61\) m, with the greatest edge elevation differences occurring along edges with grading, retention walls, or forest. The fused DEM had a vertical RMSE of 0.5 m and the USGS NED DEM a had RMSE of 3.71 m based on the surveyed stormwater inlet rim elevations (Table A3). The large error in the USGS NED  
Figure 10: WebODM processed data generated from the UAS flight: (**A**) orthomosaic, (**B**) DSM, (**C**) DTM at 15 cm resolution.  
Figure 9: The priority queue vector map shows that Site 1 transitioned from a (**A**) forested area to a (**B**) forest clearing in preparation for new development in the headwaters of the Walnut Creek watershed.  
DEM reflects the impact of topographic change on the accuracy of the available DEMs and highlights the need for DEM updates in rapidly developing regions.  
Elevation profiles of the USGS NED DEM, UAS DEM (i.e., registered, but not fused), and the fused DEM were run through two cross-sections of Site 1 (Figure 13). The profiles showed where the fusion process had to make adjustments to compensate for differences between the USGS DEM and UAS DEM. One difference of note occurs near FACILITYID DP77305094 where there was a significant difference (5.22 m) between the vertically corrected UAS DEM and the USGS NED DEM where grading had occurred on the hillside.  
Slope maps derived from the USGS NED DEM and the fused DEM highlight the topographic changes when natural hillslopes with headwater streams are replaced by a flat area with structural features with steep slopes such as sedimentation ponds, cut-slopes, and retaining walls (Figure 14). The slope maps also show were portions of the natural topography were incorporated into the constructed environment with only partial modifications. These topographic changes increased the average slope of Site 1 from \(4.59\pm 4.01\) to \(5.31\pm 6.73\) degrees. The results indicated that fused DEM was appropriate to model surface water runoff to explore how the changes to the urban topography modified the urban stream channels and basins.  
Figure 11: UAS DEM resampled to 3 m resolution with vegetation and structures removed before fusion with the USGS NED DEM.  
Figure 12: The difference in elevation of the fused DEM and the USGS NED DEM revealed how construction has modified Site 1’s topography.  
Figure 13: The elevation map with the location of profiles AB and CD, which illustrate the changes in elevation before and after the site grading (or construction) and the fusion of USGS NED DEM and UAS DEM along the site edges. Profile AB captures one of the sediment control ponds while Profile CD includes one of the surveyed inlets.