## 3 Results  
### Vegetation Volume Estimation Results Based on DSM and DTM  
Stereophotogrammetry uses several overlapped images to estimate the three-dimensional coordinates of points, creating point clouds and a digital surface model (DSM) that shows the height values of trees or buildings. After the drone photography, a DSM with a resolution of 3.3 cm was created based on the produced point cloud (Figure 5b), and the surface elevation values of the research area, mostly composed of vegetation, appeared to be between 40.0 m and 52.5 m.  
Figure 4: Example of volume estimation by original shape and alpha shape method according to radius (\(\alpha\)).  
Figure 5: (a) Orthographic image of the research area in Gam stream, (b) digital surface model created from the point cloud, and (c) digital surface model for vegetation.  
The created point cloud was sorted into the following categories based on machine learning techniques incorporating geometry and color information: soil, road, vegetation, building, and artificially constructed objects [46]. After classifying the created point cloud into the noted categories, a DTM was constructed based on the classification results. In the area of study for this research, a DTM with a resolution of 1 cm was produced based on the points classified as vegetation and soil. Boundaries for the section to calculate the vegetation volume were initially extracted from the post-processed point cloud at different resolutions to directly compare with the vegetation volume estimated using the alpha shape. Ultimately, to compare with the vegetation volume calculated using the point cloud, the height of the vegetation was determined by subtracting the DTM from the DSM in the area of interest (Figure 6).  
To validate the height of vegetation, the field survey and the reference of Basic Plan for the Gam Stream Development and Management and actual field survey results were used. Based on the Basic Plan for the Gam Stream Development and Management report, the height from riverbed to bridge was about 42 m to 49 m (right side in Figure 6b). Most trees (Figure 6d), which are inconvenient to access and difficult to measure in the field without  
Figure 6: (**a**) Result of subtracting DTM from the DSM of the research area, (**b**) distance information from riverbed to bridge, (**c**,**d**) photos of study area, and (**e**) grass height field survey height.  
specific measuring equipment, have a height of approximately 3 to 6 m (Figure 6a); based on the report, these results seem reasonable. Additionally, for some trees with a height of 8-10 m, the red square area in Figure 6c was well delineated, and the remaining grasslands with a height of approximately 1 m were also well represented, which was the result of actual measurements at accessible locations (Figure 6d). From these results, it appears that the survey results reflect the information on actual vegetation well, and the volume of the vegetation was calculated by multiplying the average difference by the area (Table 3).  
When the boundary of the research area was extracted at high resolution, areas without point cloud generation were excluded from the analysis, resulting in a reduced area (Figure 7a-c). However, the areas without point clouds and which were not included in the area calculation were mostly areas without vegetation, so the average height of the vegetation (DTM-DSM) increased (Figure 7d). For this reason, the vegetation volume calculated using the area and average height of the vegetation did not show significant differences due to changes in resolution (Table 3).