Edges

Edges are perhaps the most important primitive. For Full topology, the following information is needed for each edge

• an ordered list of coordinates; the list must at least have start and end coordinate pairs, and may contain any number of intermediate coordinate pairs
• a reference (also termed a foreign key) to the face on the right side of the edge (as viewed when traversing from start to end)
• likewise, a reference to the left face
• the foreign key of the next edge for when the right face is being circumnavigated
• similarly, the foreign key of the next edge for when the left face is being circumnavigated
• the foreign key of the associated feature (only if there is just one line feature for each edge, and not mandatory even then)

The VPF specification states that the foreign keys of start and end nodes must also be included. This is not strictly necessary, and in fact is not done in the DCW. All that is needed are the start and end coordinate-pairs, and these are included in the coordinate list.

Circumnavigation is only supported in one direction - in the case of an outer ring, it is clockwise (which implies that inner rings are traversed anticlockwise). Thus there is only one next edge for each of the two faces which are bounded by the edge. Note that the direction of an edge is arbitrary and has a 50/50 chance of matching the direction of circumnavigation. If the start comes before the end when traversing the outer ring of one face clockwise, it must necessarily be the other way around if the outer ring of opposite face is also traversed clockwise. This is winged-edge topology.

on to Edge example

back to Vector Product Format and the Digital Chart of the World