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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.<br>
<br>
thanks,<br>
mike<br>
<br>
<a class="moz-txt-link-abbreviated" href="mailto:david.bitner@gmail.com">david.bitner@gmail.com</a>
wrote:
<blockquote cite="mid4ECE4FEECC.0EB06C4FTFSVEVOZ@gmail.com" type="cite">
<pre wrap="">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 <a class="moz-txt-link-rfc2396E"
href="mailto:mfrumin@rpa.org"><mfrumin@rpa.org></a> wrote:
</pre>
<blockquote type="cite">
<pre wrap=""> 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
</pre>
</blockquote>
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</pre>
<br>
<hr size="4" width="90%"><br>
<br>
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:<br>
select *,distance_sphere(hospital1geom,hospitals2geom) from hospitals2
where distance_sphere(hospital1geom,hospitals2geom)<25*1609 order by
distance_sphere(hospital1geom,hospitals2geom) asc; <br>
<br>
Just add a limit 1 to the end if all you want is the closest.<br>
<br>
No buffer necessary at all.<br>
<br>
<br>
David<br>
<br>
<br>
<div><span class="gmail_quote">On 4/2/07, <b class="gmail_sendername">Michael
Frumin</b> <<a href="mailto:mfrumin@rpa.org">mfrumin@rpa.org</a>>
wrote:</span>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div bgcolor="#ffffff" text="#000000">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?<br>
<br>
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.<br>
<br>
How egregious would you expect the errors to be if I simply use the
projection for the UTM zone that represents, say, Central time?<br>
<br>
thanks,<br>
mike<br>
<br>
</div>
</blockquote>
</div>
</blockquote>
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