Why pressure altimeter altitude is different from GPS-derived altitude

There is a difference between altitudes displayed on the aircraft pressure altimeter and a GPS-derived altitude.

A pressure altimeter is an aneroid barometer that measures the air pressure where the aircraft is located.

• The altimeter is tested and certified per FAR Part 43 Appendix E.
• Pilots can verify the operation of the altimeter per AIM 7-2-3.

Many expect that the current altimeter setting will compensate for irregularities in atmospheric pressure at all altitudes, but this is not correct.

• The altimeter setting is broadcast by ground stations and is representative of the ground station pressure corrected to mean sea level.
• Ground-based altimeter settings do not account for the irregularities at higher altitudes, particularly the effects of nonstandard temperature.
  • Over high, mountainous terrain, certain atmospheric conditions cause the altimeter to indicate an altitude of 1,000 feet or more higher than the actual altitude.
    • Nonstandard temperatures and variations of air pressure around mountainous terrain can cause a pressure altimeter to be inaccurate regarding altitude indication.
      
    • A generous margin of altitude should be allowed for possible altimeter error.
      

All aircraft are required to have a pressure altimeter and navigate based on the pressure indicated altitude. When all pilots in a given area are using the same altimeter setting, each altimeter will equally be affected by temperature and pressure variation errors, which maintains the desired vertical separation between aircraft.

Though not certified as a pressure altitude source, GPS receivers can generate a GPS-derived altitude value for the receiver based on triangulation of the received GPS satellite signals. This triangulation formula is used at all times and provides the same accuracy at all altitudes. The GPS-derived altitude value is not affected by temperature or air pressure variations.

• Refer to the FAA's Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25A) and other FAA documentation for more information.