Creating a map topology
Last modified August 17, 2007
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A map topology is a simple topology that you can impose upon simple features on a map during an edit session. Although you can't create or edit geodatabase topologies with ArcView (only ArcEditor and ArcInfo), you can create and edit map topologies in ArcView.
A map topology allows you to simultaneously edit simple features that overlap or touch each other in ArcMap. You can use the Topology Edit tool on the Topology toolbar and the Modify Edge and Reshape Edge edit tasks to edit the features in a map topology. The features can be in one or more feature classes and may have different geometries. Line features and the outlines of polygon features become topological edges when you create a map topology. Point features, the endpoints of lines, and the places where edges intersect become nodes.
A map topology can be applied to simple features in a shapefile or to simple feature classes in a geodatabase. The feature classes that participate in the map topology must be in the same folder or geodatabase. A map topology cannot be applied to feature classes that participate in a geometric network.
A map topology creates topological relationships between the parts of features that are coincident. You can specify the feature classes that you want to participate in the map topology. You can also choose the distance, or cluster tolerance, that defines how close together edges and vertices must be in order to be considered coincident. When you create a map topology, the cluster tolerance that you specify is used to determine which parts of the features are coincident and which edges and nodes in the topology are shared. The cluster tolerance is typically a small actual ground distance. Setting large cluster tolerances can result in features being collapsed or distorted when vertices with a given feature snap together.
There are two steps to creating a map topology:
Geodatabase topology is a data object created and stored in a geodatabase. A geodatabase topology defines a set of rules about the relationships between feature classes in a feature dataset. Geodatabase topology is created in ArcCatalog and can be added to ArcMap as a layer, just like any other data. After editing has been performed on the feature classes, you validate the geodatabase topology to see if the edits break any of the topology's rules. Any errors can be fixed or marked as exceptions. An ArcEditor or ArcInfo license is required to create, edit, or validate geodatabase topology. To use geodatabase topology when you edit data from a geodatabase, the geodatabase topology in which the data participates must be in your map. If you are using an ArcView license, you can open a map containing a geodatabase topology, but you cannot start an edit session on feature classes that participate in a geodatabase topology.
Map topology is temporary and only lasts for the duration of your edit session. Unlike geodatabase topologies, map topologies are not stored permanently or represented as layers in the map. Map topologies have no rules and there is no validation process. The tools and commands for validating topology and fixing errors are disabled in the Topology toolbar when you work with a map topology. In previous versions of ArcGIS, map topology functionality was invoked using the Integrate command. This command has been replaced with the Map Topology command.
Map topology is useful because it enables you to perform topological editing in situations where you can't use geodatabase topology:
After you create a map topology, you can use the Topology Edit tool to edit the edges and nodes shared by the features. Editing an edge or node shared by two or more different features results in each feature being modified. This lets you move a border to update two forest polygons or move a corner vertex and update several parcel polygons and a few lot boundaries at the same time.
You do not specify any topology rules for a map topology. All edges or vertices of features in the map topology that fall within the cluster tolerance are considered to be topologically shared. You edit shared edges and vertices in a map topology in the same way and with the same tools as you would edit a geodatabase topology. Since there are no topology rules, there is no need to validate a map topology, and there is no creation of error features.
At the geometry level, topologies are about simple relationships—such as coincidence, covering, and crossing—between the geometric primitives that make up features. While all simple feature class geometries (point, line, polygon) may participate in topologies internally, the types of geometry that are acted on when editing a topology are: