Terrain information or topographic features can be extracted from DEM.
The following terrain information are useful for various applications of spatial analysis in GIS.
Slope and Aspect
The steepest slope(s) and the direction from the east (q
) can be computed from 3 x 3 as shown in Figure 3.16. The aspect that is, the slope faced to azimuth is 180°
opposite to the direction of q (see Figure 3.15).
Convex and Concave
Convex and concave of the terrain shape are represented by the second order differentials, that is computed by Laplacian operator (see Figure 3.17). Convex is positive Laplacian while concave is negative.
Surface Specific points
+ is assigned if the height of the central point is higher than the one of the eight neighbors and - if lower. A peak can be detected if all the eight neighbors are lower as shown in Figure 3.18 (a), while a pit is formed if all the eight neighbors are higher as shown in Figure 3.18 (b). A pass can be extracted if the + and - alternate around the central point with at least two complete cycle as shown in the example of Figure 3.17 (c) and (d).
Drainage Network and Watershed
The lowest point out of the eight neighbors is compared with the height of the central point to determine the flow direction as shown in Figure 3.18 (a).
Accumulated count of the flow pass at a point will give the catchment area or watershed as shown in Figure 3.18 (b).
Hill Shading
The effect of hill shading on the assumption of an ideally diffused reflecting surface (called Lambertian surface) can be computed as follows.
Relative shading = cos q
where q: angle between incident light vector s and surface normal n as shown in Figure 3.19.
For human psychological recognition of relief effect, the effect of hill shading is performed with the incident light of 45 elevation angle from the North West, which results in brighter (larger cos q ) North West face surface and darker (smaller cos q ) at South East face surface. The detail is described in 3-8.
Various extracted terrain information are demonstrated in the front pages