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Creating a geodatabase topology to clean up data errors


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The CAD lot lines data that you loaded in the previous exercise needs some quality checking, editing, and other processing for you to have useful parcel polygon features for your geodatabase.

You will create a simple geodatabase topology rule to help you find digitizing errors in the lot line data, then use the topology and editing tools to fix these errors. Once the problems, mostly lines that do not close to form polygons, are fixed, you will create a new polygon feature class from the lot lines. You’ll add the polygons to the topology, then use the topology to identify and resolve other errors in the data.

If you have not loaded the lot lines, a duplicate of this feature dataset with the lot lines already loaded may be found where the tutorial data is installed at: C:\ArcGIS\ArcTutor\Editor\ExerciseData\TopologyEdits\TopologyTutorial.gdb.

You must close ArcMap before building the topology to release the lock on the database.

Topologies must be created within feature datasets.

The first panel of the New Topology wizard describes what the wizard does.

On the second panel of the wizard you can name the topology and set the cluster tolerance. The cluster tolerance is the minimum distance that separate parts of features can be from each other. Vertices and edges of features that fall within the cluster tolerance are snapped together. By default, the wizard gives the smallest possible cluster tolerance, which is determined by the precision of the spatial reference of the dataset. The precision of a dataset defines how many system units can be stored per unit of linear measure and controls how precisely coordinates are stored in the dataset.

On the third panel of the wizard you can choose which feature classes in the dataset to include in the topology. In this case you'll only add one feature class to the topology.

When you have more than one feature class in a topology, you can give them different ranks. When vertices or edges of features fall within the cluster tolerance of each other, the feature class ranks control which is moved to the other’s location. Feature classes of a lower rank will be snapped to feature classes of a higher rank. The highest rank is 1; the lowest is 50. Parts of features of the same rank that fall within the cluster tolerance are geometrically averaged.

When you build a topology, you can pick the rules that will govern the allowable spatial relationships between features. In this case you'll add one rule to the topology. The rule is that LotLines must not have dangles.

Dangles are the endpoints of lines that are not snapped to other lines in the feature class. You will want to find the dangles in the LotLines feature class because they represent places where the imported CAD line work will not produce closed polygons.

After you finish the Topology Wizard, you get a message that the topology is being built, then another asking whether you want to validate the topology now.

Validating a topology checks the features against the topology rules to find topology errors.

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