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

[David William Bitner]

If you're only concerned with 25 miles, you could use the method to
determine which UTM zone your data is in for your first hospital and then
use that for the calculation and buffer creation.  You could convert Pedro's
PHP script into PL/PGSQL to make it easy to work with in the db.

On 4/2/07, Michael Frumin <mfrumin at rpa.org> wrote:
>
>  correct.  I wanted to calculate the buffer so that I can render a map
> using GeoServer, which sits right on top of PostGIS, which would show the
> first set of hospitals, the 25 mile radius, and the second set.
>
> thanks,
> mike
>
> david.bitner at gmail.com wrote:
>
> 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> <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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>
>
> ------------------------------
>
>
> 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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>
>


-- 
************************************
David William Bitner
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