Meh, I like the compass/altimiter because it looks good. I never actually read it. I suppose I'd be just as happy if it were a good-looking clock.Like I said, maybe it is GPS based (post 872). But the overall accuracy is not the issue it is out to be. And not sure how losing the GPS signal causes it to reset. Like many things Grenadier just another nonsensical item.
As for altitude, GPS can get it 2 ways.
1) It can assume you are on the ground and give you an altitude based on your position and a topographical database.
B) If there are enough satellites in view, it can triangulate altitude. (necessary for aircraft since they might not be on the ground)
I'd guess INEOS uses the first way. It could remember the last lat/lon if it looses GPS and display that as altitude.
Somebody going through the Eisenhower Tunnel should keep an eye on the altimeter and see what it does while traveling underground.