[MetaCRS] Z values & vertical datum transforms

[Frank Warmerdam]

On Mon, Apr 2, 2012 at 7:25 PM, Richard Greenwood
<richard.greenwood at gmail.com> wrote:
> There has been a thread on the postgis list regarding changing the Z
> value when a 3 or 7 parameter horizontal transform is done. This seems
> totally wrong to me. In my mind horizontal and vertical datums, and
> their associated transformations, are independent of each other.
> 1. What does the Z value returned by a cs2cs 3 or 7 parameter
> horizontal transform represent?

Richard,

Elevation above the ellipsoid.

> 2. Are there cases in which it is appropriate to apply this to
> orthometric or ellipsoidal heights?

It is effectively an ellipsoidal height.

> 3. What's the general status of vertical transformations in Proj.4/cs2cs?

Elevation transformation is preserved through 3/7 parameter
datum shifts though the meaningfulness of that is questionable.

There is also support for one step conversion between ellipsoidal
and geoidal heights using geoid shift files when explicitly requested
with the +geoidgrids= directive.

There is some discussion at:

  http://trac.osgeo.org/proj/wiki/VerticalDatums

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Software Developer