[postgis-users] merging dual carriageway motorway lines

[Stephen Woodbridge]

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