[postgis-users] creating a spatial index for finding objects withing a certaindistance in miles based on lat/lon

[Paul Ramsey]

Daniel Ceregatti wrote:

> This is good news.
> 
> The post you linked to seems to cover the "conversion from miles to
> degrees" in the query, but I still need to get my index built using
> degrees, where all I have are lat/lon. I think I saw something on the
> list regarding this, so I'll search further.

Daniel, lat/lon *are* degrees. Degrees are the units of a lat/lon data 
set. You can pretend that lat/lon coordinates are planar for the 
purposes of indexing and bounding box querying, and use the geodetic 
distance function to directly measure distance off of them once you have 
a result set. Thusly:

select * from tbl where geom && <LatLonBoundingBox> and 
distance_spheroid(geom,<LatLonQueryPoint>,<Spheroid>) < <Radius>

I wonder if we should add in a "distance_sphere" for a less 
computationally intensive conversion from lat/lon to distances than the 
general spheroid case.

Paul

> Regarding Oracle, I'm not entirely sure how it does it either. I
> followed a simple "howto" like set of instructions that created the
> index based on lat/lon and was subsequently queryable using the sdo_*
> functions native to oracle, one of which was the sdo_within_distance ()
> mentioned earlier.
> 
> Thanks for all your help, Paul and Chris.
> 
> Daniel Ceregatti
> 


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