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About topology

Release 9.1
Last modified January 25, 2006
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NOTE: You can create simple, temporary topological relationships between features in ArcView. Creating or editing geodatabase topology requires an ArcEditor or ArcInfo license.

Topology has historically been viewed as a spatial data structure used primarily to ensure that the associated data forms a consistent and clean topological fabric. With advances in object-oriented GIS development, an alternative view of topology has evolved. The geodatabase supports an approach to modeling geography that integrates the behavior of different feature types and supports different types of key relationships. In this context, topology is a collection of rules and relationships that, coupled with a set of editing tools and techniques, enables the geodatabase to more accurately model geometric relationships found in the world.

Topology, implemented as feature behavior and rules, allows a more flexible set of geometric relationships to be modeled than topology implemented as a data structure. It also allows topological relationships to exist between more discrete types of features within a feature dataset. In this alternative view, topology may still be employed to ensure that the data forms a clean and consistent topological fabric, but also more broadly, it is used to ensure that the features obey the key geometric rules defined for their role in the database.

Why use topology?

Topology is used most fundamentally to ensure data quality and allow your geodatabase to more realistically represent geographic features. A geodatabase provides a framework within which features can have behavior such as subtypes, default values, attribute domains, validation rules, and structured relationships to tables or other features. This behavior enables you to more accurately model the world and maintain referential integrity between objects in the geodatabase. Topology may be considered an extension of this framework for behavior that allows you to control the geometric relationships between features and maintain their geometric integrity. Unlike other feature behavior, topology rules are managed at the level of the topology and dataset, not for individual feature classes.

How do I work with topology?

Different people work with topology in different ways, depending upon their role in an organization and its GIS design and management work flow.

Initially, creating a topology requires a geodatabase designer. A topology organizes the spatial relationships between features in a set of feature classes. The designer analyzes an organization's data modeling needs, identifies the key topological relationships required in the geodatabase, and defines the rules that will constrain different features' topological relationships.

Once the participating feature classes have been added to the topology and the rules defined, the topology is validated. Data quality managers use the topology tools to analyze; visualize; report; and, where necessary, repair the spatial integrity of the database after it is initially created, as well as after editing. Topology provides these users with a set of validation rules for the topologically related features. It also provides a set of editing tools that let users find and fix integrity violations.

As the geodatabase is used and maintained, new features are added, and existing features are modified. Data editors update features in the geodatabase and use the topology tools to construct and maintain relationships between features within the constraints imposed by the database designer. Depending on the work flow of the organization, the topology may be validated after each edit session or on a schedule.
You can also impose a map topology on your data. A map topology allows you to simultaneously edit simple features that overlap or touch each other. You can create a map topology with an ArcView license (creating a geodatabase topology requires an ArcInfo or ArcEditor license) and apply a map topology to simple features in a shapefile or to simple feature classes in a geodatabase.

Follow these links to learn how to work with topology in ArcMap:

Basic topology tasks
Creating a map topology
Editing features in a topology
Editing shared geometry
Correcting topology errors
Finding features with shared topology elements
Clearing selected topology elements
Changing the symbology for topology features
Making new features with topology tools

Follow these links to learn more about geodatabase topology:

An overview of topology in the geodatabase
About creating a topology
Creating a new topology

Geometric elements of a topology

When you create a topology, you specify the feature classes that participate in the topology. These feature classes may have point, line, or polygon features in them. In the topology, the geometric relationships are between the parts of the features rather than the features themselves. Polygons in a topology have:

Similarly, line features are made up of an edge, at least two nodes that define the endpoints of the edge, and vertices that define the shape of the edge. Point features behave as nodes when they are coincident with other features in a topology.

When features in the topology have parts that intersect or overlap, the edges and nodes that define these parts are shared. You can use the Topology Edit tool Topology Edit tool to move nodes and whole edges that are shared between features or to move the vertices that define the shape of shared edges.

When you move nodes or vertices, you can choose whether you want the segment between the vertex and the closest vertex to be stretched or whether you want the whole edge to be proportionately stretched.

You can also temporarily add new topology nodes to split edges. This simply splits the edge for the topology; it does not break the feature into two features. This can be useful when you want to move one part of an edge without affecting other parts of the edge or when you want to create a new node to snap to.

You can find out which features share a given topology element and control whether or not the geometry should be considered shared with the Show Shared Features tool.

If two or more features share an edge or node, you can use the Show Shared Features tool Show Shared Features tool to turn off geometry sharing for one or more of the features. Changes you make to that topology edge or node with the Topology Edit tool will only affect the features for which the geometry is still shared.

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