Aviation altitude (vertical distance)

  1. http://web.archive.org/web/20130626035126/http://www.gmat.unsw.edu.au/snap/gps/clynch_pdfs/coordcvt.pdf page 5
  2. http://www.esri.com/news/arcuser/0703/geoid1of3.html “what is MSL”
NameDescription
True AltitudeThis is the actual, physical distance between the aircraft and the mean sea level. It is what you would see if you could actually measure the distance with a very big ruler. In the real world, pilots usually do not use this, as it can usually only be measured approximately by GPS. In the virtual world (which is what swift is modelling), it is what we generally use when talking to FSD and the simulators.
Absolute Height AGLThis is the actual, physical distance between the aircraft and the ground directly beneath it. This could be as measured by a radar altimeter.
Altitude QNHThis is the altitude above mean sea level as measured by a pressure altimeter on the aircraft. In order to relate this to any other type of altitude, two data must be known: the actual atmospheric QNH (e.g. obtained by METAR), and the QNH setting which the pilot has dialled into their altimeter; these two data might be different, so both must be known.
Flight LevelBesides the fact that the unit of measurement is always hundreds of feet, the only difference between Flight Level and Altitude QNH is that the QNH setting is always fixed at 1013.2hPa or 29.92inHg. Therefore one datum must be known: the actual atmospheric QNH (e.g. from METAR).
Height QFEThis is the height above a reference elevation at e.g. an airport. Height QFE differs from Altitude QNH only in that the reference point is an elevation above or below mean sea level, rather than mean sea level itself (MSL+0). Therefore three data must be known: the actual atmospheric QNH, the pilot’s QNH setting, and the QFE reference elevation.

In addition, an altitude can be either an indicated altitude of an actual aircraft or other object in real time, or a nominal altitude such as entered in a flight plan. These two groups, multiplied by the five above, gives a total of ten.

This matrix shows the data which would be needed to convert between different types of altitude:

True AltitudeAbsolute Height AGLAltitude QNHFlight LevelHeight QFE
True AltitudeGround elevationActual QNH and set QNHActual QNHActual QNH, set QNH, and QFE elevation
Absolute Height AGLGround elevationGround elevation x2Ground elevation, actual QNH, and set QNHGround elevation and actual QNHGround elevation, actual QNH, set QNH, and QFE elevation
Altitude QNHActual QNH and set QNHGround elevation, actual QNH, and set QNHActual QNH x2 and set QNH x2Actual QNH x2 and set QNHActual QNH x2, set QNH x2, and QFE elevation
Flight LevelActual QNHGround elevation and actual QNHActual QNH x2 and set QNHActual QNH x2Actual QNH x2, set QNH, and QFE elevation
Height QFEActual QNH, set QNH, and QFE elevationGround elevation, actual QNH, set QNH, and QFE elevationActual QNH x2, set QNH x2, and QFE elevationActual QNH x2, set QNH, and QFE elevationActual QNH x2, set QNH x2, and QFE elevation x2