[postgis-users] merging dual carriageway motorway lines

[Stephen V. Mather]

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
svm at clevelandmetroparks.com
clevelandmetroparks.com




-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Stephen
Woodbridge
Sent: Monday, December 19, 2011 9:21 AM
To: postgis-users at postgis.refractions.net
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
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