Re: SRID for analyzing a USA national data set in Meters

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@... 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@...> 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