Marc Ramsey wrote:

GPS altitude can not be corrected to pressure altitude with reasonable
error bounds, unless specific meteorological data is provided for the
time and place of the flight. Pressure altitude can not be corrected to
geometric altitude with reasonable error bounds, unless specific
meteorological data is provided for the time and place of the flight.
Without making these meteorological corrections, geometric and
calibrated pressure altitude can differ by as much as 1000 feet for a
Diamond altitude gain.

So what's so bad about 1000ft of error? That's 8% for a diamond gain.
If one uses a GPS, make 'em go 1000ft higher than the requirement...

4281 ft for silver, 10843 ft for gold, 17404 ft for
diamond... would that be enough to make up for any error?

Sounds good to me...still quite silly, since we are talking about
a gain when measured by the same device for badges (not a
comparison of pressure to geometric altitude).

Really accurate altitude and the distinction only has meaning
for situations where altitude, rather than gain or loss of
altitude, is a factor. Requiring pressure altitude for badges
makes no sense at all, IMHO, if fairly accurate GPS altitudes
are available...and I believe this is available and verifiable under
the current system (although the FAA has been slow allowing
GPS vertical guidance for approaches as a matter of caution
and safety, RAIM is available)...
altitude gain or loss
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Mark Boyd
Avenal, California, USA