## 2 Materials and Methods
### Road DEM Construction Method
#### 2.3.1 Determination of Elevation Information of Road Skeleton Line  
The sidelines and road centerlines of the two roads were the control lines of the road's macroundulation shape. After the elevation of the sidelines and the centerlines was determined, the road interior could be interpolated according to road surface morphology characteristics. Therefore, the elevation calculation of the sidelines and the centerlines became the primary task of the road DEM construction. Due to the high density of LIDAR elevation points and high elevation accuracy, the calculation of elevation information on the sidelines and centerlines could be directly interpolated from LIDAR points near them, in theory. However, the shape characteristics of the road should be considered in this study. Therefore, while ensuring elevation accuracy, the elevation information of sidelines and centerlines should be further adjusted to avoid local elevation point oscillation. Based on the above ideas, the implementation process for determining the elevation information of the sidelines and centerline of the road was as follows:  
Figure 2: Split local results.  
1. Read in the sideline and centerline of the road, take the grid size of the road DEM to be constructed as the step size, and insert nodes on the sideline and centerline of the road (Figure 3(1));
2. For the road sideline, start from the first node, take the node as the center of the circle, take the DEM grid size of the road as the radius to draw the semicircle in the road surface (Figure 3(2)), count the average elevation value of Lidar points falling into the semicircle, and assign it to the center node as the initial elevation value;
3. For the road centerline, start from the first node, take the node as the center of the circle, draw the circle in the road surface with the DEM grid size of the road as the radius (Figure 3(3)), count the average elevation value of Lidar points falling into the circle, and assign it to the center node as the initial elevation value;
4. Select a road sideline and further adjust the elevation value of the nodes on the sideline, as follows: 1 First, mark the first and last two points (A and B) of the sideline as nonetilde, and then calculate the distance from the remaining middle nodes to the line AB. After calculation, select the corresponding node (C) with the largest distance, and mark point C as nonetilde (Figures 3 and 4);
5. Determine a distance threshold parameter and calculate the lengths of section AC and section CB. If the length of a section is greater than the distance threshold parameter, consider the section a new road sideline and then return to step 5; if the length of the segment is less than the distance threshold parameter, smooth the remaining nodes inside the segment according to step 7;
6. Starting from the second node, process each node. For the current processing node, select its left node and right node and calculate the average elevation of the three nodes together with the current processing node as the adjusted elevation value of the current processing node. If the current node has no valid elevation value, it will not be processed. Similarly, when calculating the average elevation value, if the left or right point of the current processing node has no valid elevation value, it will not be counted;
7. For the sideline and centerline of the other road, adjust the node elevation value according to the operation method in step (4).