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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>David,</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>This is very nice, I think this
could also be expanded, with some math, to do the extent case as well.
You make the assumption that the UTM zone of geometry a is a better fit than
geometry b’s zone, but I think that is a reasonable starting point, though
arbitrary.</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Will diddle around with this one. Thanks!</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>r.b.</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<div>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Robert W. Burgholzer</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Surface Water Modeler</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Office of Water Supply and Planning</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Virginia</span></font><font size=2
color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:navy'> </span></font><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'>Department</span></font><font
size=2 color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:navy'> of Environmental Quality</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><a
href="mailto:rwburgholzer@deq.virginia.gov">rwburgholzer@deq.virginia.gov</a></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>804-698-4405</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Open Source Modeling Tools:</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><a
href="http://sourceforge.net/projects/npsource/">http://sourceforge.net/projects/npsource/</a></span></font></p>
</div>
<p class=MsoNormal style='margin-left:.5in'><font size=2 face=Tahoma><span
style='font-size:10.0pt;font-family:Tahoma'>-----Original Message-----<br>
<b><span style='font-weight:bold'>From:</span></b>
postgis-users-bounces@postgis.refractions.net
[mailto:postgis-users-bounces@postgis.refractions.net] <b><span
style='font-weight:bold'>On Behalf Of </span></b>David William Bitner<br>
<b><span style='font-weight:bold'>Sent:</span></b> </span></font><font size=2 face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>Thursday, May
15, 2008</span></font><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma'> </span></font><font size=2 face=Tahoma><span
style='font-size:10.0pt;font-family:Tahoma'>3:49 PM</span></font><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'><br>
<b><span style='font-weight:bold'>To:</span></b> PostGIS Users Discussion<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [postgis-users]
Newbie questions: SRIDs, function return values</span></font></p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p class=MsoNormal style='margin-right:0in;margin-bottom:12.0pt;margin-left:
.5in'><font size=3 face="Times New Roman"><span style='font-size:12.0pt'>This
is where a slightly less easy to use implementation that I put in the PostGIS
Wiki as a PL/PGSQL function presents a slightly more robust approach to the
problem. The use of this function simply returns the UTM SRID for the
appropriate zone for a point that you pass in (note: this could be expanded to
be center of extent or whatever pretty easily). So the use of this would be:<br>
select blah from a,b where st_dwithin(transform(a.the_geom,utmzone(a.the_geom)),transform(b.the_geom,utmzone(a.the_geom)),100);<br>
<br>
While being quite a bit more verbose, at least this approach holds the zone
constant. Granted it is still rife with the problem of geometries being a
distance similar to the size of a UTM zone or more apart or sizes similar to
UTM zones, but it at least holds to one zone per comparison.<br>
<br>
David</span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>CREATE OR REPLACE FUNCTION utmzone(geometry) RETURNS
integer AS </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>$BODY$ </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> declare </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> geomgeog geometry; </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> zone int; </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> pref int; </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> begin </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> geomgeog:=transform($1,4326); </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> if (y(geomgeog))>0 then </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> pref:=32600; </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> else </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> pref:=32700; </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> end if; </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> zone:=floor((x(geomgeog)+180)/6)+1;
</span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> return zone+pref; </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> end; </span></font></p>
<p class=MsoNormal style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>$BODY$ LANGUAGE 'plpgsql' immutable;</span></font></p>
<p class=MsoNormal style='margin-right:0in;margin-bottom:12.0pt;margin-left:
.5in'><font size=3 face="Times New Roman"><span style='font-size:12.0pt'> </span></font></p>
<div>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>On Thu, </span></font>May 15, 2008 at 9:57 AM, Paul Ramsey <<a href="mailto:pramsey@cleverelephant.ca">pramsey@cleverelephant.ca</a>>
wrote:</p>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>The danger of this approach is that it falls apart as
people start<br>
pushing the limits...<br>
<br>
select blah from a, b where st_dwithin(st_utm(a.geom),st_utm(b.goem),100)<br>
<br>
It's a pandora's box... as are all things geodetic<br>
<font color="#888888"><span style='color:#888888'><br>
p</span></font></span></font></p>
<div>
<div>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'><br>
On Thu, </span></font>May 15, 2008 at 7:45 AM, Markus Schaber <<a
href="mailto:schabi@logix-tt.com">schabi@logix-tt.com</a>> wrote:<br>
> Hi, Paul,<br>
><br>
> "Paul Ramsey" <<a href="mailto:pramsey@cleverelephant.ca">pramsey@cleverelephant.ca</a>>
wrote:<br>
><br>
>> There's no universal answer to this, but I have long thought that a<br>
>> simple answer suitable for may people would be a ST_UTM(geom) wrapper<br>
>> on transform that picks the appropriate UTM zone for a given geometry.<br>
>> It would work perfectly well for any collection of small objects. It<br>
>> would fall apart for larger things like states and countries that are<br>
>> of similar size to a UTM zone.<br>
><br>
> The function could simply use the centroid of the bounding box, that<br>
> shoul be fast enough.<br>
><br>
><br>
> Regards,<br>
> Markus<br>
><br>
> --<br>
> Markus Schaber | Logical Tracking&Tracing International AG<br>
> Dipl. Inf. | Software Development GIS<br>
><br>
> Fight against software patents in Europe! <a href="http://www.ffii.org"
target="_blank">www.ffii.org</a><br>
> <a href="http://www.nosoftwarepatents.org" target="_blank">www.nosoftwarepatents.org</a><br>
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<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'><br>
<br clear=all>
<br>
-- <br>
************************************<br>
David William Bitner </span></font></p>
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