# An Integrated Algorithm for Extracting Terrain Feature-Point Clusters Based on DEM Data
## 2 Materials and Methods
### Methods
#### 2.2.2 Terrain Feature-Point Cluster Extraction  
Terrain feature points such as peaks, saddles and runoff nodes are the basic frameworks that constitute the surface topography and undulation changes. The candidate peaks and their control areas are obtained by the runoff simulation method. Then, the ridgelines are intersected with the boundaries of the peaks' control area to obtain the candidate saddles. Meanwhile, the candidate runoff nodes are extracted by self-intersecting the peak control areas' boundaries. Finally, the integrated terrain feature-point clusters are obtained by filtering and merging these feature points. The workflow of the proposedmethod is shown in Figure 4, including two steps: peak extraction by space segmentation and feature-point cluster extraction and verification. **Peaks extraction by space segmentation**. The hydrological view believes that each watershed has its own control area, and there are multiple levels of sub-watersheds within this watershed. The watershed and its sub-watersheds at all levels are affected by specific topography, geomorphology, and lithology, showing a typical and irregular pattern. Correspondingly, the mountainous area can be considered as a positive geomorphic system consisting of the individual mountain, which controls different regional patches, respectively. Therefore, the original positive geomorphic mountains can be reversed to a negative watershed system, and the corresponding peaks can evolve into sinks in each watershed (Figure 5). Based on this viewpoint, the space segmentation method [33] is adopted to extract the peaks and their control areas. The original terrain is reversed into an anti-terrain. The anti-terrain is sink-filled under the Height Difference between Peaks and Saddles (HDPS) threshold limit, and the flow direction is calculated based on the sink-filled anti-terrain. Then, the peaks and their control area's boundaries can be obtained by the anti-terrain watersheds extracted from the sink-filled flow direction. Among them, the sink filling's threshold can be regarded as the peak control area's height difference limit. The processes of this method are as follows: (1) calculate the anti-terrain, let \(InvertDEM=DEM\times(-1)\); (2) fill sink on the anti-terrain with the given HDPS threshold; (3) calculate the flow direction of the filled anti-terrain; (4) extract sinks based on the filled anti-terrain flow direction as the candidate peaks of the original terrain; (5) extract the watershed of anti-terrain based on the flow direction and sinks of anti-terrain, as peak's control area of the original terrain; and (6) obtain the peaks by searching the maximum elevation value of the original DEM in each peak's control area. The needed peaks and  
Figure 4: Processes of the terrain feature-point cluster extraction [33]. their control areas are finally obtained through the above processes. It is worth noting that the \"watershed extract\" here is performed on the anti-terrain data, and its results are the candidate peaks' control areas. Therefore, this step's input data and output results are different from the \"watershed extraction\" in the step 3 of Section 2.2.1. **Feature-point cluster extraction and verification**. The feature-point clusters include peaks, saddles, and runoff nodes, among which peaks are obtained by the space segmentation method described above. The saddles and runoff nodes are obtained by intersecting the ridgelines with the peak control areas' boundary and self-intersecting the peak control areas, respectively. The point cluster verification is realized by positive and negative terrain filtering. According to the geomorphological viewpoint, the peaks and saddles are the convex points of the local terrain, which should be located in the positive terrain. Correspondingly, the runoff nodes should be located in the negative terrain. Therefore, the pseudo peaks and saddles can be eliminated by positive terrain checking, and pseudo runoff nodes can be eliminated by negative terrain checking.