# Soil Depth Prediction Model Using Terrain Attributes in Gangwon-do, South Korea
## 2 Methodology
### Statistics Analysis  
#### 2.2.1 Correlation Analysis  
Public drilling data from the Geotechnical Information DB System is used to acquire soil depth data of the study area. This database includes drilling information for 332,889 holes in Korea [51]. In order to secure the reliability of variables which are overly sensitive to the streamline among TAs, data near the streamline were excluded. In addition, too old data (produced before 2000) or data, in which location information of digital maps and boring test data is inconsistent were excluded, since it can be considered an outlier.  
[PERSON] [41] used the soil depth ratio (SR = soil depth/slope height) derived by the slope height for representative rock types in Korea for a total of 373 slopes. It was evaluated and reported that the average soil depth ratio (SR) was 0.09 for sedimentary rocks, 0.17 for metamorphic rocks, and 0.3 for igneous rocks. As such, the weathering depth will be different depending on the rock types, and the soil depth will also be different for this reason. Therefore, in this study, the Boring Test data in study area was classified into igneous, metamorphic, and sedimentary rocks, which are representative rock types in Korea, and the correlation between soil depth and TAs are analyzed. After the excluding outliers, a total of 297 sites were obtained. They were classified into 101 sites for igneous rock, 101 sites for metamorphic rock, and 95 sites for sedimentary rock. Descriptive statistics for soil depth by representative rock types are shown in Figure 2. The used soil depth data range from 0.2 to 0.68 m, and the average depth ranges from 2.63 to 3.03 m. In particular, the sedimentary rock data is distributed relatively wide compared to the data for the other two types of rock.  
Correlation analysis is performed between the soil depth of the study area and the calculated TAs. The correlation analysis results for each of the representative rock types are shown in Figures 3-5 in the form of a correlation matrix. It shows the histograms of each parameter along the central diagonal, the scatter diagram for each pair of parameters in the lower triangle, and each Pearson correlation coefficient and significance level (\(p\)-value) is shown in the upper triangle. The results of the correlation analysis indicate that the correlation between soil depth and SLOPE is the highest in all three rock types, and the correlation between Curvature and SCA (Specific catchment area) is relatively low.  
Figure 1: Location of study area and distribution of soil depth sampling sites.  
Figure 3: Correlation matrix for igneous rock (* Significant at the 0.05 probability level. ** Significant at the 0.01 probability level).  
Figure 2: Descriptive statistics on three representative rock types sampled in the study area (Gangwon region).  
Figure 4: Correlation matrix for metamorphic rock (*: Significant at the 0.05 probability level. **: Significant at the 0.01 probability level).  
Figure 5: Correlation coefficient for Sedimentary rock (*: Significant at the 0.05 probability level. **: Significant at the 0.01 probability level).