a google search on "GPS altitude discrepancies" comes up with a lot of
information...
the best explination I found is below, so it is a quote from another
source. BT

1) GPS altitude is TRUE altitude, not barometric altitude, above sea level
in reference to the center of the satellite's orbit, which should be the
center of the earth, but it isn't (see #2 below),

2) The idea of TRUE is in reference to a few different models of the
"geoid," The models have to fudge an average to get an idea of where
"ground" is. It is NOT the height above the surface of the earth - including
sea level. The geoid is is not a sphere. The geoid is not exactly where sea
level is. Where the real sea level is determined by gravitational
abnomalities.

http://www.esri.com/news/arcuser/0703/geoid1of3.html

BUT to some extent what we call MSL now, the barometric altitude is based on
the same inaccurate models with more fudging. We already know that the
altimeter setting where my plane is is going to be slightly off from what
the ATC is telling us to use - which was taken at a point in time earlier at
a place not where we are.

3) GPS receivers need WAAS to get good accuracy in the Z-axis. They are much
more accurate in the other two axi.

As was mentioned, some GPS receivers have also barometric altitude
capability so they they can reference the number that you and ATC expect.

You wanted an easy explanation?


"Pablo" wrote in message
oups.com...
I've been using a Garmin GPS396 unit for a couple of months now. I am
amazed at the accuracy of the elevation information. It always seems
to be within a few feet of the listed airport's elevation, for example.
I took it up in my old Cessna 170. The elevation at the airport was
right on. At altitude, however, it was off by as much as 600' from my
altimeter at 14,000'. Initially, I figured that my altimeter was off.
Then, I took it up in a friend's Malibu, and discovered similar
discrepancies from his altimeter. Again, the GPS is very accurate at a
number of ground points. I'm at a loss to explain this. I'm using
altimeter settings very near my location - so why would the altitudes
vary by so much?

In following up on this, I checked the GPS altitudes in the jet I fly,
but it was inconclusive. Using a nearby altimeter setting up in the
flight levels (around 35,000') the GPS altitude was different by a
couple hundred feet, and seemed to vary +/- 200'. But I don't know
how accurate the elevation is in this unit (Airbus 320), or even
whether it is corrected for differential. It is used primarily for
lateral navigation, and since the unit is about 5 years old, it may not
even have the accuracy of the latest from Garmin.

Does anybody out there have some thoughts about this, or maybe similar
experience?