[postgis-users] How to eliminate small gaps produced by ST_union?

[pcreso at pcreso.com]

My impression is that working topologies is slow, especially with large datasets - lots of computation involved.

How effective would this be with hundreds of thousands or millions or small polygons?

Just running ST_Union() takes hours to days.

Thanks

Brent Wood

--- On Fri, 10/26/12, Sandro Santilli <strk at keybit.net> wrote:

From: Sandro Santilli <strk at keybit.net>
Subject: Re: [postgis-users] How to eliminate small gaps produced by ST_union?
To: "PostGIS Users Discussion" <postgis-users at postgis.refractions.net>
Date: Friday, October 26, 2012, 6:04 AM

You may try to construct a topology within PostGIS-2.0, using
a small tolerance, and then get the geometries back. 

--strk;

On Thu, Oct 25, 2012 at 05:54:05PM +0100, Alexandre Neto wrote:
> I'm trying to Aggregate\Dissolve a set of adjacent, but not touching
> polygons in Postgis. The geometries were created editing the Postgis table
> in QGIS created, with snapping and avoid overlap in the layer.
> 
> I used ST_Union like this:
> 
> SELECT ST_Union(the_geom)
> FROM table;
> 
> Small gaps similar to lines appear in the result. This happens when the
> adjacent polygons do not share the exact same vertices.
> 
> [image: Inline image 2]
> Is there a way to eliminate this "gaps" or "small angles" within Postgis?
> 
> I know that v.clean in GRASS can eliminate small angles, and correct
> topology, but that would mean I needed to copy the features to GRASS,
> correct them and copy it back to Postgis.
> 
> Thanks in advance,
> 
> Alexandre Neto


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