Altitude is the elevation of an object from a known level or datum.
Common datums are mean sea level and the surface of the WGS-84 geoid, used by GPS.
In the United States and the UK aviation altitude is usually measured in feet.
Everywhere else in the world the altitude is measured in metres. Atmospheric pressure decreases as altitude increases.
This principle is the basis of operation of the pressure altimeter, which is an aneroid barometer calibrated to indicate altitude instead of pressure.
It is the fall in pressure that leads to a shortage of oxygen (hypoxia) in humans on ascent to high altitude.
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