Vector Rules (Was: donuts - Vector format flaw!(?))

[Darrell McCauley]

Chris W Skelly (gewcs at jcu.edu.au) writes on 20 Oct 93:
>While we on this topic, can anyone confirm or
>deny the exact structure used in GRASS's vector
>format?  Particularly whether the arc-node
>topology a-la ARC/INFO is used?

maybe this might be helpful to Chris and others (this is from
the fine programming manual):

Vector Topology Rules

1. Arcs should not cross each other (i.e., arcs which would cross must
be split at their intersection to form distinct arcs).

2. Arcs which share nodes must end at exactly the same points (i.e.,
must be snapped together).  This is particularly important since nodes
are not explicitly represented in the arc file, but only implicitly as
endpoints of arcs.

3. Common boundaries should appear only once (i.e., should not be
double digitized).

4. Areas must be explicitly closed.  This means that it must be
possible to complete each area by following one or more area edges
that are connected by common nodes, and that such tracings result in
closed areas.

5. It is recommended that area features and linear features be placed
in separate layers. However if area features and linear features must
appear in one layer, common boundaries should be digitized only once.
An area edge that is also a line (e.g., a road which is also a field
boundary), should be digitized as an area edge (i.e., arc type "A" to
complete the area.  The area feature should be labeled as an area
(i.e., feature type "A" in the dig_att file).  Additionally, the
common boundary arc itself (i.e., the area edge which is also a line)
should be labeled as a line (i.e., feature type "L" in the dig_att
file) to identify it as a linear feature.