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 would hope that an aeronautical GPS receiver would perform this
adjustment!) As for my decidedly non-aeronautical-non-WAAS-enabled
Magellan 410 GPS, it rarely correlates very closely to known altitude
benchmarks when I try it, even with many satellites 'locked.' God help
anyone who attempts a precision approach with my GPS!

Steve


bob mologna wrote:
I was on top of Lugnaqilla mountain in Ireland today and the GPS (Meridiam
Plat) put it at 925 - 924 metres. The topo puts it at 925. Just lucky
maybe...

"Craig Davidson" wrote in message
.net...

Jay Honeck wrote in message ...

How accurate is GPS for determining altitude, now that WAAS is active?

I was in the air over five hours yesterday, and while droning along found
that our new AvMap's altitude readout was just about dead-nuts

accurate --

it was maybe 50 feet off, at times.

In fact, due to the constantly changing barometric pressure along our

route

of flight, there were times when I might have trusted it before my
altimeter.

Anyone know how precise it is now?
--

Jay, The FAA seems to say 3 meters vertical throughout the majority of the
continental U.S. and portions of Alaska:

http://gps.faa.gov/Programs/WAAS/waas.htm


But the site below says 6 meters (95 percent of the time) with a worst

case

of 15.7 meters.

http://users.erols.com/dlwilson/gpswaas.htm



Here are some more interesting sites:

http://waas.stanford.edu/old_metrics.html

http://waas.stanford.edu/

http://www.edu-observatory.org/gps/dgps.html#WAAS