Document: What is Map Topology?
Created: 10/13/2004 (Tom Evans)

Map topology is a method to create shared features between feature classes. When editing features in ArcMap, you can select feature classes to participate in Map Topology. You also specify a cluster tolerance that is a threshold used to define when features are coincident across feature classes. After you create features that are shared across feature classes, any topological edits to these shared features will result in modification to both feature classes simultaneously. This is extremely helpful for keeping data layers in synch when there are coincident features across feature classes.

For example, assume you have two feature classes - one for a State Forest Boundary (polygon) and another for a Roads layer (line). Some of the State Forest Boundary edges should be exactly coincident with the Roads layer, but some of them are not.

You can create Map Topology to create shared features across two feature classes. The steps are as follows:

1) Add feature classes to table of contents

2) Start Editing (open Editor Toolbar, click Editor : Start Editing)

3) Open the Topology Toolbar, click the Map Topology icon, select the layers to participate in the topology and set the cluster tolerance

4) From the Topology Toolbar, select the Topology Edit Tool

5) Double click the location where you want to make a single coincident feature from two pre-existing features (in two different feature classes)

6) Move a vertex or make a small edit and this modification will snap the pre-existing features to the new shared feature location.

7) Any edits made to this feature with the Topology Edit tool will modify both feature classes. Any edits made using the editing tools from the Editor Toolbar will affect only a single feature class (and break the topology/sharing for that feature location).

How does Map Topology create shared features?

In the case of two feature classes with pre-existing features, the short answer is that it finds an average position for vertices across the two feature classes. Thus the new shared feature is not 'snapped' to the location of the pre-existing feature in either feature class. Rather, a new 'middle' location is created for the shared feature. See this figure.