It is indicated altitude corrected for non-standard temperature and pressure. True altitude is the actual elevation above mean sea level. Also referred to as radar height or feet/meters above ground level (AGL). It can be measured using a radar altimeter (or absolute altimeter). In UK aviation radiotelephony usage, the vertical distance of a level, a point or an object considered as a point, measured from mean sea level this is referred to over the radio as altitude.Ībsolute altitude is the vertical distance of the aircraft above the terrain over which it is flying. Indicated altitude is the reading on the altimeter when it is set to the local barometric pressure at mean sea level. There are several types of altitude in aviation:
When flying at a flight level, the altimeter is always set to standard pressure (29.92 inHg or 1013.25 hPa). Pressure altitude divided by 100 feet (30 m) is the flight level, and is used above the transition altitude (18,000 feet (5,500 m) in the US, but may be as low as 3,000 feet (910 m) in other jurisdictions) so when the altimeter reads 18,000 ft on the standard pressure setting the aircraft is said to be at Flight level 180.
On the flight deck, the definitive instrument for measuring altitude is the pressure altimeter, which is an aneroid barometer with a front face indicating distance (feet or meters) instead of atmospheric pressure. Aviation altitude is measured using either mean sea level (MSL) or local ground level (above ground level, or AGL) as the reference datum. Parties exchanging altitude information must be clear which definition is being used. In aviation, the term altitude can have several meanings, and is always qualified by explicitly adding a modifier (like true altitude), or implicitly through the context of the communication.
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