If you have an perfectly calibrated barometric altimeter and
have a perfect altimeter setting taken just a few seconds
before, there are mechanical errors. Altimeter settings on
the ground will vary in just a few miles. The lapse rate is
not constant. But the GPS errors are very small and are
calculated and corrected in real time. But aircraft
separation is based on barometrics, so GPS is not used to
establish cruise altitudes.

GPS could be used for instrument approaches since the ground
and obstructions are fixed and generally known quantities.
GPS is used for photography and surveying.

GPS and terrain mapping will keep you safe from the ground
and the barometric altimeter is still needed to keep the
cruise altitudes set.


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

--
The people think the Constitution protects their rights;
But government sees it as an obstacle to be overcome.
some support
http://www.usdoj.gov/olc/secondamendment2.htm
See http://www.fija.org/ more about your rights and duties.


"Chris W" wrote in message
news:q2x0g.917$9c6.179@dukeread11...
| It is my understanding that even if a GPS is reading
perfectly and your
| standard altimeter is reading perfectly even on a standard
day that the
| 2 values will not be the same. Does anyone know about how
far off they
| are and if there is a way to calculate what the difference
should be?
| My guess is that at different points around the earth, the
correction
| will be significantly different, making a generic formula
difficult if
| not impossible to write.
|
| --
| Chris W
| KE5GIX
|
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