The FAA and, I would guess, all other airspace control agencies, assign
altitudes according to the local altimeter setting, not GPS. Not all
aircraft are equipped with GPS (yet).Â* Above 18,000' MSL, all aircraft
set their altimeters to 29.92 in Hg (or the metric equivalent).Â*
Regardless of your GPS data points, your barometric altimeter is the
controlling instrument in flight.Â* Note that I did not bother to address
the lowest usable flight level (when atmospheric pressure is below
29.92") as I've never seen that actually used.

I've been told that the Oudie can run XCSoar should you want to do so.Â*
From my XCSoar days (and I actually still run it on the right side of
my Stemme's cockpit), I recall that, when an airspace boundary is
approached, the user is presented with a warning and buttons to ACK the
warning once (which will reset the sensing) or ACK Day (which
acknowledges the warning for the day and it won't bother you again).Â* Of
course you'll then be responsible for monitoring your own position just
like in the good old days when men were men and women were women.

My ClearNav only allows me to silence the Controlled Airspace warning
and, after pressing the "OK" button, is reset for another go.Â* That can
be annoying when a thermal is right at the side of a Restricted Area as
every time the glider's heading is towards the area, the warning goes
off again.Â* I'll have to check my manuals to see if CN has an ACK Day
ability.

Bottom line to the above long-winded story is don't worry about your GPS
track getting you into trouble on altitude so long as your altimeter is
good.Â* It's a different story with the horizontal boundaries of
controlled airspace.

On 2/14/2019 7:46 AM, son_of_flubber wrote:
If you have ADS-B out or TABS, the FAA has a record of your airspace compliance (both barometric and GPS altitudes).

https://www.faa.gov/nextgen/programs/adsb/faq/#g7

ATC's network of ADS-B receivers provides coverage in many places where radar is blocked by mountains. ADS-B Performance reports are eye-opening wrt coverage https://adsbperformance.faa.gov/paprrequest.aspx

--
Dan, 5J