Creating map topology

A map topology is created during an edit session in ArcMap. You specify which feature classes in a single geodatabase you want to include and, if necessary, change the cluster tolerance.

The cluster tolerance defines how close together features must be in order to be considered coincident. The default cluster tolerance is the minimum possible cluster tolerance based on the precision of your data.

Once you have created the map topology, spatial relationships are discovered on the fly for the current map extent when you build the topology cache. The geometric relationships in a topology are between the parts of the features rather than the features themselves. Therefore, when you edit features in a topology, the types of geometry that are acted on are edges, nodes, and pseudo-nodes.

The shared border between the countries of Chad and Sudan is treated as one element (an edge) in a topology. The point where the borders of Chad, Sudan, and Libya come together is also a single element (a node) in a topology.

With a map topology, the spatial relationships discovered during an edit session are temporary. They do not persist after you stop editing.

How are shared feature parts stored in a geodatabase?

In a map topology or geodatabase topology, line and polygon topology consists of features with coincident segments. Each feature stores all of the coordinates for itself; therefore, the geometry for the coincident segments is stored twice in the geodatabase.

In this example, the two polygons are adjacent because they both have a segment that is made up of coordinates 100.50, 200.50 and 100.50, 220.00. Each coordinate pair is stored twice in the geodatabase.

Point features behave as nodes when they're coincident with other features (and an x,y coordinate pair is stored for each feature).