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Regarding altimeter, GPS and encoder readings. They can be all be in perfect
working order and indicate different altitudes. The encoder indicates the
altitude at constant pressure (29.92 in Hg), controller's scopes compensate
for the current actual pressure so they will tell you what the compensated
altitude reading is. If the altimeter is set correctly it's reading will
correspond to the encoder. However the encoder's output is in 100 foot
quantums. Finally GPS is the most unreliable of all (in the case of a VFR
GPS). The altitude is determined (as is the position) by the difference in
timing signals from satellites. If the only satellites in view are clustered
above the receiver the time differences will be small and thus the
positional fix will be less accurate and the error is worst in the altitude
axis. So for altitude use the altimeter as everyone else around you is using
theirs. All errors in reported altimeter setting will cancel each other.

Pat
N747PW
"Only Three Short Years To Go!"
http://members.home.com/dylanw12


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