> -----Original Message----- > From: [email protected] > [mailto:[email protected]] On > Behalf Of Picavet Vincent > Sent: Wednesday, October 07, 2009 4:26 AM > To: PostGIS Users Discussion > Subject: Re: [postgis-users] Merge lines > > > Hello, > > > Does this look correct? > > > > select st_linemerge(st_collect(cs1.the_geom)) > > from ( > > select the_geom, st_startpoint(cs_s.the_geom) as linepoint > > from mylinetable cs_s > > UNION ALL > > select the_geom, st_endpoint(cs_e.the_geom) as linepoint > > from mylinetable cs_e > > > > ) as cs1 > > group by linepoint > > having count(*) = 2 > > Be careful with group by linepoint, as it uses the = > operator, which works on bounding boxes and not on > geometries. The bounding box of a point is not equal to the > point itself. This could lead to points being very close to > each other being grouped by, while being different. If you > want to group by strictly identical geometries, you could do a > group by st_asbinary(linepoint)
How would the bounding box on a point be a problem with group by? I am aware of the classic example of a line 0,0-5,5 and 0,5-5,0 being treated as same when grouped. But does this really apply to points? > > Hope this helps, > Vincent > > > > > > > > > -----Original Message----- > > > From: [email protected] > > > [mailto:[email protected]] On > Behalf Of > > > Martin Fafard > > > Sent: Tuesday, October 06, 2009 11:21 AM > > > To: PostGIS Users Discussion > > > Subject: [postgis-users] Merge lines > > > > > > > > > Hi > > > > > > Is there a way to merge lines that are contiguous but stop > > merging at > > > an intersection? > > > I have join an image. > > > Thank you > > > > > > Martin F > > > > > _______________________________________________ > > 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 > _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
