About creating a map topology
A map topology is a topology that you can impose on simple features on a map during an edit session. Although you cannot 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, Modify Edge, and Reshape Edge tools on the Topology toolbar 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 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:
- Specify which feature classes on the map will participate in the topology.
- Specify a cluster tolerance. The cluster tolerance is a distance within which features will be coincident.
When you have defined a map topology, it is shown on the Topology toolbar in the Topology drop-down list as an entry called <Map Layers>. Only one map topology can be defined for an edit session.
Editing a map 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.
Map topology versus geodatabase 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 the Catalog window or 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. You do not specify any topology rules for a map topology. Since there are no topology rules, there is no need to validate a map topology, and there is no creation of error features.
When to use a map topology
Map topology is useful because it enables you to perform topological editing in situations where you cannot use geodatabase topology:
- Map topology can be defined for shapefiles, which cannot participate in a geodatabase topology.
- If you are using an ArcView license, you cannot create or edit geodatabase topology, but you can define map topology for any data on which you can start an edit session (such as shapefiles and simple feature classes in file and personal geodatabases). You cannot create map topology on feature classes that participate in a geodatabase topology because you need an ArcEditor or ArcInfo license to edit them.
- If you are using an ArcEditor or ArcInfo license, you can create a map topology for any shapefile or feature class data, including feature classes that participate in a geodatabase topology. For example, you might want to perform topological editing on two feature classes that participate in two separate geodatabase topologies. In this case, you can define a map topology containing both of these feature classes.