# Data Mining Approaches for Landslide Susceptibility Mapping in Unyeonsan, Seoul, South Korea
## 3 Data  
Given that data mining models are a data-driven methodology, accurate data are essential to build a suitable database. Accurate data on the state of the earth's surface can be obtained rapidly using aerial photographs, which also effectively capture complicated urban structures. Moreover, despite the high cost of aerial photographs, it is cost-effective to acquire the images of and continuously monitor urban areas due to their dense nature. To acquire accurate data on the landslide occurrence, aerial photographs from before and after the Umyeonsan landslides were used (Figure 2a,b). Location data for the Umyeonsan landslide area were acquired through comparative analysis, as traces of landslides are visible for several years after the events. In addition, a digital topographic map (1:5000) generated by the National Geographic Information Institute (NGII) was obtained, and geometric correction was performed. Overlay and comparative analysis of the aerial photographs and the 1:5000 digital topographic map indicated the landslide occurrence area (Figure 2c). The mapped landslide occurrence data were finally confirmed by comparison with data surveyed in the field by the Seoul metropolitan government [39]. Figure 2: Cont. Slopes are established by erosion and sedimentation of terrain surfaces and have a key effect on landslide occurrence as they affect the flow of water and the soil characteristics [42]. Therefore, among the factors influencing landslides, topographic factors were preferentially considered and were used as input data to identify the relationships with landslide occurrence (Figure 3). Prior to calculation of the topographic factors, a triangulated irregular network (TIN) was produced from a pre-generated topographic map, and a digital elevation model (DEM) was created using ArcGIS 10.1 (ESRI, Redlands, LA, USA). All topographic factors were calculated from the DEM using ArcGIS software (ESRI, Redlands, LA, USA). The slope, indicated as the angle to the horizontal plane or the ratio of the vertical height to the horizontal distance as a percentage, was calculated using the SLOPE tool. The ASPECT tool was used to calculate the aspect, the compass direction of the slope faces. The curvature values of the terrain surface were calculated using the CURVATURE tool. The curvature represents the morphological characteristics of the study area; an upwardly convex surface possesses a positive value, while an upwardly concave surface possesses a negative value. Figure 2: Data of landslide occurrence locations from aerial photographs: (**a**) before landslides in 2011; (**b**) after landslides in 2011; (**c**) training and validation data; (**d**) damage situation in Southern Circular Road [40]; (**e**) Gyungnam apartment buildings in Seocho-gu [41]. Figure 3: Topographical factors related to landslides used to construct a spatial database: (**a**) slope gradient; (**b**) slope aspect; (**c**) curvature; (**d**) topographic wetness index (TWI); (**e**) stream power index (SPI); (**f**) slope length factor (SLF); (**g**) standardized height; (**h**) valley depth; (**i**) downslope distance gradient. The topographic wetness index (TWI) is defined for a steady state. The index is commonly used for hydrological processes to quantify the disposition of water flow and the impact on the topography [43]. The TWI is calculated by the run-off model as follows, which reflects the accumulation of water at a specific point in the catchment area and the force of gravity on the water in the area [43]. \[TWI=In\bigg{(}\frac{a}{\tan\beta}\bigg{)} \tag{1}\]  
where \(\alpha\) is the cumulative upslope area drained per unit contour length in a cell, and \(\beta\) is the local slope angle of the cell. The stream power index (SPI) estimates the erosion power of a stream, which affects the stability of the area. Therefore, it is generally used to determine the locations for performing soil conservation measures to reduce the erosive effects of concentrated surface runoff.