Stephen, 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.
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): SRTEXT = GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 8 4",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0 ],AUTHOR ITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT[ "degree" ,0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4326"]] Specifically, the parameter: UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]] 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). So, the real question may be, "can PostGIS be told to measure distance in a specified unit other than by using a transform?". Robert W. Burgholzer Surface Water Modeler Office of Water Supply and Planning Virginia Department of Environmental Quality [EMAIL PROTECTED] 804-698-4405 Open Source Modeling Tools: http://sourceforge.net/projects/npsource/ -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Stephen Lee Sent: Tuesday, May 06, 2008 3:35 AM To: PostGIS Users Discussion Subject: Re: [postgis-users] question on returning area size using st_area. Thanks Robert, 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: SELECT st_area(transform(the_geom, 3385)) / 10000 as hectares FROM mal_state WHERE name = 'KELANTAN' thou i still have no way of verifying them, will try to verify it somehow. thanks again for your fast and helpful response. 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? Start reading the PostGIS manual, really appreciate if someone can help answer these questions or point me the right way. Thanks in advance. Best regards, Stephen On Tue, May 6, 2008 at 3:02 AM, Burgholzer,Robert <[EMAIL PROTECTED]> wrote: Stephen, 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: SELECT st_area(transform(the_geom, 26918)) / 10000 as hectares FROM mal_state WHERE name = 'KELANTAN' 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. If anyone else has better, more comprehensive insight on this, I for one would like to hear about it, Robert W. Burgholzer Surface Water Modeler Office of Water Supply and Planning Virginia Department of Environmental Quality [EMAIL PROTECTED] 804-698-4405 Open Source Modeling Tools: http://sourceforge.net/projects/npsource/ -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Stephen Lee Sent: Monday, May 05, 2008 2:27 PM To: postgis-users@postgis.refractions.net Subject: [postgis-users] question on returning area size using st_area. hi there, i've just started using postgis, i have loaded a sample map of malaysia using the following command: F:\PROGRA~1\PostgreSQL\8.3\bin>shp2pgsql -c -s 4326 G:\Maps\Malaysia\Detailsmaps\state_region public.mal_state > state.sql Shapefile type: Polygon Postgis type: MULTIPOLYGON[2] the projection file in the prj is as follow so I assume the SRID is 4326 : GEOGCS["Lat Long WGS84",DATUM["D_WGS84",SPHEROID["World_Geodetic_System_of_1984",6378137, 298.257222932867]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943 295]] 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 (4.6.2.2). the sql statement i issue is: SELECT ST_Area(the_geom)/10000 AS hectares FROM mal_state WHERE name = 'KELANTAN'; hectares ------------------ 0.000122167591783352 (1 row) 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. best regards, Stephen _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
_______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
