steve wrote:

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.

Most do but some do not. The computation required to derive true
altitude is not complex but requires a table of data that can be very
large. The math is simply to interprolate between table locations and
provide a simple addition or subtraction to the ellipsoid altitude. You
can determine if your receiver does this by looking at the NMEA data.
Check my web site for an article on how to decode this information. You
can compare the computed value with the accurate data that is avaialbe
on government web sites to see how good the table is.

Dale

--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs