[postgis-users] Indices for ST_distance_sphere

[Paul Ramsey]

If you're in a planar projection, then

select * from footable where st_dwithin(the_geom, 'POINT(foo bar)', DISTANCE)

will return based on an indexed search for arbitrary foo and bar. You
need to be in a planar projection so the spatial index works in the
same cartesian domain as the distance calculation.

Every planar projection introduces distance/area distortion, the
question is "do you care". Is it big enough to be concerned about,
given your use case.  Variance of +/- 1cm in questions that are need
10m accuracy aren't important.

P.

On Tue, Oct 14, 2008 at 5:31 PM, Stephen Baillie
<steve at alliancesoftware.com.au> wrote:
>
>> What about 3112 (# GDA94 / Geoscience Australia Lambert) or 3577
>> (Australian Albers), those should cover things fine, no?
>>
>
> Possibly, although I'm finding it hard to find hard data on what those
> actually mean (let alone what other alternatives are supported by PostGIS).
>  It looks like they still produce range distortion, which answers to my
> previous question on this list gave me to believe would be a problem for
> anything except true spherical distances.
>
> Closer to the point, though, the choice of projection is of lesser
> importance to my question than that of indexing.  PostgreSQL tells me that
> the spherical distance calculations increase full table scan time by about
> two-thirds when I use those as the where clause of my search instead of
> using a lat/long bounding box, but my question is actually about indexing
> before doing such distance calculations:
>
> Is it possible to create an index that can be used to speed searches based
> on distance from an arbitrary point, or should I just use a lat/long
> bounding box?
>
> Thanks,
>
> Steve.
>
> --
> Stephen Baillie
> Developer
> Alliance Software
>
> 1/234 Whitehorse Road
> Nunawading, VIC 3131
> Australia
>
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>
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