[postgis-users] Problems with GeomUnion

[Bill Binko]

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