Topology, or overlay analysis shows the spatial relationships between two features – do they touch each other? Is one feature contained by another? What do they have in common? Bentley Map provides tools to investigate these relationships, and can perform a variety of overlay operations to find the answers.

The result is what is called an overlay feature. When you create an overlay between two polygons, a new layer is created containing the resulting graphical elements. In addition, the properties of both contributing layers are copied to the overlay layer. This allows you to edit and report on the overlay features using Bentley Map tools, such as the Data Browser.

Only two layers can be overlaid at one time.

When both layers are of type Polygon, the Polygon Operation section is active in the dialog.  The following polygon operation options are available:

Intersect: Only areas that are inside both the layers will be saved	Union: All the areas of both layers will be saved.
Subtract: The second layer will be subtracted from the first layer and the resulting areas saved.	Exclusive Or: The areas that are not common to both layers will be saved.

To perform overlay analyses with two polygons:

In the Map Manager, select the two layers to compare.

1. Right-click on the layer and select Overlay

2. The Topology Analysis dialog opens and the selected layer or layers are automatically added to the Layers to be overlaid list on the right side of the dialog.
3. Use the arrow buttons to select other layers, or change the combination of layers to be compared.

4. By selecting the operation to perform on the layers, you can see the difference in the results. The topology icon changes depending on the results that are produced.

5. Type a name for the overlay in the Result field. Click Ok. The results are added as a new feature class to the Map Manager.

Data Browser showing individual layers selected for overlay analysis:

        Municipalities Feature Properties                         Hydrography Feature Properties

Data Browser showing the results of the overlay layer.  Note the combined properties.