Altitude, Flight Level and Height
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The vertical distance of an object measured from mean sea level.
Flight Level (FL)
A surface of constant atmosphere pressure which is related to a specific pressure datum, 1013.2hPa, and is separated from other such surfaces by specific pressure intervals.
Altitude above sea-level in 100 feet units measured according to a standard atmosphere. Strictly speaking a flight level is an indication of pressure, not of altitude. Only above the transition level (which depends on the local QNH but is typically 4000 feet above sea level) are flight levels used to indicate altitude; below the transition level feet are used.
e.g. FL250 = 25,000 feet above mean sea level when the pressure at sea level is 1013.2 mb.
Elevation or Height
The vertical distance of a level, a point, or an object considered as a point, measured from a specified datum.
Units of Measurement
The primary unit of measurement of altitude and elevation or height is the metre. However, the most widely used unit of measurement in aviation is the foot. Metric altitudes and flight levels are used in certain countries. ICAO Annex 5 and Supplement give a partial listing of the units of measurement used in most countries. If any doubt exists, the Aeronautical Information Publication (AIP) of the country in question should be consulted.