On Sat, 26 Jun 2004 22:27:34 GMT, "Wyatt Emmerich"
wrote:

My Garmin 296 indicates 330 MSL at KHKS, which is correct. My Echo Flight
GPS indicates 140 MSL. Both are locked on to WAAS.

How can I have such a large discrepancy? Both units seem to be functioning
properly other than this.


Actually, neither one may be accurate.

Depending on where you are on the field, I see charted elevations of
306-342'.

Airport elevations and GPS elevations are determined differently.
Actually, there are over twenty different standards for determining
altitudes. And how accurately a particular WAAS enabled GPS receiver
translates it's GPS altitude to "airplane" altitude can vary depending on
the software algorithms being used.

Although I would expect that TSO 146 certified GPS receivers would perform
the conversion in the same way, I would not have that expectation with
regard to portable, non-certified GPS receivers.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)