-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Kevin Neufeld Sent: Monday, March 10, 2008 9:51 PM To: PostGIS Users Discussion Subject: Re: [postgis-users] Turn a Polygon into Lines
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.h tml) 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 >> _______________________________________________ >> postgis-users mailing list >> [email protected] >> http://postgis.refractions.net/mailman/listinfo/postgis-users >> > _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users __________ Information from ESET Smart Security, version of virus signature database 2935 (20080310) __________ The message was checked by ESET Smart Security. http://www.eset.com _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
