Exercise 8: Creating a topology

Complexity: Beginner Data Requirement: ArcGIS Tutorial Data Setup Goal: Define a topology to define the spatial relationships between two feature classes.

In Exercise 5: Building a geometric network, you created a geometric network. A geometric network is a specialized type of topological relationship that allows network tracing, analysis, and editing. In this exercise, you will create a geodatabase topology. A geodatabase topology allows you to specify rules that control the spatial relationships of features in a dataset. There are a variety of topology rules that you can apply to your data, depending on your organization's requirements. You will only apply two topology rules to this dataset.

Creating a topology

You will create the topology to regulate two types of spatial relationships in this dataset. The first is that parcels should not overlap, and the second is that parcels that have been classified as residential must fall within blocks that are also classified as residential.

Steps:

1. Start ArcCatalog by clicking Start > All Programs > ArcGIS > ArcCatalog 10.
2. In the Catalog tree, navigate to the Landbase feature dataset in the Montgomery geodatabase.

This dataset contains several feature classes. You will create a topology using two feature classes—Parcels and Blocks.

3. Right-click the Landbase feature class, point to New, then click Topology.

   The New Topology wizard starts.

The first page provides a brief description of the wizard.

4. Click Next.

The wizard presents a default name and cluster tolerance for the topology. You will accept the default name that the wizard provides.

The default cluster tolerance is based on the XY tolerance of the Landbase dataset.

5. Type 0.01 to set the new cluster tolerance.
6. Click Next.
7. Check Blocks and Parcels.

   These feature classes will participate in the Landbase topology.

One of the topology rules that you will create will concern the Parcels feature class, and the other will be between one subtype of Parcels and one subtype of Blocks; thus, both Blocks and Parcels feature classes must participate in the topology. If one of these feature classes were already participating in another topology or a geometric network, or if they were registered as versioned in a multiuser geodatabase, it would not appear in the list of available feature classes to add to this topology.

8. Click Next.

The next page of the wizard allows you to set the number of topology ranks and the rank of each feature class in the topology.

Ranks allow you to ensure that more accurately collected features are not snapped to the position of less accurately collected ones when the topology is validated. For example, if you were including a feature class that was collected using a survey-grade Global Positioning System (GPS) unit and a feature class digitized from a 1:1,000,000-scale source map in the same topology, you would likely assign the GPS feature class a rank of 1 and the 1:1,000,000-scale source feature class a rank of 5. If you were to validate the topology, parts of features that fell within the cluster tolerance would snap together, with the less accurate ones moving to the location of the more accurate ones. The GPS features would not be moved to the position of the 1:1,000,000-scale features.

You can assign up to 50 different ranks, with 1 being the highest rank. In this topology, you will assume that all the feature classes are based on equally accurate data, so you will not assign more than one rank. Parcels and Blocks have equivalent levels of accuracy, since the Blocks feature class was derived from the parcel features.

9. Type 1 for the number of ranks.
10. Click Next.
11. Click Add Rule.

Topology rules allow you to define the permissible spatial relationships of features within and between feature classes that participate in the topology.

Landownership parcels are usually not allowed to overlap each other. You will add a rule to prevent your parcel features from overlapping each other.

12. Click the Features of feature class drop-down arrow and click Parcels.
13. Click the Rule drop-down arrow and click Must Not Overlap.
14. Click OK.

    You have created a rule governing the topological relationship of features within the same feature class.

Next you will create a topology rule governing the topological relationship of features in particular subtypes of two different feature classes. Specifically, you'll make sure that residential parcels are covered by or contained within blocks also designated as residential.

15. Click Add Rule.
16. Click the Features of feature class drop-down arrow, click the plus sign to expand Parcels, then click Residential.

    Residential is a subtype of the Parcels feature class that the planning department uses to represent parcels where people live.

17. Click the Rule drop-down arrow and click Must Be Covered By.
18. Click the Feature class drop-down arrow, click the plus sign to expand Blocks, then click Residential.
19. Click OK.

    The topology rule is added to the list of rules for this topology.

20. Click Next.
21. Review the summary information for the topology to be sure it is correct.
22. Click Finish.

After the topology is created, you have the opportunity to validate it. You do not need to validate the topology immediately after creating it. Depending on your data and your workflow, it may make sense to assign different areas to data editors to validate and edit within ArcMap.

23. Click No.

The topology appears in the Landbase feature dataset.