Spatial Thinking in Planning Practice: An Introduction to GIS

50 spot elevations on the ground, usually in feet or meters. Care must be taken when using grid-based DEMs due to the enormous volume of data that accompanies these !les as the spatial extent covered in the image begins to increase. DEMs are referred to as digital terrain models (DTMs) when they represent a simple, bare-earth model and as digital surface models (DSMs) when they include the heights of landscape features such as buildings and trees. From the elevation data in each pixel of the raster DEM layer, you are able to produce output layers to portray slope (inclination), aspect (direction), and hillshading (Figure 9.5). "ese topographic functions are typical neighborhood processes; each pixel in the resultant layer is a product of its own elevation value as well as those of its surrounding neighbors. % Slope layers exhibit the incline or steepness of the land. It is the change in elevation over a de!ned distance. % Aspect is the compass direction in which a slope faces. From north, it is usually expressed clockwise from 0 to 360 degrees. % Hillshading , which is cartographically called shaded relief, is a lighting e#ect which mimics the sun to high- light hills and valleys. Some areas appear to be illuminated while others lie in shadows. Figure 9.5. Topographic Functions. "e DEM creates the slope, aspect, and hillshading layers. While these functions are raster processes, most can be mimicked in a vector environment by Triangulated Irregular Networks (TIN). In addition, topographic functions can derive vector isolines (contours). Source: GIS Commons (http://giscommons.org/analysis/ ) CONNECTIVITY ANALYSIS Connectivity analyses use functions that accumulate values over an area traveled. Most o$en, these include the analysis of surfaces and networks. Connectivity analyses include network analysis, spread functions, and vis- ibility analysis. "is group of analytical functions is the least developed in commercial GIS so$ware, but this situation is changing as commercial demand for these functions is increasing. Vector-based systems generally focus on network analysis capabilities. Raster-based systems provide visibility analysis and sophisticated spread function capabilities. SPREAD FUNCTIONS )SURFACE ANALYSIS* Spread functions are raster analysis techniques that determine paths through space by considering how phe- nomena (including features) spread over an area in all directions but with di#erent resistances. You begin with an origin or starting layer (a point where the path begins) and a friction layer, which represents how di&cult— how much resistance—it is for the phenomenon to pass through each cell. From these two layers, a new layer Chapter 9: Raster Data Models