Indicated vs GPS altitude: which should I use?

In real-world aviation and flight simulation, indicated altitude comes from the barometric altimeter and depends on its pressure setting, while GPS altitude is a geometric satellite-derived value. Use indicated altitude for ATC clearances, airspace, flight levels and published procedures; treat GPS altitude as a cross-check unless a specific approved system or procedure says otherwise.

What do indicated altitude and GPS altitude measure?

The two readings derive altitude differently and may not use the same vertical reference.

A barometric altimeter does not measure physical distance from sea level. It measures pressure and assumes a standard relationship between pressure and height. With the correct QNH selected, its indication is designed to match the altitude system used by pilots, ATC and published charts.

A GPS receiver first calculates geometric height relative to an ellipsoid such as WGS 84. Many aviation units convert that to an approximate mean-sea-level altitude, but labels and implementations vary. Integrated avionics may also blend GNSS and air-data information, while a simulator may expose a raw, corrected or game-engine altitude value.

Why do indicated altitude and GPS altitude disagree?

A difference is normal because pressure, temperature, vertical datum and sensor accuracy all affect the comparison.

• Incorrect or stale QNH: If atmospheric pressure changes or the aircraft moves into an area with a different setting, indicated altitude drifts away from geometric altitude.
• QFE instead of QNH: QFE makes the altimeter read approximately zero at its reference aerodrome. GPS altitude normally remains referenced to sea level, producing a difference close to the aerodrome elevation.
• Standard pressure selected: With 1013.25 hPa or 29.92 inHg set, the altimeter shows pressure altitude for flight-level operations, not actual height above mean sea level.
• Non-standard temperature: QNH corrects for pressure but not the full temperature profile. In air colder than standard, true altitude is lower than indicated; in warmer air, it is higher.
• Different GPS references: Ellipsoid height and geoid-corrected MSL altitude are not interchangeable. The difference can be substantial if the display or simulator variable is misunderstood.
• Normal measurement error: GPS vertical accuracy is generally poorer than horizontal accuracy. Barometric systems also have instrument, static-source and modelling errors.
• Simulator discrepancies: Injected weather, simulator ATIS, an add-on aircraft and the scenery elevation database may not agree. A mismatch affecting one aircraft often points to its avionics or air-data implementation; a mismatch across every aircraft more often indicates weather or pressure configuration.

Which altitude should I use?

Use the barometric altimeter for normal aircraft operation because aviation’s vertical separation system is built around pressure altitude.

• ATC assignments: Fly the indicated altitude using the pressure setting supplied by ATC, ATIS or the applicable weather report.
• Published procedures: Observe charted crossing altitudes, minimum altitudes and barometric minima on the altimeter unless the procedure explicitly specifies another approved source.
• Flight levels: Set standard pressure when required by the local transition rules and use the barometric indication as a flight level. On descent, restore the appropriate QNH at the transition level or when instructed.
• Traffic circuits: Set QNH and add the circuit height to aerodrome elevation. Our practical circuit-altitude example shows how MSL altitude and height above the field relate.
• GPS navigation: Use GPS altitude for situational awareness and approved equipment-specific functions, not as a replacement for the barometric altimeter.

A GPS approach does not automatically make the GPS altitude readout the controlling altitude source. Follow the published vertical guidance, altitude restrictions and minima exactly as specified for the installed equipment and procedure.

Does GPS altitude show height above the ground?

Ordinary GPS altitude is not height above ground, and neither is indicated altitude when QNH is set.

An AGL figure on a moving map is often calculated by subtracting terrain-database elevation from the aircraft’s geometric position. A radio altimeter measures actual height over the surface directly, but only within its operating range. These three values can disagree over sloping ground, buildings or inaccurate scenery.

How can I troubleshoot a large altitude difference?

A systematic reference check usually identifies whether the mismatch comes from pressure, datum, weather or the simulated aircraft.

1. Identify each field. Confirm whether the display says barometric altitude, GPS altitude, MSL, ellipsoid height, radio altitude or AGL. If the panel labels are unfamiliar, our MSFS instrument-reading explanation distinguishes the main altitude references.
2. Set the correct pressure. Enter local QNH below the transition altitude, checking that hPa and inHg have not been confused. Our guide to setting QNH correctly in a simulator covers the pressure-setting workflow.
3. Check on the ground. With QNH set, the indicated altitude should be reasonably close to the published aerodrome elevation. A scenery elevation mismatch, GPS uncertainty or equipment tolerance may prevent an exact match.
4. Check for standard pressure. If STD is still selected after descent, the difference from GPS altitude may be several hundred feet when local pressure is far from standard.
5. Look at the pattern of the error. A nearly constant aerodrome-elevation offset suggests QFE or a datum mismatch. An error that changes with weather suggests stale QNH; one that grows in very cold air may be temperature-related; a fluctuating GPS value points towards vertical-position uncertainty.
6. Compare another aircraft or weather mode. This separates an aircraft-specific avionics problem from a wider simulator weather or scenery issue.

Do not adjust the pressure setting merely to make the altimeter agree with GPS. Set the pressure from the operational source, then investigate why the second value differs.