[postgis-users] Problems with GeomUnion

[Martin Davis]

In JTS buffer is pretty tolerant of invalid topology.  That's why I
suggested using it to clean up bad geometry.  AFAIK it should have
similar behaviour under GEOS.  It's not 100% guaranteed to work,
however, so if there are failures some other approach might be needed
for them.

Fixing topology errors is usually not a simple single-step process - it
can require some deviousness to deal with the infinite number of ways
that geometry can be bad.

Martin Davis, Senior Technical Architect
Vivid Solutions Inc.      www.vividsolutions.com
Suite #1A-2328 Government Street Victoria, B.C. V8T 5G5
Phone: (250) 385 6040 - Local 308 Fax: (250) 385 6046


> -----Original Message-----
> From: Bill Binko [mailto:bill at binko.net] 
> Sent: April 8, 2005 8:55 AM
> To: PostGIS Users Discussion
> Subject: RE: [postgis-users] Problems with GeomUnion
> 
> 
> On Fri, 8 Apr 2005, Mark Cave-Ayland wrote:
> 
> > Hi Bill,
> > 
> > > So, obviously, I have some real issues in this data.
> > 
> > This is because you are running a buffer() on a column containing 
> > invalid geometries - you need to check the validity 
> *before* you pass 
> > the geometry to a GEOS function. My guess is that you want 
> something 
> > like:
> > 
> > SELECT oid from parcels where ndims(parcel_shape, 0) != 2)
> 
> I was running buffer() on it to FIX the invalid shapes (per Martin's 
> suggestion).
> 
> I had tried your query before, and it returned zero rows (which makes 
> sense given the constraint). The buffer() seemed to be 
> converting them to 
> non-2d.
> 
> > 
> > Which will return the oids for all geometries in the parcel_shape 
> > column which don't have 2 dimensions.
> > 
> Right: there were none.
> 
> > > I thought I might get to the bottom of it with this query:
> > > 
> > > gis=# select oid from (select oid,
> > > isvalid(buffer(parcel_shape, 0)) as valid from parcels) as 
> > > foo where valid = false;
> > > 
> > > But I got:
> > > 
> > > ERROR:  invalid memory alloc request size 0
> > > 
> > > That can't be good. :-(
> > > 
> > > Anyway, I'm still plugging at it, so any help is fabulous.
> > 
> > Again, this query fails for the same reason. The following 
> SQL should 
> > return the oids of all invalid geometries in the 
> parcel_shape column 
> > of your parcels table:
> > 
> > select foo.oid from (select oid, isvalid(parcel_shape) AS valid from
> > parcels) as foo where foo.valid = 'f';
> 
> This one gave me a list of 829 invalid shapes (out of 330881).  Since 
> you're suggesting that buffer() can't be run on them, do you have any 
> suggestions on how to correct them?  I got lots of 
> 
> NOTICE: Self-Intersaction
> and 
> NOTICE: Ring Self-Intersection 
> 
> messages while the query was running, along with a few uglier 
> lines about null elements.  Should I try to match up the 
> stderr messages with the OIDs 
> (on stdout) and only run buffer() on the ones with NOTICE: 
> Self-Intersection?
> 
> (Don't get me wrong: I appreciate the help, I'm just flying a 
> bit blind 
> here.)
> 
> 
> Bill
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