[postgis-users] Postgis topology help

pcreso at pcreso.com pcreso at pcreso.com
Wed Apr 4 12:14:13 PDT 2012

Thanks again for a prompt reply.

QGIS is showing the results of the inserts & polygonise in the attached image. So, yes, just as you sketched it.

I had assumed Polygonize was constructing polygon TopoGeometries ?? Obviously I've missed something somewhere.

I can find no info at all on the ST_TopologySummary() function. Not in the docs, & even Google cannot find a single reference. Where do I look for some explanation of this function (& any others that might be relevant )? 



--- On Thu, 4/5/12, Sandro Santilli <strk at keybit.net> wrote:
From: Sandro Santilli <strk at keybit.net>
Subject: Re: [postgis-users] Postgis topology
To: pcreso at pcreso.com, "PostGIS Users Discussion" <postgis-users at postgis.refractions.net>
Date: Thursday, April 5, 2012, 5:49 AM

On Wed, Apr 04, 2012 at 09:57:10AM -0700, pcreso at pcreso.com wrote:
> Following on from the reply by strk (thanks!!) I'm still working through how to use topologies.
> I have a script which creates a topology, adds 5 linestrings looking like:
>    |      |      |
>  ---------------
>    |      |      |
>  ---------------
>    |      |      |
> validates the result and then Polygonizes it, creating two faces. 

Like this

     |      |      |
     |      |      |
     |      |      |
> I'm using the SVN docs 
>http://www.postgis.org/documentation/manual-svn/Topology.html ) 
> as my reference, as this is the best I have found so far. 
> What I want this script to do is run a query listing/describing the faces that have been created, then extract them as polygon geometries into a Postgis geometry column to be used as conventional polygons, but I can see no way in the docs of accomplishing this.

ST_TopologySummary gives you a good

You can use GetFaceGeometry to extract the faces polygons,
maybe use it in a query to <yourtoponame>.face to see them
all. Remember that face_id=0 is the universe face, so you
want to skip that from the query.

> Once I get the script able to complete this operation I figure I should have a much better understanding of how Postgis topology works, & will stick it on the wiki as a working example. I'll also insert more commands to illustrate other operations, as I learn this stuff.
> Any suggestions as to how to accomplish these next steps?

I think your next step should be looking at constructing TopoGeometry
objects :)


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