GPS altitude vs altimeter altitude

The barometric altimeter is calibrated to zero at the wheel
contact point with the ground. The GPS measures at the
antenna. Like all things made by man, there are tolerances.

Reason the baro-altimeter is wheel height, think of a 747,
do you want the cockpit or the wheels to clear the trees at
the end of the runway?



--
James H. Macklin
ATP,CFI,A&P

wrote in message
oups.com...
| To respond to the original posters question. I have often
asked the
| same thing. My observations are this. My plane is sitting
in the hangar
| at 6412.37 msl. The airport was just recently surveyed so
I am sure
| this is a real close number. I set my altimeter to the
current setting
| and it shows within 3 feet. I fire up my King KMD150 MFD
and it will
| almost always show 8- 12 feet higher then that, never seen
it lower. It
| must be that mathmatical modeling thing. Then I fire up my
Garmin 196
| and it varies between 15 feet higher and 10 feet lower. I
attribute
| that to a less sensitive circuitry in the handheld unit
altho I might
| be wrong surmising that. I would be curious to see what
others see in
| their respective areas.?
|
| Ben
| www.haaspowerair.com
|

Jim Macklin wrote:
The barometric altimeter is calibrated to zero at the wheel
contact point with the ground. The GPS measures at the
antenna. Like all things made by man, there are tolerances.

Reason the baro-altimeter is wheel height, think of a 747,
do you want the cockpit or the wheels to clear the trees at
the end of the runway?




Having been on an overloaded business Cessna that took off just clearing
the trees (he raised the gear in ground effect to get a little more
speed to make it) I'd hope that commercial aircraft piloting would be a
little less "bold". :-)

Richard

Good point. My GPS antenna is 5' 7" off the ground... The error is less
then I thought. I am VERY impressed now.!!!!!!!!!!