> That's also the way to set up the pressure altimeter in a GPS receiver.
> Here's a decent article about GPS satellite-based altitude:
> http://gpsinformation.net/main/altitude.htm
>
> I love the last line: "Those who use GPS altitude to aid in landing
> their small plane should have their insurance policies paid up at all
> times."
>
As a pilot, I've heard lots of other pilots questioning the
discrepancy between the altimeter and GPS. It's two main factors... the
difference between the mathematical model of the earth the GPS uses (not
spherical) being one, but the non-standard pressure lapse rate with
altitude. That's primarily due to non-standard temperature, but can be
affected by other things such as windshear with winds aloft. Of course
the GPS must also support manually entering the current barometric
altimeter setting or else the resulting barometric reading is crap. (e.g.
standard day is 29.92 inHg, but good-weather high might be 30.30 inHg)
Pilots deal with this more often then most hikers since they
often fly higher than most hikers, and also fly through weather systems
(rather than hikers waiting for weather systems to pass them).
One interesting tidbit that's taught to pilots in Alaska and
Canada but not in the rest of the US deals with cold-weather compass
errors. Consider a pilot beginning a long descent to an airport at low
elevation, but there's high terrain along the way. The colder the weather
is from standard (e.g. arctic conditions) and the higher the terrain is
above the airport, the more inaccurate the pressure altimeter is relative
to *true* (i.e. "not hitting the rocks") altitude shown on aeronautical
charts. Most of the times it's not an issue, but it can be under the
right circumstances.
-Cory
--
*************************************************************************
* Cory Papenfuss, Ph.D. Electrical Engineering, PPSEL-IA *
* Research Associate, Vibrations and Acoustics Laboratory *
* Mechanical Engineering *
* Virginia Polytechnic Institute and State University *
*************************************************************************
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