ST_Intersection is perhaps a bit more complete than most users would
like. If two polygons share a boundary what is their intersection? Not
a polygon, but that linear, shared boundary: a line. Similar for
touching at a point.
Anyhow, probably a cleaner approach would be an extra optional
argument to ST_Intersection allowing users to specify the
dimensionally of the return, but in the meanwhile you can go to
ST_CollectionExtract and use that to pull out just the polygons.
P.
On Wed, Mar 7, 2012 at 9:58 AM, Rebecca Clarke rebe...@clarke.net.nz wrote:
Hi there
I have 2000 polygons in table A and I want to show only the polygons that
intersect with the bounding box specified in table B . So I do this:
SELECT the_geom
FROM A
WHERE st_intersects(the_geom, (select the_geom from B))
AND ST_GeometryType(the_geom) = 'ST_Polygon'
This query returns all the intersecting polygons. When I run st_geometrytype
they are all St_Polygon.
Some of the polygons are large and exceed the boxes boundary so I use this
query to trim them:
SELECT st_intersection(the_geom, (select the_geom from B)) as the_geom
FROM A
WHERE st_intersects(the_geom, (select the_geom from B))
AND ST_GeometryType(the_geom) = 'ST_Polygon'
The problem I'm having is that when I use the st_intersection, instead of
returning just St_Polygon's, it's returning other types as well:
ST_LineString
ST_MultiLineString
ST_MultiPolygon
ST_Polygon
I need to output the results to a SHP file and this is preventing me from
doing so seeing there is more than one Geom Type.
Any ideas how I can keep the results of the st_intersection as just
St_Polygons? Am I doing something wrong?
Regards
Rebecca
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