Barometric vs. Geometric Altitude: How Flight Trackers Calculate True Aircraft Height

1. The Fundamental Difference in Altitude Measurements

When tracking an aircraft online, users often notice subtle differences between reported altitude readings. Aviation safety relies on two distinct methods of height measurement:

• Barometric Pressure Altitude: Measured using static pressure ports and altimeters referenced to standard sea-level pressure (1013.25 hPa / 29.92 inHg). Above the transition altitude (e.g., 18,000 ft in North America, FL060 in Europe), all aircraft set their altimeters to 29.92 inHg to maintain vertical separation measured in Flight Levels (e.g., FL370 = 37,000 ft).
• Geometric GPS Altitude: Calculated by the aircraft's GNSS receiver based on WGS-84 ellipsoid mathematics. It represents the physical distance above mean sea level regardless of atmospheric pressure changes.

2. Barometric Altimeter Settings: QNH vs. QFE

Pilots adjust altimeter sub-scale settings based on flight phase:

• QNH: Altimeter setting that displays height above mean sea level (MSL). When parked at an airport, the altimeter reads airport elevation.
• QFE: Setting that displays height above the specific airfield threshold. When landed, the altimeter reads 0 feet.
• QNE (Standard): 1013.25 hPa setting used in cruise above transition altitude.

See airport elevations and the latest METAR weather reports on our Srinagar (SXR) and Dubai International (DXB) pages.