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

[Dave Potts]

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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>
> --
>
>  http://www.cartodb.com - Map, analyze and build applications with your
> data
>
>                                        ~~ http://strk.keybit.net
>
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