## 3 Results and Discussion
### Road DEM Construction with Functional Expression Priority  
Using the road DEM correction method described in the methods section of this paper, a typical road surface with catchment function in the study area was selected to carry out DEM correction experiments. The road section was located under a hillside, and there were several outlets of underground drainage channels on one side of the road. During rainfall, water on the hillside flows to the road surface and then into the underground drainage channels through the outlets on the side of the road. To analyze and verify the construction effect of the modified road DEM more intuitively and clearly, its resolution was set to 0.2 m, and the modeling data were obtained from the road DEM and constructed based on the outlet of orthophoto image acquisition and the traditional interpolation method (IDW). The position of the water outlet in the orthophoto image is shown in Figure 9.  
To analyze the effect of road DEM correction, the DEM before correction (Figure 9(a)) was taken as the object of comparative analysis. The figure shows that the elevation difference boundary of the DEM after correction (Figure 9(b)) was obvious, but there was no obvious difference in elevation. The maximum elevation of the DEM after correction did not change, and the minimum elevation difference was only 1 cm, indicating that although the original road shape changed after correction, this had little impact on the elevation accuracy of the road. To verify the road DEM modified by the method in this paper and its functional characteristics, the DEMs constructed by the two methods were used to simulate the catchment process. The results are shown in Figure 11. The catchment path simulated by the original DEM data (Figure 10(a)) was disordered, with many small flows, and could \begin{table}
\begin{tabular}{c c c c c c c} \hline  & \multicolumn{3}{c}{**Traditional Method (IDW)**} & \multicolumn{3}{c}{**Method Used in This Paper**} \\ \cline{2-7}  & **Minimum** & **Maximum** & **Average** & **Minimum** & **Maximum** & **Average** \\ \hline Flat road & 0.05 & 7.89 & 1.47 & 0.03 & 4.00 & 0.97 \\ Undulating road & 0.01 & 11.61 & 3.24 & 0.05 & 8.95 & 3.08 \\ \hline \end{tabular}
\end{table} 
Table 2: Statistics of road gradient index (\({}^{\circ}\)). Figure 10: Correction of DEM results. not clearly express the flow direction into the outlet. The DEM-simulated catchment path (Figure 11b), processed by the correction method proposed in this study, was able to eliminate the disordered catchment path, ensure that the catchment path pointed to the outlet in an orderly manner, and reasonably simulate the catchment process. This result indicates that the modified DEM can ensure the correct catchment function, although it has led to changes in morphology from the original DEM data. Furthermore, this study calculated the difference between the modified road DEM and the original DEM data (Figure 12) and computed statistics on the distribution of grid data values in Figure 12, where the maximum value was 0.032 m and the average value was 0.009 m. The proposed DEM correction algorithm did not have an appreciable impact on the elevation accuracy of the original DEM data. For the road section examined in this study, the area where the road DEM correction algorithm changed the original road DEM greatly was on the left side of the road section. The field survey revealed that this area was slightly concave. Therefore, as shown in Figure 11a, the catchment path in this area pointed to the lower boundary of the road. In the actual precipitation process, although this area was concave, when there was too much water in the road area, the water in this area still flowed to the drain. Therefore, the function of this modified algorithm was to appropriately increase the grid unit elevation value in this area so that the modified DEM could correctly simulate the flow path. The results of this study have great application value in actual road construction and maintenance.