altimeter - / ˈæltɪmiːtə / -
noun
Pocket device, usually digital, to measure altitude. Most altimeters used in outdoor activities are in fact digital barometric pressure sensors, these measure air pressure to calculate altitude and can be more reliable than GPS in certain situations, for example where there is no signal or when available satellites are near the horizon.
\\ I can't see a thing in this cloud, I guess we aren't far from the summit, what's your altimeter showing?
