[postgis-users] Third dimension units

[Olivier Courtin]

On Nov 12, 2009, at 12:06 PM, Gilles Bassière wrote:

Hi Gilles,

> I've always thought that WGS84 is expressed in decimal degrees for  
> *all*
> dimension (even if 100° deep does not make sense to me).

EPSG:4326 is only defined for 2D, for 3D with WGS84 you should
have a look on EPSG:4979 <http://spatialreference.org/ref/epsg/4979/>

> Does this means that Z coordinates are always expressed in meters?

Yes, at least the case for EPSG:4979

Not really clear indeed (for me) if it's above the mean sea level,
or above WGS84 ellipsoid.

> How does ST_Transform normally behaves with 3rd coordinate?

My guess is that ST_Transform just don't care about the Z dimension,
as altitude should be the same before and after point reprojection.

SELECT ST_AsEWKT(ST_Transform(
		ST_GeomFromEWKT('SRID=4326;POINT(5 5 100)'), 4979));

Give: SRID=4979;POINT(5 5 100)

HTH,

--
Olivier