text: Others before me said air pressure is used. That is correct.\n\nAir pressure decreases at a steady rate as you go up in altitude. Using this fact, along w/ an instrument that measures air pressure, height above sea level can be determined.\n\nAltimeters are commonly used on airplanes to give them altitude readings; although they do have to reset to zero according to current elevation to give them correct readings. This instrument is also used to find elevation.\n\nMean (average) sea level is used as a starting point for 0 feet elevation.