Spatial Thinking in Planning Practice: An Introduction to GIS

22 Figure 3.1. "e geographic primitives include nodes, edges, and faces. Source: Department of Geography, "e Pennsylvania State University. Adapted from DiBiase (1997). "e following illustration shows how a layer of polygons can be described and used: % As collections of geographic features (points, lines, and polygons) % As a graph of topological elements (nodes, edges, faces, and their relationships) Figure 3.2. Relationship between geographic feature and the topological elements. Source: ESRI Help: http:// webhelp.esri.com/arcgisserver/9.3/java/index.htm#geodatabases/topology_basics.htm TOPOLOGICAL RELATIONSHIPS We have learned how coordinates, both geometric and geographic, can de!ne points and nodes, how nodes can build edges, and how edges create faces. We will now consider how nodes, edges, and faces can relate to one another through the concepts of containment, connectedness, and adjacency. A fundamental property of all topological relations is that they are constant under continuous deformation: re-projecting a map will not alter Chapter 3: Topology and Creating Data