[postgis-users] Re: srid / area

[Michael Fuhr]

On Wed, May 17, 2006 at 02:04:22PM +0200, Christo Du Preez wrote:
> Yes, I'm quite sure my data is in decimal degrees.
> 
> I ran [select UpdateGeometrySRID('public', 'cdsmriverarea', 'the_geom', 
> 4148);] on my data. I did notice if you look below that there is no 
> units specified at the end. So I tried using transform with srid 2046, 
> which does specify units=m. And I'm still getting the same result.

SRID 2046 has proj=longlat so coordinates in that system are
apparently degrees.  I don't know if that's correct but you can see
that transforming to that SRID yields degrees:

SELECT AsEWKT(Transform(GeomFromEWKT('SRID=4148;POINT(18 -33)'), 2046));
         asewkt          
-------------------------
 SRID=2046;POINT(18 -33)
(1 row)

That would explain why areas still aren't in the desired units
(square meters).

Does transforming to an appropriate UTM zone give results closer
to what you're expecting?  If SRID=4148;POINT(18 -33) is a
representative location then try SRID 32733 or 32734 (WGS 84 / UTM
zone 33S or 34S).  Offhand I don't know if there's a more appropriate
area-preserving SRID.

-- 
Michael Fuhr