[postgis-users] Indices for ST_distance_sphere

[Stephen Baillie]

Paul Ramsey wrote:

>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.
>  
>
Ooh, that looks to be just what I was looking for.  Thanks!

Now that I know what function to look for, the documentation says that 
it will use indexes "if available".  Do I need to do anything other than 
AddGeometryColumn() to ensure that indexes are available?

>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.
>  
>
I'm just going on the advice I've received from this list previously, 
which was that using UTM zone 53 projection for my target area (all of 
Australia) would give "EXTREME distortion at the edges".  I haven't 
tried to quantify that; I shall find some numbers using UTM 53 and the 
other projections you suggested.

Thanks again,

Steve.

-- 
Stephen Baillie
Developer
Alliance Software

1/234 Whitehorse Road
Nunawading, VIC 3131
Australia

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