On Wed, 13 Feb 2008, Old Central wrote:

IIRC the use of GPS to determine heights is a complex topic. You need to
determine the spheroid and geoid separation in relation to the grid used
and so on. Remember that many countires use by the different versions of
these for their mapping and with different origins.

If you want to know the height above local sea level, then yes, you need a
map of the geoid. But nobody uses that. In the UK, we use height above the
OSGB36 datum, which can be computed from the WGS84-based GPS height fairly
easily (not trivially, but a computer can do it without breaking a sweat).
Ditto for any other reference frame.

Additionally as identified above there are satellite fix issues.

I think this is the killer. There's just so much inaccuracy in a typical
height measurement that it's not very useful.

I suspect then, as a relative method it would be reliable but more
difficult to be reliable as an absolute method.

Interesting point.

tom

--
Me ant a frend try'd to WALK the hole unterrgrand but was putting off -
sometime we saw a trane! -- Viddler Sellboe