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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'>Stephen,</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 a classic, and I think it must be
in some FAQ some where. The only way I have done it is to re-project
using the transform function to something whose units are in meters, since the
4326 SRID has distance units in meters. Thus, something like the following:</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'>SELECT st_area(transform(the_geom, 26918))
/ 10000 as hectares </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>FROM mal_state</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>WHERE name = ‘KELANTAN’</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'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Now, in this example, I have chosen SRID
26918, which off the top of my head is like NAD 1983, UTM Zone 17. This
is a fairly appropriate projection for my area (eastern US), however, I don’t
know enough about projections to say without reservation that you could use any
old projection without concerning yourself with whether or not the projection
you use is valid in the area that you are operating on in DD.</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'>If anyone else has better, more
comprehensive insight on this, I for one would like to hear about it, </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'> </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 Department 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>
<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=3 face="Times New Roman"><span style='font-size:
12.0pt'> </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>Stephen Lee<br>
<b><span style='font-weight:bold'>Sent:</span></b> Monday, May 05, 2008 2:27 PM<br>
<b><span style='font-weight:bold'>To:</span></b>
postgis-users@postgis.refractions.net<br>
<b><span style='font-weight:bold'>Subject:</span></b> [postgis-users] question
on returning area size using st_area.</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'>hi
there,<br>
<br>
i've just started using postgis, i have loaded a sample map of malaysia using
the following command:<br>
<br>
F:\PROGRA~1\PostgreSQL\8.3\bin>shp2pgsql -c -s 4326
G:\Maps\Malaysia\Detailsmaps\state_region public.mal_state > state.sql<br>
<br>
Shapefile type: Polygon<br>
Postgis type: MULTIPOLYGON[2]<br>
<br>
the projection file in the prj is as follow so I assume the SRID is 4326 :<br>
<br>
GEOGCS["Lat Long WGS84",DATUM["D_WGS84",SPHEROID["World_Geodetic_System_of_1984",6378137,298.257222932867]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]<br>
<br>
the map is successfully loaded and can be viewed using uDig. The problem occurs
when I tried to use the st_area command as state in the PostGIS (1.3.3) manual
(<a href="http://4.6.2.2">4.6.2.2</a>). the sql statement i issue is:<br>
<br>
SELECT<br>
ST_Area(the_geom)/10000 AS hectares<br>
FROM mal_state<br>
WHERE name = 'KELANTAN';<br>
<br>
hectares <br>
------------------ <br>
0.000122167591783352 <br>
(1 row)<br>
<br>
is the return result measure in the unit of the WGS84 projection? if so how can
i change it to the correct measurement? i have attached my map file in this
email, can someone please shine some light? thanks in advance.<br>
<br>
best regards,<br>
<br>
Stephen</span></font></p>
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