Altimeter vs. GPS Altitude in Aviation

It is common for a cockpit altimeter and GPS altitude readout to show different numbers. That does not automatically mean one is broken. They measure altitude in different ways, using different references.

For pilots, the practical rule is straightforward: fly assigned altitudes and traffic pattern altitudes using the pressure altimeter, set correctly. Use GPS altitude as helpful situational awareness, not as the primary reference for altitude compliance with ATC instructions or published procedures.

How a Pressure Altimeter Works

A traditional aircraft altimeter is a pressure instrument. Static air enters the pitot-static system and reaches the altimeter. Inside, sealed wafers expand or contract as pressure changes. The instrument turns that pressure change into an altitude indication.

As the aircraft climbs, outside pressure decreases. The altimeter senses that pressure drop and displays a higher altitude. As the aircraft descends, pressure increases and the displayed altitude decreases.

The altimeter must be set to the proper pressure reference. In normal U.S. flying below the transition altitude, pilots use the local altimeter setting so the instrument displays altitude above mean sea level. In higher-altitude operations using flight levels, pilots use the standard setting.

How GPS Altitude Works

GPS altitude is based on satellite positioning. The receiver measures signals from multiple satellites and calculates the aircraft's position in three dimensions.

That vertical position is geometric. It is tied to a model of the earth, not to the same pressure reference used by a barometric altimeter. Because the systems use different methods and datums, the numbers can disagree even when both are operating normally.

GPS can be very accurate, but it is not the reference ATC expects you to use for altitude assignments in normal operations.

Why the Numbers Differ

Pressure altimeters are affected by pressure setting, temperature, static system errors, instrument errors, and blocked or disturbed static sources.

GPS altitude can be affected by satellite geometry, signal quality, receiver processing, database references, and the vertical model being used. Vertical GPS accuracy is also often less intuitive to pilots than lateral GPS accuracy.

The result is simple: the altimeter and GPS can disagree by tens or hundreds of feet. The amount may change with weather, location, equipment, and altitude.

Which One Should You Use?

Use the pressure altimeter for flying assigned altitudes, pattern altitudes, minimum altitudes, and altitude communication with ATC. Other aircraft and ATC systems are built around barometric altitude references.

Use GPS altitude as a backup awareness tool. It can help you notice a possible altimeter-setting error, understand terrain relationship, or cross-check what the airplane is doing. But if ATC assigns 4,500 feet, you do not level at a GPS-derived 4,500 while your altimeter says something else.

Common Student Mistakes

One mistake is forgetting to update the altimeter setting. If the pressure changes or you fly far enough from the last setting, your indicated altitude may become less accurate. Get updated weather and set the altimeter carefully.

Another mistake is using GPS altitude to judge traffic pattern height. Pattern altitude is flown by the aircraft altimeter unless your instructor or procedure gives a specific reason otherwise.

A third mistake is ignoring the static system during preflight. Static ports matter. A blocked or damaged static source can affect the altimeter, vertical speed indicator, and airspeed indication.

A Good Cockpit Habit

When you receive an altimeter setting, set it deliberately and read it back if appropriate. Compare the altimeter to known field elevation before takeoff. If the indication is unreasonable, stop and investigate.

In flight, cross-check. If GPS altitude, altimeter, terrain, and vertical speed do not make sense together, do not just pick the number you like. Treat it as a warning that something needs attention.

Pressure altitude and GPS altitude are both useful, but they are not interchangeable. The skilled pilot knows what each one is showing and uses the right tool for the job.

Related Reading

• Airspeed and Altitude Control Simplified
• Airspace Classes Explained
• How to Read an IFR Approach Chart

Official References

• eCFR Title 14
• FAA Reference
• FAA Reference
• FAA Aeronautical Information Manual
• 14 CFR Part 91 General Operating Rules