Spatial Thinking in Planning Practice: An Introduction to GIS

25 Source: ESRI Help : http://webhelp.esri.com/arcgisserver/9.3/java/index.htm#geodatabases/topology_basics. htm Galdi (2005) describes the very speci!c rules that de!ne the relations of entities in the vector database: 1. Every edge must be bounded by two nodes (start and end nodes). 2. Every edge has a le$ and right face. 3. Every face has a closed boundary consisting of an alternating sequence of nodes and edges. 4. "ere is an alternating closed sequence of edges and faces around every node. 5. Edges do not intersect each other, except at nodes. Compliance with these topological rules is an aspect of data quality called logical consistency. In addition, the boundaries of geographic areas that are related hierarchically — such as blocks, block groups, tracts, and coun- ties - are represented with common, non-redundant edges. Features that do not conform to the topological rules can be identi!ed automatically, and corrected. Topology is fundamentally used to ensure data quality of the spatial relationships and to aid in data compilation. Topology is also used for analyzing spatial relationships in many situations such as dissolving the boundaries between adjacent polygons with the same attribute values or traversing along a network of the elements in a topology graph. Topology can also be used to model how the geometry from a number of feature classes can be integrated. Some refer to this as vertical integration of feature classes. Generally, topology is employed to do the following: % Manage coincident geometry (constrain how features share geometry). For example, adjacent polygons, such as parcels, have shared edges; street centerlines and the boundaries of census blocks have coincident geometry; adjacent soil polygons share edges; etc. % De!ne and enforce data integrity rules (such as no gaps should exist between parcel features, parcels should not overlap, road centerlines should connect at their endpoints). Chapter 3: Topology and Creating Data