Definition

1. A <Feature Node> FN has the following relationship with its <Feature Edge> instances, if any exist.

   1.1	For each <Feature Edge> that has FN as a starting node, FN shall be associated to that <Feature Edge>.
   1.2	For each <Feature Edge> that has FN as an ending node, FN shall be associated to that <Feature Edge>.
   1.3	If FN is neither the starting nor ending node of a given <Feature Edge>, FN shall be associated to that <Feature Edge>.
   1.4	Consequently, for any given <Feature Edge> FE of which FN is a starting or ending node, FE shall appear among the associates of FN either: once, if FN is FE's starting node and not its ending node, once, if FN is FE's ending node and not its starting node, twice, if FE is a loop.

2. A <Geometry Node> GN has the following relationship with its associated <Geometry Edge> instances, if any.

   2.1	For each <Geometry Edge> that has GN as a starting node, GN shall be associated to that <Geometry Edge>.
   2.2	For each <Geometry Edge> that has GN as an ending node, GN shall be associated to that <Geometry Edge>.
   2.3	If GN is neither the starting nor ending node of a given <Geometry Edge>, GN shall not be associated to that <Geometry Edge>.
   2.4	Consequently, for any given <Geometry Edge> GE of which GN is a starting or ending node, GE shall appear among the associates of GN either: once, if GN is GE's starting node and not its ending node; once, if GN is GE's ending node and not its starting node; or twice, if GE is a loop.

Rationale

The associations between <Feature Node> and <Feature Edge> implement a boundary/co-boundary relationship. This topological relationship must be consistent, and must be consistent with the geometric representation of these objects.

The associations between <Geometry Node> and <Geometry Edge> implement a boundary/co-boundary relationship. This topological relationship must be consistent, and must also be consistent with the geometric counterparts of these objects.

Example

1. Consider a <Feature Edge> instance E1 that has distinct starting and ending <Feature Node> instances N1 and N2. N1 shall be associated to E1, and N2 shall also be associated to E1.

2. Consider a <Geometry Edge> E1 that has distinct starting and ending <Geometry Node> instances N1 and N2. N1 shall be associated to E1, and N2 shall also be associated to E1.

FAQs

For a <Feature Edge> that forms a loop (that is, a <Feature Edge> for which the starting <Feature Node> and ending <Feature Node> are the same <Feature Node>), how many times does that <Feature Edge> appear in that <Feature Node>'s list of associated <Feature Edge> instances?

Twice, with opposite <Edge Direction> values. A <Feature Edge> with distinct starting and ending <Feature Node> instances would appear only once.

For a <Geometry Edge> that forms a loop (that is, a <Geometry Edge> whose starting <Geometry Node> and ending <Geometry Node> are the same <Geometry Node>), how many times does that <Geometry Edge> appear in that <Geometry Node>'s list of associated <Geometry Edge> instances?

Twice, with opposite <Edge Direction> values. A <Geometry Edge> instance with distinct starting and ending <Geometry Node> instances would appear only once.