# An Integrated Algorithm for Extracting Terrain Feature-Point Clusters Based on DEM Data
## 2 Materials and Methods  
### Study Area and Data  
The study area is Shaanxi Province (105\({}^{\circ}\)29\({}^{\prime}\)E to 111\({}^{\circ}\)15\({}^{\prime}\)E, 31\({}^{\circ}\)42\({}^{\prime}\)N to 39\({}^{\circ}\)35\({}^{\prime}\)N), which is located in the hinterland of northwest China, with a land area of 205,700 km\({}^{2}\). The northern mountains and the Qinling Mountains divide Shaanxi Province into three natural regions: the northern part is the northern Shaanxi plateau, including the wind-sand landscape and the Loess Plateau (including loess tablelands, hills and ridges), with sparse precipitation and serious soil erosion; the central part is the Guanzhong Plain, which is high in the west and low in the east, and is the urban agglomeration in the province with strong human activities; and the southern part is the Qinba Mountains, which has the landform structure of \"two mountains sandwiching one river\", and there are serious natural hazards such as landslides and flash flooding. The various geomorphic differentiation causes the spatial heterogeneity of natural disasters and human disturbances, providing a case basis for the study of terrain point clusters and geomorphic differentiation (Figure 2a).  
In the selection of experimental sample areas, considering various factors such as the representativeness of the experimental sample areas, the richness of landform types and the existing research bases, the six counties of Hanyin (HT), Mizhi (MZ), Ningshan (NS), Yanchuan (YC), Zhen'an (ZA) and Zhenba (ZB) were selected as typical sample areas in this study. These six sample areas represent different periods of loess landform evolution to a certain extent and cover the main landform types in Shaanxi Province [38]. The overview of these typical sample areas is shown in Table 1 and Figure 2b.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample Area Name** & **Landform Type** & **Geomorphological Region** \\ \hline Ningshan County & Qinling middle and high mountains & Qinling Mountains \\ Zhen’an County & Qinling middle mountains & Qinling Mountains \\ Zhenba County & Daba middle mountains & Daba Mountains \\ Hanyin County & Low hills and mountains & Hanzhong low hill and basin area \\ Yanchuan County & Loess ridge & Loess Plateau \\ Mizhi County & Loess hill & Loess Plateau \\ \hline \hline \end{tabular}
\end{table}
Table 1: Overview of the typical sample areas.  
Figure 2: Study area and typical sample areas: (**a**) the overview of the study area; (**b**) the locations of the six typical sample areas in the DEM imageries.  
In this study, the DEM data are Shuttle Radar Topography Mission (SRTM) DEM, which were jointly measured by the National Aeronautics and Space Administration (NASA) and the US National Geospatial-Intelligence Agency (NGA), and covered almost 80% of the land surface area (from 60 N to 60 S). This time, we use the SRTM1 V3 downloaded from the United States Geological Survey (USGS) as the experimental data. The spatial resolution of these data is one arcsecond (30 m along the equator) with a nominal planimetric accuracy of \(\pm\)20 m and an elevation accuracy of \(\pm\)16 m, and the WGS84 coordinate system and EGM96 elevation datum (orthometric) are adopted. The study area and DEM data are shown in Figure 2.