[postgis-users] odd results from intersection and geomunion

[Dylan Beaudette]

Hi,

I am performing some simple geometry operations on two data sets which are 
both of type POLYGON and all have valid geometries.

Here is the general idea:

select geomunion(
	intersection(geom_A,
		buffer( collect(geom_B ))
		)
	)

group by ...


This query produces two unexpected results:

1. without the GeomUnion: among the POLYGON geomtypes I am also getting some 
others...

ogc_fid | isvalid |       area       |    geometrytype    | numgeometries 
---------+---------+------------------+--------------------+---------------
 1051918 | t       |                0 | GEOMETRYCOLLECTION |             0
 1051930 | t       | 99567.4770585895 | MULTIPOLYGON       |             2
 1072895 | t       |                0 | GEOMETRYCOLLECTION |             0
 1073017 | t       | 566721.275024042 | MULTIPOLYGON       |             2


.. not sure how these other types are getting in there - artifacts from the 
digitization? Adding a check for geometrytype  != GEOMETRYCOLLECTION  filters 
out some of the noise, but I cannot figure out why they are there in the 
first place.


2. with the GeomUnion: I get an error:
NOTICE:  TopologyException: side location conflict 
(-2.11292e+06,-140347,-140347)
ERROR:  GEOS union() threw an error!


System information:
Linux Debian/Unstable
All components of PostGIS built from source

GEOS: 2.2.3
Postgresql: 8.1.3
PostGIS: 1.2.1

Any ideas?

thanks in advance

Dylan

-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341