About the parcel fabric and topology

This topic applies to ArcEditor and ArcInfo only.

In ArcGIS, topology is the model used to describe how features share geometry and is the mechanism for establishing and maintaining spatial relationships between features. An example of a topological relationship in ArcGIS is the one between building polygons and parcel polygons: you might need to make a rule that building polygons are always contained within parcel polygons.

A topology is stored in a geodatabase as a set of relationships that defines how the features in one or more feature classes share geometry. A topology consists of a set of rules structuring the relationship between a collection of features in one or more feature classes in a feature dataset.

When a topology is validated, features participating in the topology are evaluated against a set of defined rules, and violations of the rules are treated as errors. For example, if the building polygon overlaps the parcel polygon, this is a violation of the rule specifying that building polygons should always be contained within parcel polygons.

Topology and the parcel fabric

In the parcel fabric, topological relationships between parcel points, parcel lines, and parcel polygons are inherently established and maintained through the use of common parcel points.

While the parcel fabric cannot be topologically edited, you can run a topology on a parcel fabric to validate the rules between the internal feature classes of the parcel fabric. For example, you can use a topology to find those linepoints that do not sit on top of their parcel lines. This might happen when parcel data is inaccurate. Parcel editing tools are then used to fix problems identified by the topology. For example, running a least-squares adjustment can be used to force linepoints back onto their parcel lines. This is different from standard feature classes where errors can be corrected through topological edits.

The parcel fabric can also participate in an ArcGIS topology to validate rules between the features of the fabric and other standard feature classes; for example, zoning polygons should always overlap parcel fabric parcel boundaries.

Rules can be established between the following feature classes of the parcel fabric:

• Control
• Parcels
• Lines
• Line points
• Points

The following list describes some examples of topology rules that can be defined between feature classes of the parcel fabric to identify data integrity problems in the parcel fabric:

• Parcel line points must be covered by parcel lines.

  Identifies line points that are not sitting exactly on their parcel lines, causing slivers.

  Identifies overlapping parcels caused by incorrect joining or floating line points.

• Parcels must not have gaps.

  Identifies overlapping parcels caused by incorrect joining or floating line points.

• Parcel polygon boundary must be covered by parcel lines.

  Identifies any unclosed parcels.

Cluster tolerance

Validating a topology starts a cracking and clustering process. The cluster tolerance of a topology is the minimum distance between the vertices of features participating in the topology. The cluster tolerance of a new topology defaults to the minimum possible cluster tolerance, based on the precision and extent of the data.

During cracking, vertices are created at the intersection of feature edges; during clustering, vertices that fall within a cluster tolerance are snapped together. Cracking and clustering are not ideal for a parcel fabric, since data in a parcel fabric should only be edited by fabric editing tools. However, while line, point, and polygon geometry in the parcel fabric can be cracked and clustered, the shape geometry will always be recalculated from the attributed survey data on the parcel points and parcel lines.

Topology coordinate ranks

Each feature class participating in a topology must have a rank assigned to it to control how much the feature class will move when the topology is validated. The rank helps to control how vertices are moved when they fall within the cluster tolerance of one another. Different feature classes have different coordinate accuracies; for example, vertices in the parcel fabric generally have a high coordinate accuracy because they are defined from survey data. Vertices on features collected from digitizing will generally have a less reliable coordinate accuracy. Coordinate ranks in a topology can allow you to ensure that reliably placed vertices are the anchor locations to which less reliable vertices are moved. A parcel fabric should generally have a high rank in a topology if it has been adjusted with control in a least-squares adjustment. This will ensure the least amount of movement of parcel fabric parcels while being validated in a topology.