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

[Alexandre Neto]

Thank you for all the answers.

I did not try the topology, as I need to study its implementation a little
better.

I did not found the st_cleanpolygon function and using (St_Buffer, 0) did
not worked:

The dilate\erode visually corrects the problem, but of course introduce a
small rounding in the angle introducing a small overlaping with other
adjacent polygons...

I guess that the tolerance with the GEOS PrecisionModel would be nice, I
think that is the method used by ArcGIS products to "insure" geometry
"topology" after aggregation operations.

Any more ideas?

Alexandre Neto






On Fri, Oct 26, 2012 at 8:40 AM, Dave Potts <dave.potts at pinan.co.uk> wrote:

> Have you considered curve smoothing or apply a small buffer around the
> orginal data/
>
> Sandro Santilli wrote:
> > 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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> > --
> >
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> > data
> >
> >                                        ~~ http://strk.keybit.net
> >
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