You might also look to straight skeletons or voronoi polygons (medial axis transform) to collapse dual lines, which would require R-stat e.g. http://www.joeconway.com/plr/, unless someone has a script kicking around to do it in pure postgis.
Some early work on the problem: http://proceedings.esri.com/library/userconf/proc96/TO400/PAP370/P370.HTM (mostly useful conceptually) BTW, these techniques work really well for collapsing smooth lines, like those made of human built infrastructure. They do not work as well for "bumpy" lines like rivers and such. For these, some filtering is needed, such as the scale axis transform http://www.balintmiklos.com/scale-axis/algorithm_siggraph_2010.html. Best, Steve Stephen Mather Geographic Information Systems (GIS) Manager (216) 635-3243 [email protected] clevelandmetroparks.com -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Stephen Woodbridge Sent: Monday, December 19, 2011 9:21 AM To: [email protected] Subject: Re: [postgis-users] merging dual carriageway motorway lines On 12/19/2011 7:12 AM, Lauri Kajan wrote: > Hi all, > > I have a road network loaded to PostGIS. > Now I'm trying to find a way to merge parallel lines of a dual > carriageway motorway to single a line? > Or is there any? Lauri, I do not think there is any automated way to do this, but here are some thoughts on how you might be able to do this. 1. collect the dual carriageway segments based on a common attribute like name 2. for each segment, extract the points of the linestring and tag them with the segments uid, you also might want to generate extra points along this linestring and tag them with the segments uid also 3. for each point find the nearest point not on this segments uid, and probably not on a connected segment within some limit distance and insert a point that is the midpoint between the reference point and the nearest point into a combine_point table. 4. stringify the combined_point table by connecting nearest points within some limit distance. Problems that you might encounter: a. ramps backs that might get included by mistake. b. loops or near loops like mountain switch that might confuse the stringify c. combine carriageway does not follow mid-point path of dual carriage because not all segments were selected or there were not enough points generated along the original paths to compute the mid point path. I have not tried to do this but it is how I would tackle the problem. Others might have some good ideas also. Thanks, -Steve _______________________________________________ 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
