[postgis-users] ST_Unioning invalid multipolygons

[Stephan Holl]

Hello Horst,

Düster Horst <Horst.Duester at bd.so.ch>, [20080829-09:02:02]:

> Mmmmm...
> >Another solution which *may* work is to replace the geometry by a
> buffer 
> >of size 0 around the geometry in your select statement 
> 
> No this is a bad idea. You do not have any control about the result of
> your buffer(0). A better way is to clean the polygon geometry with the
> cleangeometry() function you can download from:
> 
> http://www.sogis1.so.ch/sogis/dl/postgis/cleangeometry.sql

This link seems to be wrong, I get a 404.
Could you post the correct link to this interesting-sounding
function, please?

> The function cleans by breaking the self-intersecting parts of your
> geometry into multigeometries.
> 
> Sometimes postgis throws topology exeptions although your geometry is
> valid. In this case geos by itself produces the error. Maybe this is
> the result of rounding problems and you don't have any chance to
> solve the problem.

best regards from Osna

	Stephan

> 
> -----Ursprüngliche Nachricht-----
> Von: Michael Michaud [mailto:michael.michaud at free.fr]
> Gesendet am: Freitag, 29. August 2008 08:24
> An: PostGIS Users Discussion
> Betreff: Re: [postgis-users] ST_Unioning invalid multipolygons
> 
> David Jantzen a écrit :
> > Hi again,
> >
> > I'm trying to ST_Union multipolygons from the Zillow neighborhood
> data.
> > When I do this I occasionally see:
> >
> > NOTICE:  TopologyException: found non-noded intersection between
> > -73.812....
> >
> > This appears to be due to invalid geometries, as the following query
> > returns over a hundred rows:
> >
> > select * from neighborhoods where not ST_IsValid(the_geom)
> >
> > Can anyone suggest a workaround here?  The resulting geometry needs
> > to be a multipolygon.  Also, what is ST_Is_valid actually testing
> > for?
> The
> > documentation isn't very explicity about the definition of validity.
> >   
> Most of time, invalid polygons are polygons with a self intersecting 
> boundaries (as in the number '8') or self-touching boundary (two non 
> contiguous points of the boundary are touching each other)
> The best solution is to correct those geometries if you can.
> Another solution which *may* work is to replace the geometry by a
> buffer
> 
> of size 0 around the geometry in your select statement (using a
> 0-buffer
> 
> is a work-around which generally works with JTS library from which 
> postgis functions are derived).
> 
> Michaël


-- 
Stephan Holl <stephan.holl at intevation.de>, http://intevation.de/~stephan
Tel: +49 (0)541-33 50 8 32 | Intevation GmbH | AG Osnabrück - HR B 18998
Geschäftsführer:  Frank Koormann, Bernhard Reiter, Dr. Jan-Oliver Wagner
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