[CF-metadata] vertical coordinates

From: Steve Emmerson <steve>
Date: Thu, 07 Aug 2008 16:13:00 -0600

Ethan Davis wrote:

> "Vertical CRSs make use of the direction of gravity to define the
> concept of height or depth. By implication therefore, ellipsoidal
> heights (h) cannot be captured in a vertical CRS: ellipsoidal
> heights cannot exist independently, but only as an inseparable part
> of the 3D coordinate tuple defined in a geodetic 3D CRS."
>
> I'm not sure I understand since I thought a vertical datum could be
> based on an ellipsoidal earth.

I think the problem lies with the fact that, in general, a vertical ray
(as determined by gravity) is not parallel to a ray at the same place
that is perpendicular to the surface of any Earth-based ellipsoid. This
is because the geoid (defined as an equipotential surface) deviates from
a true elipsoid (it has dips and bumps).

--Steve
Received on Thu Aug 07 2008 - 16:13:00 BST