I concur, it sounds like you're after topology support in PostGIS.
Regina's function will simply provide you with the raw lines that make
up a polygon (the first half of Martin's first bullet item). Topology
in PostGIS is very much in a pre-alpha stage. There is an underlying
topology datamodel, but that's about it. The stored procedures Martin
was referring to has yet to be developed.
(see this thread
http://postgis.refractions.net/pipermail/postgis-users/2007-October/017530.html)
Cheers,
Kevin
Martin Davis wrote:
It sounds like what you are looking for is a way to turn a set of
non-overlapping polygons and turn them into a polygonal coverage (or
at least produce the arcs in that coverage).
I don't think there's any easy way of doing this in PostGIS. I'm not
sure that Regina's & Kevin's suggestions will produce the non-shared,
unique node-to-node arcs that are required.
The usual algorithm for doing this is:
- extract all line segments from each input polygon, labelled with the
polygon ID on the appropriate side
- merge duplicate segments, also merging the labels (on the
appropriate side). To identify duplicate segments and merge them,
segments will need to be reoriented so that they are in a consistent
direction (and the labels flipped accordingly)
- "sew" the resulting set of line segments together to create
linestring edges from 3-node to 3-node (e.g. eliminating
"non-topologically significant" 2-nodes
You might be able to create some SQL and/or stored proc code to do
this - it would be interesting to see how complicated and performant
this would be.
Lee Hachadoorian wrote:
I'm looking through the PostGIS reference, and I can't seem to find a
way to take a geometry of polygons and turn it into lines. What I'm
looking for is something like the ArcGIS Feature to Line
geoprocessor, which will create a line shapefile where each feature
is an arc representing the boundary between neighboring polygons with
a field indicating the ids of the polygon on either side.
Functions like ST_MakeLine require point geometries, and I don't see
anything else that seems to be what I'm looking for. Any ideas would
be welcome.
Thanks,
Lee Hachadoorian
PhD Student in Geography
Program in Earth & Environmental Sciences
CUNY Graduate Center
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