The pilot knows your altitude through an altimeter that correlates altitude with the atmospheric pressure outside the aircraft.
An altimeter is a device used to measure the height of objects above a fixed level. Height measurements are called altitude measurements and are related to the term altimetry, which is a measurement of depth in water.
The altimeter is a device that measures altitude (distance from sea level). Most altimeters are barometers. That is, it calculates the barometric pressure at the location and measures the altitude.
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