GPS does give relatively accurate altitude/speed but it does have
error and is nowhere near as accurate as a calibrate barometer (since
an aircraft altimeter is just a barometer with a different face).
Assuming someone has a data plan you can get the barometric pressure
from a nearby airport (most have a weather station, all data is pooled
by the FAA online**). Wouldn't be sure about other countries but I bet
France/most sizable would have something similar. That or you could
assume the "standard" 29.92 in/Hg and 1000ft per 1 in/Hg.
An even more desperate thing you could do is call 1800-WX-BRIEF and
ask what the baro is at a local airport.
**They also maintain a very detailed list of all airports available as
an XLS file.
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