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

[Michael Frumin]

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> 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
>>
>>
>>     
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>   
>
> ------------------------------------------------------------------------
>
>
> 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 <mailto: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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20070402/9e2db891/attachment.html>