Hi Lelo,
I don't think that will help - ST_Envelope only works on geometry and I
need this calculated on the sphere. Besides ST_Intersects() already does
a bounding box check.
Evan
On 02.07.2013 18:18, Lelo - Luiz Rogério De Pieri wrote:
Hi Evan,
What about instead of verify the intersection with Simplified polygon,
verify the intersection with the BBox ?
*ST_Envelope
*http://postgis.refractions.net/documentation/manual-1.4/ST_Envelope.html
I've been using this strategy on my system but using geotools and
gives me nice results.
Good luck
On Tue, Jul 2, 2013 at 10:39 AM, Evan Martin
<[email protected] <mailto:[email protected]>>
wrote:
Hi,
I have tables of ~25,000 polygons and ~80,000 lines and I want to
find which lines intersect which polygons using PostGIS 2.1. Both
are geographies and can span the dateline. Doing this the simple
way using ST_Intersects(geog, geog) takes about 3 hours on my
machine and I'd to see if there's a way to speed this up.
I already have indexes on the geography columnsand one of them is
being used (the one on the lines). Each line only has 2 points,
butthe polygons have anywhere from 4 to 20,000 points and some of
them are very large. It would be OK to miss some of the smaller
intersections (ie. where the two only just barely intersect), but
I wouldn't wantthe query to return false positives. In fact,
ideally, I'd like to find only the lines that "substantially"
intersect a polygon, eg. at least x km or x% of the line is in the
polygon, but finding any intersections at all would be a start.
One trick I tried is ST_SimplifyPreserveTopology. I used that to
create simplified versionof the polygons (at least those that
don't span the dateline) and check those first, then if they
intersect then check the real polygons. This seems to work, but
the performance gains are marginal compared to the simple approach.
Is thereanother trick I can use to do this faster? I know
ST_Intersects() internally calls ST_Distance(), which calculates
the distance to a fraction of a metre. I don't need that kind of
precision, so surely there's some "shorcut" to be found?
Thanks,
Evan
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--
Rogério De Pieri (Lelo)
MBA em Gerenciamento de Projetos - FGV
SCJP 5
Buscando melhorar a cada dia
Áudio, Hardware & Software
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