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

[Sandro Santilli]

It will be _very_ slow.
So much that you may prefer to make the topology persistent.

Another approach I'd like to see implemented would be to expose a tolerance
to the GEOS implemented functions. Basically exposing the GEOS
PrecisionModel. By doing that you could specify a tolerance in the overlay
operations and the result should never have gaps bigger than the given
tolerance. 

--strk;

On Thu, Oct 25, 2012 at 10:28:19AM -0700, pcreso at pcreso.com wrote:
> 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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