8.6 Terrain Mapping for Spatial Analysis
Surface analysis is often referred to as terrain (elevation) analysis when information related to slope, aspect, viewshed, hydrology, and volume are calculated on raster surfaces such as DEMs (digital elevation models; Chapter 5 “Geospatial Data Management,” Section 5.3.1 “Vector File Formats”). In addition, surface analysis techniques can also be applied to more esoteric mapping efforts, such as the probability of tornados or the concentration of infant mortalities in a given region. This section discusses a few methods for creating surfaces and standard surface analysis techniques related to terrain datasets.
Several standard raster-based neighborhood analyses provide valuable insights into the surface properties of the terrain. For example, slope maps (part (a) of Figure 8.12 “(a) Slope, (b) Aspect, and (c and d) Hillshade Maps”) are excellent for analyzing and visualizing land-form characteristics and are frequently used in conjunction with aspect maps (defined later) to assess watershed units, inventory forest resources, determine habitat suitability, estimate slope erosion potential, and more. They are typically created by fitting a planar surface to a 3-by-3 moving window around each target cell. When dividing the horizontal distance across the moving window (determined via the spatial resolution of the raster image) by the vertical distance within the window (measured as the difference between the most significant cell value and the central cell value), the slope is relatively easily obtained. The output raster of slope values can be calculated as either percent slope or degree of slope.
Any cell that exhibits a slope must, by definition, be oriented in a known direction. This orientation is referred to as an aspect. Aspect maps (part (b) figure 8.12 “(a) Slope, (b) Aspect, and (c and d) Hillshade Maps”) use slope information to produce output raster images whereby the value of each cell denotes the direction it faces. This is usually coded as either one of the eight ordinal directions (north, south, east, west, northwest, northeast, southwest, southeast) or in degrees from 1° (nearly due north) to 360° (back to due north). Flat surfaces have no aspect and are given a value of −1. To calculate the aspect, a 3-by-3 moving window is used to find the highest and lowest elevations around the target cell. For example, if the highest cell value is located at the top-left of the window (“top” being due north) and the lowest value is at the bottom-right, it can be assumed that the aspect is southeast. The combination of slope and aspect information is of great value to researchers such as botanists and soil scientists because sunlight availability varies widely between north-facing and south-facing slopes. Indeed, the various light and moisture regimes resulting from aspect changes encourage vegetative and edaphic differences.
A hillshade map (part (c) of Figure 8.12 “(a) Slope, (b) Aspect, and (c and d) Hillshade Maps”) represents the illumination of a surface from some hypothetical, user-defined light source (presumably, the sun). Indeed, the slope of a hill is relatively brightly lit when facing the sun and dark when facing away. Using the surface slope, aspect, angle of incoming light, and solar altitude as inputs, the hillshade process codes each cell in the output raster with an 8-bit value (0–255), increasing from black to white. As you can see in part (c) of Figure 8.12, “(a) Slope, (b) Aspect, and (c and d) Hillshade Maps,” hillshade representations are an effective way to visualize the three-dimensional nature of land elevations on a two-dimensional monitor or paper map. Hillshade maps can also be used effectively as a baseline map when overlaid with a semitransparent layer, such as a false-color digital elevation model (DEM; part (d) of Figure 8.12 “(a) Slope, (b) Aspect, and (c and d) Hillshade Maps”).
Viewshed analysis is a valuable visualization technique that uses the elevation value of cells in a DEM or TIN (Triangulated Irregular Network) to determine those areas that can be seen from one or more specific location(s) (part (a) of Figure 8.13 “(a) Viewshed and (b) Watershed Maps”). The viewing location can be either a point or line layer and placed at any desired elevation. The output of the viewshed analysis is a binary raster that classifies cells as either 1 (visible) or 0 (not visible). For example, in the case of two viewing locations, the output raster values would be 2 (visible from both points), 1 (visible from one point), or 0 (not visible from either point).
Additional parameters influencing the resultant viewshed map are the viewing azimuth (horizontal and vertical) and viewing radius. The horizontal viewing azimuth is the horizontal angle of the view area and is set to a default value of 360°. However, the user may want to change this value to 90° if, for example, the desired viewshed included only the area seen from an office window.
Similarly, the vertical viewing angle can be set from 0° to 180°. Finally, the viewing radius determines the distance from the viewing location to be included in the output. This parameter is typically set to infinity (functionally, this includes all areas within the DEM or TIN under examination). However, it may be decreased if, for instance, you only wanted to include the area within a radio station’s 100 km broadcast range.
Similarly, watershed analyses are a series of surface analysis techniques that define the topographic divides that drain surface water for stream networks (part (b) of Figure 8.13 “(a) Viewshed and (b) Watershed Maps”). In geographic information systems (GISs), a watershed analysis is based on a “filled” DEM input. A filled DEM contains no internal depressions (as seen in a pothole, sink wetland, or quarry). A flow direction raster is created to model the direction of water movement across the surface from these inputs. A flow accumulation raster calculates the number of cells contributing to each cell from the flow direction information. Cells with a high flow accumulation value represent stream channels, while cells with low flow accumulation represent uplands. A network of rasterized stream segments is created. These stream networks are based on some user-defined minimum threshold of flow accumulation. For example, it may be decided that a cell needs at least one thousand contributing cells to be considered a stream segment. Altering this threshold value will change the density of the stream network. Following the creation of the stream network, a stream link raster is calculated whereby each stream segment (line) is topologically connected to stream intersections (nodes). Finally, the flow direction and stream link raster datasets are combined to determine the output watershed raster as seen in part (b) of Figure 8.13 “(a) Viewshed and (b) Watershed Maps” (Chang, 2008). Such analyses are invaluable for watershed management and hydrologic modeling.
