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<<When my GPS-reported altitude closely tracks my altimeter-reported
altitude (usually only 25 to 75 feet difference) and further, when my
calibrated transponder reports an altitude that corresponds to my
altimeter (so reported by controllers), are these adequate indications
that my instruments (GPS, altimeter and transponder all three) are
correct and well-calibrated?>>

Yes and no, mostly no.  The information that comes both from the altitude
encoder and independently from the altimeter are nothing more than
representations of the static pressure measured at the static port.  If this
is actually the ambient pressure at your altitude then you should get an
accurate reading.  If not, the resulting error is called "position error"
(There's also an instrument error, but this is checked biennially as required
by the FAR's).

If the altimeter doesn't match the reported altitude (via the encoder and the
transponder) then one of those two gizmos is out of calibration.  However, if
the static pressure does not match the ambient pressure (because the static
port is in the wrong place) then both can agree perfectly and you could still
be way off your actual or "tapeline" altitude.

Using the GPS as a reference is not a bad idea as a rough check, but I don't
know how accurate GPS altitude is.  If GPS altitude matches the altimeter
that's a good thing, but if it's 150 feet off there's no way to tell which is
correct without additional testing (which could fill up lots of e-mail space
and I won't get into here).

- Rob Wolf
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