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Topology is a collection of rules that, coupled with a set of editing tools and techniques, enable the geodatabase to more accurately model geometric relationships. ArcGIS implements topology through a set of rules that define how features may share a geographic space and a set of editing tools that work with features that share geometry in an integrated fashion. A topology is stored in a geodatabase as one or more relationships that define how the features in one or more feature classes share geometry. The features participating in a topology are still simple feature classes—rather than modifying the definition of the feature class, a topology serves as a description of how the features can be spatially related.
Why topology?
Topology has long been a key GIS requirement for data management and integrity. In general, a topological data model manages spatial relationships by representing spatial objects (point, line, and area features) as an underlying graph of topological primitives—nodes, faces, and edges. These primitives, together with their relationships to one another and to the features whose boundaries they represent, are defined by representing the feature geometries in a planar graph of topological elements.
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 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.
Ways that features share geometry in a topology
Features can share geometry within a topology. Here are some examples among adjacent features:
- Area features can share boundaries (polygon topology).
- Line features can share endpoints (edge-node topology).
In addition, shared geometry can be managed between feature classes using a geodatabase topology, for example:
- Line features can share segments with other line features.
- Area features can be coincident with other area features. For example, parcels can nest within blocks.
- Line features can share endpoint vertices with other point features (node topology).
- Point features can be coincident with line features (point events).
Two views: Features and topological elements
A layer of polygons can be described and used in the following ways:
- As collections of geographic features (points, lines, and polygons)
- As a graph of topological elements (nodes, edges, faces, and their relationships)
This means that there are two alternatives for working with features—one in which features are defined by their coordinates and another in which features are represented as an ordered graph of their topological elements.