Thanks, Paul, but do you have any suggestions on how I can cut up the
polygons properly? As I posted, ST_Intersection doesn't work properly on
the ones spanning the dateline and I don't know how else to do it. Maybe
there's some obvious trick I'm missing. (I haven't looked at the code,
but it seems a bit strange to me that ST_Intersects works, but not
ST_Intersection - so PostGIS can somehow figure out that they intersect,
but can't figure out where?)
By the development work do you mean ticket 1796 or something else?
Either way, any estimate on when it might get done? :)
Regards,
Evan
On 30/05/2012 11:35 PM, Paul Ramsey wrote:
Evan,
Beyond the cutting-up-of-large-things as Stephen suggests, there is
the extra development work to optimize the edge searching routines
with an internal index on the objects (and caching that index once
built). Something for 2.1 :)
P.
On Mon, May 28, 2012 at 10:08 AM, Evan Martin
<postgre...@realityexists.net> wrote:
Hi all,
I have a table of ~350 multi-polygons and ~185,000 points and I need to find
which points are inside which multi-polygons. Some polygons are quite large
and span the dateline, so I'm using geography ST_DWithin for this (with a
tolerance of 100m). My initial query looks like this (simplified):
SELECT ...
FROM points, polygons
WHERE ST_DWithin(point, real_area, 100)
This works, but it takes about 90 minutes. I'm trying to make it faster by
using ST_SimplifyPreserveTopology. That worked very nicely for my "adjacent
polygons" problem
[http://postgis.refractions.net/pipermail/postgis-users/2012-January/031992.html],
because all polygons were modified in the same way, but this is trickier.
Since I'm modifying the polygon, but not the point, the results are
different. So I thought maybe I could do this in two phases: if the point is
well within or well outside the polygon then take the result of the
"simplified" check as correct, but if it's close to the border then check it
properly, ie.
SELECT ...
FROM points, polygons
WHERE ST_DWithin(point, simple_area, 20000)
AND (ST_Distance(point, simple_border)> 20000 OR ST_DWithin(point,
real_area, 100))
simple_area = ST_SimplifyPreserveTopology(real_area::geometry,
0.01)::geography and simple_border = the exterior ring of simple_area.
This takes about 18 minutes (a 5x improvement) and gives very similar
results, but not quite the same. It falls down on polygons that have
rhumblines along parallels, because they get turned into great circle lines.
Eg. the original polyon may have a rhumbline approximated as (24 10,25 10,26
10,27 10), ST_SimplifyPreserveTopology does its job and simplifies it to (24
10,27 10) and then ST_DWithin on geography treats it as a great circle line,
giving an incorrect result. I tried inserting extra points to "unsimplify"
the rhumblines, but that itself is very slow and also quite a hack, because
I can't really be sure which lines were supposed be rhumblines when looking
at the simplified polygon. I feel like I'm so close and this is a very silly
little problem - but it is a problem.
Could anyone suggest a way to work around this? Or a different approach to
the whole thing?
Thanks,
Evan
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