[postgis-users] Speeding up point-in-polygon search using ST_SimplifyPreserveTopology?

[Paul Ramsey]

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
<postgresql at 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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