[postgis-users] SRID for analyzing a USA national data set in Meters

[David William Bitner]

It is a common mis-assumption that you need to do a buffer calculation.  To
do this type of analysis, all you need is the distance_sphere() calculation:
select *,distance_sphere(hospital1geom,hospitals2geom) from hospitals2 where
distance_sphere(hospital1geom,hospitals2geom)<25*1609 order by
distance_sphere(hospital1geom,hospitals2geom) asc;

Just add a limit 1 to the end if all you want is the closest.

No buffer necessary at all.


David


On 4/2/07, Michael Frumin <mfrumin at rpa.org> wrote:
>
>  Right, I should be more specific from the outset.  I did do some
> searching thru the PostGIS archives, and didn't find the answer I was
> looking for; is there a PostGIS FAQ somewhere?
>
> As for my problem, my inputs are two sets of geocoded hospitals, and I
> just want to be able to identify for each hospital in the first set, the
> hospitals in the second set within approximately 25 miles.  I will the map
> these sets, with a 25 mile buffer around the first set, using Geoserver.
> So, distance and area are both somewhat important, heading not at all.
> distance_sphere(oid), sounds good for the calculation, but won't help with
> the buffering because it doesn't tell me the 'distance' in lat/lng space
> that would equate to 25 miles (because of course this varies over the
> globe).  To achieve this I would need to reproject into something that is in
> meters, and buffer around that.
>
> How egregious would you expect the errors to be if I simply use the
> projection for the UTM zone that represents, say, Central time?
>
> thanks,
> mike
>
>
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