[postgis-users] ST_Unioning invalid multipolygons

[Michael Michaud]

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
> Thanks,
> David
>
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