<html>
<head>
<meta http-equiv=Content-Type content="text/html; charset=us-ascii">
<meta name=Generator content="Microsoft Word 10 (filtered)">
<style>
<!--
/* Font Definitions */
@font-face
{font-family:Tahoma;
panose-1:2 11 6 4 3 5 4 4 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman";}
a:link, span.MsoHyperlink
{color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{color:blue;
text-decoration:underline;}
p
{margin-right:0in;
margin-left:0in;
font-size:12.0pt;
font-family:"Times New Roman";}
span.EmailStyle18
{font-family:Arial;
color:navy;}
@page Section1
{size:8.5in 11.0in;
margin:1.0in 1.25in 1.0in 1.25in;}
div.Section1
{page:Section1;}
-->
</style>
</head>
<body lang=EN-US link=blue vlink=blue>
<div class=Section1>
<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'>I am shooting in the dark a little here,
so, take what I say with a grain of salt. Hopefully, someone will see my
mangled attempt to understand this, and chime in with something sensible.
</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'>My guess is that, even though the spheroid
is WGS84, the units are set in the SR_text, as follows (for SRID 4326 from my
PostGIS db):</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'>SRTEXT = GEOGCS["WGS
84",DATUM["WGS_1984",SPHEROID["WGS 8</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>4",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHOR</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>ITY["EPSG","6326"]],PRIMEM["</span></font><font
size=2 color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:navy'>Greenwich</span></font><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'>",0,AUTHORITY["EPSG","8901"]],UNIT["degree"</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>,0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4326"]]
</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'>Specifically, the parameter: UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]]</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'>Tells me that it wants the units to be in
degrees? So then, what is the 0.0174xx number there? Once again, I can
only speculate, but I am assuming that perhaps this is a conversion to meters
or some other standard unit at some point on the map (perhaps at the prime
meridian). </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'>So, the real question may be, “can
PostGIS be told to measure distance in a specified unit other than by using a
transform?”.</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>
</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> Tuesday, May 06, 2008 3:35
AM<br>
<b><span style='font-weight:bold'>To:</span></b> </span></font><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>PostGIS Users
Discussion</span></font><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma'><br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [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'>Thanks
Robert,<br>
<br>
I have run a query on spatial_ref_sys to check all projection related to
Malaysia and have found quite a few, by trying a few projection listed there I
have some result that is more likely to be true:<br>
<br>
<br>
SELECT st_area(transform(the_geom, 3385)) / 10000 as hectares<br>
<br>
FROM mal_state<br>
<br>
WHERE name = 'KELANTAN'<br>
<br>
<br>
thou i still have no way of verifying them, will try to verify it somehow.
thanks again for your fast and helpful response.<br>
<br>
i do have another question, sorry if this is a newbie question, why do we need
to reproject it again if its in WGS84? i thought of putting all types of free
maps into one central database and open it to anyone who want to use them, if
this is the case, lets say someone contribute a map of Thailand, how do i
perform a spatial query if map in different geographic location has to be
manually handled? take for example, if a query involve both map in Thailand and
Malaysia, what kind of re-projection should i used? And how come the sample
data in PostGIS tutorial do not require a re-projection?<br>
<br>
Start reading the PostGIS manual, really appreciate if someone can help answer
these questions or point me the right way. Thanks in advance.<br>
<br>
Best regards,<br>
<br>
Stephen<br>
<br>
</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 Tue, May 6, 2008 at 3:02 AM, Burgholzer,Robert <<a
href="mailto:rwburgholzer@deq.virginia.gov">rwburgholzer@deq.virginia.gov</a>>
wrote:</span></font></p>
<div link=blue vlink=blue>
<div>
<p style='margin-left:.5in'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'>Stephen,</span></font></p>
<p style='margin-left:.5in'><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 style='margin-left:.5in'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p style='margin-left:.5in'><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 style='margin-left:.5in'><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 style='margin-left:.5in'><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 style='margin-left:.5in'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p style='margin-left:.5in'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p style='margin-left:.5in'><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 style='margin-left:.5in'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p style='margin-left:.5in'><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 style='margin-left:.5in'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p style='margin-left:.5in'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'> </span></font></p>
<div>
<p style='margin-left:.5in'><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 style='margin-left:.5in'><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 style='margin-left:.5in'><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 style='margin-left:.5in'><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 style='margin-left:.5in'><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" target="_blank">rwburgholzer@deq.virginia.gov</a></span></font></p>
<p style='margin-left:.5in'><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 style='margin-left:.5in'><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 style='margin-left:.5in'><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/" target="_blank">http://sourceforge.net/projects/npsource/</a></span></font></p>
<p style='margin-left:.5in'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
</div>
<div>
<div>
<p style='margin-left:1.0in'><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> <a
href="mailto:postgis-users-bounces@postgis.refractions.net" target="_blank">postgis-users-bounces@postgis.refractions.net</a>
[mailto:<a href="mailto:postgis-users-bounces@postgis.refractions.net"
target="_blank">postgis-users-bounces@postgis.refractions.net</a>] <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> <a
href="mailto:postgis-users@postgis.refractions.net" target="_blank">postgis-users@postgis.refractions.net</a><br>
<b><span style='font-weight:bold'>Subject:</span></b> [postgis-users] question
on returning area size using st_area.</span></font></p>
<p style='margin-left:1.0in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p style='margin-right:0in;margin-bottom:12.0pt;margin-left:1.0in'><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" target="_blank">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>
</div>
</div>
</div>
</div>
<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'><br>
_______________________________________________<br>
postgis-users mailing list<br>
<a href="mailto:postgis-users@postgis.refractions.net">postgis-users@postgis.refractions.net</a><br>
<a href="http://postgis.refractions.net/mailman/listinfo/postgis-users"
target="_blank">http://postgis.refractions.net/mailman/listinfo/postgis-users</a></span></font></p>
</div>
<p class=MsoNormal style='margin-left:.5in'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
</div>
</body>
</html>