Topology

The model of topology that has been implemented is the traditional node/arc/area approach.

Areas are bounded by a list of arcs.
Areas can have internal holes that also have arc boundaries. The holes are known as rings.
Arcs consist of a series of vertices, and start and end at nodes.
Arcs never intersect or overlap, but do meet at their endpoints (at nodes).
Nodes are points at which two or more arcs end (or start).
Nodes, arcs, and areas all have independent attributes.
Each Arc borders on, at most two areas, one on its left and one on its right.
Arc direction is significant, therefore one end node is designated the start node, and another the end node.

We keep track of the following as attributes of the nodes, arcs, and areas within GeoGateway:

For each arc: the ShapeId of a start node and end node.
For each arc: the ShapeId of a left area (or -1), and the ShapeId of a right area (or -1).
For each node: the list of ShapeIds of all arcs entering/leaving the node, in clockwise order (this can be derived from the arc information).
For each area: the list of ShapeIds of all arcs bounding the area in clockwise order, with an indication of whether the arc needs to be transversed forward or backward in order to proceed in a clockwise direction (this can be derived from the arc information).

A full topology consists of three vector layers (segments). These are: a node layer with the points of intersection of arcs, an arc layer with the lines defining boundaries between areas, and an area layer representing the polygon regions.

A key difficulty then, is to keep the three layers together, or at least to ensure that they can be found when necessary. This is dealt with through the use of inter layer references stored as metadata, on each layer in the topology grouping.

Topological Arc Layers
In addition to the topological metadata information, a Topological arc layer must contain the following attributes:

StartNodeId: Attribute containing the GDBShapeId of the starting node.

EndNodeId: Attribute containing the GDBShapeId of the ending node for this arc.

LeftAreaId: Attribute containing the GDBShapeId of the left area polygon, or GDBNullShapeId if there isn't such an area.

RightAreaId: Attribute containing the GDBShapeId of the right area polygon, or GDBNullShapeId if there isn't such an area.

An arc must have at least 2 vertices.

An arc layer must have NODE_REF, and AREA_REF metadata pointing to the related node, and area layers. It must also have a LAYER_TYPE of TOPO_ARCS indicator in metadata.

Topological Node Layers
In addition to the topological metadata information, a topological node layer must have the following attribute:

ArcIdList: (GDBFieldTypeCountedInt) List of arc ShapeIds starting or ending at this node.

Each node should also have one vertex defining the position of the node.

A node layer must have ARC_REF, and AREA_REF metadata pointing to the related arc, and area layers. It must also have a LAYER_TYPE of TOPO_NODES indicator in metadata.

Topological Area Layers
In addition to the topological metadata information, a topological area layer must have the following attribute:

ArcIdList: (GDBFieldTypeCountedInt) List of arc ShapeIds forming the border of this area in clockwise order. Each ring is separated by a GDBNullShapeId entry in the ArcIdList.

A topological area shape should have either zero or one vertex. If it has one, it will be presumed to be an internal label point for the area.

An area layer must have NODE_REF, and ARC_REF metadata pointing to the related node and arc layers. It must also have a LAYER_TYPE of TOPO_AREAS indicator in metadata.

Parent Topic: Vectors (VEC)
About PCI Help Gateway