Also keep in mind: Pressurized airliners have a cabin altitude of
8-10,000 feet.

I am absolutely no expert on this subject, so take this as a query, not
a correction: my Casio altimeter watch can easily be a hundred feet or
more off, especially if not calibrated at the start of a trip, but in a
fair amount of transcontinental and trans-Atlantic flying I've never
seen it indicate above maybe 6800 feet at most . . . ???

On the subject of cabin pressure, I have an interesting story to relate.

My father is a mechanical engineer, and he occasionally had to travel with
test equipment, to work in factorys at his destination.

On a trip to Europe and back, he had his equipment with him as usual, and
the return flight went into the wee hours of the night. He had a report to
write, so he was using the time on the flight to get it done. He started
getting sleepy, and out of curiosity, he took out an accurate altitude
barometer, and it showed an unusually high altitude. I don't remember for
sure what the number was, but I think it was in the 10,000 foot range. He
called a steward over, and told him to tell the captain to turn the cabin
pressure up to where it was supposed to be, that making everyone sleepy to
make the cabin attendant's life easier was not supposed to be happening.
The steward said, "Oh, they don't do that," to which my dad said, "Bull."
"I have equipment to prove it."

A short time later, the cabin pressure was back to normal.
--
Jim in NC