## 3 Modeling Methodology
### Data Collection and Preparation
#### 3.1.2 Landslide Conditioning Factors  
Modeling of landslide susceptibility is the process of investigating the associations between historical landslides and a suit of explanatory variables known as landslide conditioning factors that typically characterize the geomorphology, hydrology, climate, and anthropogenic conditions of the research area [49; 53]. For modeling the landslide susceptibility in the Nam Dan commune, ten conditioning factors were selected and used: slope aspect, slope degree, elevation, curvature, topographic wetness index (TWI), weathering crust, river density, fault density, distance from roads, and geomorphology. Maps of these factors are presented in Figures 3 and 4. Currently, the development of data processing techniques and the data-sharing policies of companies, such as NASA, has facilitated the collection of such conditioning factors. Slope aspect is a widely used topography factor for landslide susceptibility mapping because this factor has a relationship with many other factors, such as solar radiation and rainfall amount, that affect landsl [54]. Slope degree is a significant factor for modeling landslide susceptibility due to its weighty effects on the sliding of material and flow of water [55]. Elevation characterizes local climate condition and the resistance of the slope-forming  
Figure 2: Flowchart of the modeling methodology proposed for the Nam Dan commune.  
materials to failure [56]. Curvature affects the accumulation and flow of water on terrain surface, thus influencing the probability of landslide occurrences [57]. TWI was selected as another conditioning factor because this factors in the relationships between topography and moisture [58]. The thematic map for these five topography factors was derived from a 10-m Digital Elevation Model (DEM) that was constructed using 1:10,000 topographic maps. A weathering crust was selected in this study to represent the difference between the weathering conditions of the rocks that in turn influence the probability of landsl [59]. Data for the weathering crust were derived from the 1:10,000 geomorphology map, prepared by the SRV-10/0026 project. Rivers generally adversely affect the stability of slopes due to erosion and scouring of the side slopes of the valley and saturation of the groundmass within flood height [60,61]. Therefore, the inclusion of the information related to rivers is vital for landslide modeling and mapping. In this study, the rivers of the Nam Dan commune were derived from the DEM and used to generate a thematic map of river density. Faults induce instability in the ground and rock mass that typically lead to landslides [62,63]. In this study, faults were interpolated from the SAR images (Cosmos Skymed with descending orbit, band X, 3m resolution) and used to generate a thematic map of fault density for the Nam Dan commune. Road construction is the main causes of landslide occurrences in many regions [64]. A thematic map of distance to roads is usually used as a proxy for explaining the effects of human activities on landsl. In this study, the road networks of the Nam Dan commune were derived from a topographic map with a scale of 1:10,000. Geology is another important factor for landslide susceptibility modeling because this factor reflects the rockmass/groundmass condition of the Nam Dan commune for the stability analysis [65]. In this study, geological units of the Nam Dan commune were identified using the geological map of the General Department of Geology and Minerals of Vietnam.  
Figure 3: Thematic map of the landslide conditioning factors selected for the Nam Dan commune.