[postgis-users] Turn a Polygon into Lines
kneufeld at refractions.net
Mon Mar 10 12:51:16 PDT 2008
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
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.
>> Lee Hachadoorian
>> PhD Student in Geography
>> Program in Earth & Environmental Sciences
>> CUNY Graduate Center
>> postgis-users mailing list
>> postgis-users at postgis.refractions.net
More information about the postgis-users