"David Megginson" wrote in message
...
steve writes:

Doesn't the GPS give you 'altitude' -- computed as height above the
surface of an ellipsoid? Or does it correct for mean sea level? There
seem to be several computations required to derive true altitude from
the raw GPS altitude coordinate and it is not clear to me if
particular GPS receivers perform this adjustment.

I think that most of them use WGS84, which is a little more complex
than a simple ellipsoid.

Some models (Garmin's included) display orthometric (above the geoide or
roughly above mean sea level) by using a look up table for the geode height,
and some other models display the ellipsoid altitude.