[postgis-users] Unexplained results when using Intersection?
kneufeld at refractions.net
Tue Oct 30 16:41:31 PDT 2007
What is it you are trying to do exactly?
Reading your query, I see that you are interested in the intersection of
all geometries in "evc" and "bnd_buf" where the geometries overlap (by
definition, this means that the interior of one polygon is both inside
and outside that of another polygon) and where "buf_dist" = 1. So,
you're not interested in cases where a polygon in "env" is contained
wholly inside "bnd_buf". Right?
Did you mean to filter with "overlaps"? Or did you mean "intersects"?
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Craig Feuerherdt wrote:
> I have been attempting to intersect on set of geometries (bio.evc)
> with another geometry (bio.bnd_buf). All the geometries are valid
> according to isvalid and the query below successfully returns a table.
> create table bio.bnd_evc as
> select a.buf_dist, b.bio_no, b.bio_code, b.evc, b.constcde,
> b.consstat, b.density, intersection(a.the_geom, b.the_geom) as the_geom
> from bio.bnd_buf as a, bio.evc as b
> where a.the_geom && b.the_geom
> and a.buf_dist = 1
> and overlaps(a.the_geom, b.the_geom)
> When I view the resulting table (bnd_evc) in QGIS there are several
> missing polygons ( image 1a
> <http://www.bendigo-orienteers.com.au/study/evc1_over.png>). One of
> the polygons lies entirely within bnd_bnf while the other has a small
> portion which overlaps the boundary.
> When I try intersecting the same geometries but this time selecting
> buf_dist = 1000 (the 1000m buffer), additional evc polygons (which are
> wholly within buf_dist=1 and therefore also within buf_dist=1000) are
> missing from the output table and the whole thing is a 'mess' nad is
> in no way a representation of what the intersection shoul dlook like (
> image 2a <http://www.bendigo-orienteers.com.au/study/evc1000_over.png>).
> The other thing that occurs in both cases is that some polygon
> geometries are created where there are no evc polygons ( image 1b
> <http://www.bendigo-orienteers.com.au/study/evc1_under.png> & 2b
> Both tables have a GIST index on the geometry column (the_geom).
> Are the issues associated with having multiple polygon geometries in
> the one table (unlikely in my opinion)?
> Is it something to do with the query?
> Are these issues associated with PostGIS or GEOS?
> Is there a work around to solve the issue?
> Thanks in advance,
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> postgis-users at postgis.refractions.net
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