[postgis-users] Third dimension units

[Suhr, Ralf]

ST_Transform will not touch Z-value unless you defined the towgs84 parameter in project definition. The towgs84 parameter reflect the difference between ellipsoids.

--

Gr 
Ralf

-----Ursprüngliche Nachricht-----
Von: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] Im Auftrag von Olivier Courtin
Gesendet: Donnerstag, 12. November 2009 13:14
An: PostGIS Users Discussion
Betreff: Re: [postgis-users] Third dimension units


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
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