text: Appendix B
  working in and around controlled airspace or providing traffic separation clearance enroute, airspace element
  personnel must consider that the AGL varies while the MSL does not. Manned aircraft determine AGL with
  a radar altimeter.
  Height Above Ellipsoid
  B-19. Sometimes referred to as Global Positioning System GPS altitude, height above ellipsoid (HAE) uses
  GPS for altitude data use. For global applications, the geodetic reference (datum) used for GPS is the WGS-
  84. When the HAE is used, the height above the ellipsoid differs from the MSL. Direct elevation readings
  for most locations can differ up to hundreds of feet. This variation is caused, in part, because the GPS
  definition of altitude does not refer to MSL, but rather to a gravitational surface called the reference ellipsoid.
  Some aircraft self-reporting systems (see Appendix C) report GPS altitude as an HAE altitude, but some
  systems convert the GPS altitude to an MSL altitude prior to reporting. Munitions maneuvering with GPS
  data generally use HAE.
  B-20. Current digital systems use the USMTF standard for ACMs, and USMTF currently does not have an
  HAE option. Airspace personnel have to use MSL as a substitute. These personnel must know any significant
  differences between MSL, AMSL, AGL, and HAE for their AO and consider these differences when
  integrating airspace users.
  Flight Level
  B-21. A flight level (FL) is a standard nominal altitude of an aircraft in hundreds of feet (such as FL 250 is
  25,000 feet). This altitude is calculated from an international standard datum pressure of 29.92 inches of
  mercury (inHg), the average sea-level pressure, and therefore is not necessarily the same as the aircraft’s true
  altitude either above MSL or AGL. Airspace personnel normally use FL for flights above the transition
  altitude of 18,000 feet in the United States and Canada. The altitude that aircraft transition to FLs is called
  the transition altitude. Transition altitudes are local, regional, or national and vary considerably between
  about 3,000ft and 18,000ft. Regardless of altitude, Identification Friend Foe Mode C altitudes will be based
  on the standard datum pressure of 29.92 inHg.