"Ron Lee" wrote in message
...
"Kev" wrote:


Mxsmanic wrote:
Marco Leon writes:
I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location?

In theory, yes. But in practice there is also a lot of inaccuracy in
altitude readings via GPS. The system is not designed to provide
highly accurate altitude, [..]

And a simple Google search with "gps altitude accuracy" will bring up
quite a few interesting write-ups on the topic. All urge extreme
caution about trying to use GPS as an altimeter.

Kev


Altitude accuracy is not as good as horizontal for reasons already
mentioned...not because of a design issue. It is not the accepted
form of altitude information because baro inputs do that. However, it
can be an indicator if somehow a pilot lost all altitude information.

Ron Lee

Never say never, as the old saw goes....
Having said that, I would never use my GPS altitude indication for
anything other than driving my car over the Rockies or Sierras.
On the other hand, the KLN-89B in the 172SP I used to fly had the
ability to dial in the local baro setting. I don't recall the delta between
the 89B and the legal altimeter being more than 20 feet with the baro date
set in. Biggest problem was the 89B altitude info was all but useless for
anything other than audible altitude deviation alert. That and having to dig
out the book and flip through the pages to remember how to input the baro
data. And, as I recall the altitude alerting was not legal for Part 91
either