It would be inconvenient; not to mention dangerous; to run a long, weighted string out the aircraft window to see how far you are above the ground. Fortunately, your plane has an instrument that measures the air pressure against a standard and tells you the difference in feet of altitude. It’s called the altimeter.
An altimeter is a barometer that measures atmospheric pressure. A barometer not only changes when a high- or low-pressure front moves through an area or when the temperature changes, it also changes when the barometer is moved higher into the atmosphere. At sea level under (rare) standard weather conditions, an altimeter will read 0 feet and the small window on it will indicate a barometric pressure of 29.92. If you then fly to a nearby airport that’s at an altitude of 600 feet the altimeter will read 600 feet at 29.92.
Knowledge Test The standard altimeter setting is 29.92 inches of mercury (Hg).
In fact, before you take off from an airport you’ll calibrate the altimeter by turning the adjustment knob until the airport elevation (posted somewhere near the main taxiway or found on aeronautical charts) is shown on the altimeter dial. The number in the small window; 29.92, 30.40, or whatever; is the “altimeter setting” for that location at that time. It’s your point of reference for your flight.
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An important point is that the altimeter reading indicates the feet above mean sea level, abbreviated MSL. It is not the distance above the ground; because the ground level changes as the terrain below changes. For example, if you’re supposed to fly over a location at 1,000 feet above ground level (AGL) and the map says ground level is 500 feet above MSL, your altimeter should read 1,500 MSL.
"Man cannot discover new oceans unless he has the courage to lose sight of the shore."
-- Andre Gide