3.2.2. Topographic Factors

Topographic factors are most frequently used in LSM. In this study, several topographic factors were created using a 12.5-m digital elevation model (DEM) obtained from the Alaska Satellite Facility (ASF) [30], which included altitude, slope gradient, slope aspect, and curvature information. Because of variations in temperature, humidity, and vegetation, the degree and type of weathering also varied with altitude. Therefore, altitude has been employed as an LCF in previous studies [8,13,15]. Considering the setting of geomorphology and landforms present in Kenya, the altitude factor was categorized as (1) <50 m, (2) 50–200 m, (3) 200–500 m, (4) 500–1000 m, (5) 1000–2000 m, and (6) >2000 m (Figure 3b). Steepness directly affects slope stability because slopes become more susceptible to landslides as the slope gradient increases. Reviews of LSM studies have suggested that the slope gradient factors is usually categorized using a 5◦ interval. Thus, the slope LCF was categorized as follows: <sup>&</sup>lt;5◦, 5–10◦, 10–15◦, 15–20◦, 20–25◦, 25–30◦, 30–35◦, 35–40◦, and >40◦ (Figure 3c). Since parameters such as sunlight, precipitation, and vegetation cover vary, slope aspects may also have an effect on landslide occurrences. Consistent with previous studies [15,16,31], the aspect factor was classified into 9 subclasses, as shown in Figure 3d. By controlling the water flow and erosion type in curved terrains, curvature is also a commonly used topographic factor that is associated with landslides. A positive curvature value indicates an upwardly convex terrain, while a negative value indicates an upwardly concave terrain. Terrains with values of zero for the curvature factor were classified as flat (Figure 3e).
