What Is True Altitude and How Do You Calculate It?

True altitude is the reference altitude for many aeronautical charts, obstacles, and other points; it is critical to know and understand it when piloting any aircraft. True altitude is defined by the FAA as “the height above mean sea level (msl).” This means true altitude is the distance between an airplane at altitude and the average height of the oceans worldwide, which is determined by using still or calm water measurements to avoid fluctuations caused by waves or wind.

Understanding true altitude allows pilots to interpret geographical elevations, determine aircraft performance, and assess atmospheric conditions. By knowing how to calculate true altitude, pilots can effectively avoid midair collisions and ensure safe flight operations from takeoff through landing.

There are two techniques used to calculate true altitude. To calculate true altitude over water, pilots can use a radar or a radio altimeter and simply measure the altitude over the water. This technique is similar to calculating the absolute altitude and may not always be completely accurate.

The other technique of calculating true altitude involves a formula using the airplane’s indicated altitude and the temperature. Per the FAA, indicated altitude is “the altitude shown on the altimeter.” The True Altitude from Indicated Altitude and Temperature formula is True Altitude = Indicated Altitude + (ISA Deviation × 4/1,000 × Indicated Altitude). This formula accounts for the loss of 4 feet of altitude for each 1°C deviation for every 1,000 feet. The ISA Deviation is the atmospheric pressure.

The atmospheric pressure decreases as altitude increases and with the altitude changes, there are often temperature changes as well. An airplane’s altimeter was designed to perform at its best at standard pressures and standard temperatures. When non-standard pressures and temperatures exist, they can affect the reading of the airplane’s indicated altitude. Because indicated altitude is used to calculate true altitude, this can cause the true altitude to be inaccurate.

While it is rare for indicated or true altitude and pressure altitude to be equal, if the local barometric pressure is 29.92 inHg, then they could be the same. All aircrafts in the U.S. and Canada flying 18,000 feet above the mean sea level are required to have the same altimeter settings of 29.92 inHg. This allows them to be on the same flight levels, even though ground and sea levels are constantly changing below them.

Temperature affects true altitude in the same way as pressure does. Temperatures that are too hot or too cold cause the airplane’s indicated altitude to be off, which in turn affects the true altitude calculations.

Colder temperatures cause true altitude to be lower than indicated altitude. The lower the temperature, the lower the true altitude. Adding in the proper temperature corrections can be crucial in ensuring an airplane clears all obstacles and terrain along its flight path.

Calibrated altitude is indicated altitude corrected for position and instrument errors. Aircraft manufacturers are required to publish a calibration table in the aircraft manual to determine the corrections to be applied to calculate the calibrated altitude. In practice, however, because the corrections are so minimal, it is often assumed that the calibrated altitude and indicated altitude are roughly equal. True altitude is then calibrated or indicated altitude corrected for temperature error.

There is one major difference between absolute altitude and true altitude. Per the FAA, absolute altitude is “the height above ground level (agl),” while true altitude is “the height above mean sea level (MSL).” While vertical distance is what is measured for both altitudes, the absolute altitude constantly changes with the ground level directly below it.

A great example of calculating true altitude comes from the resources at PPLA Education. 

True Altitude = Indicated Altitude + (ISA Deviation × 4/1,000 × Indicated Altitude)

Example: Indicated Altitude is 20,000 feet.SAT is minus-35°C. What is True Altitude?

ISA at FL 200 is minus-25°C, so our SAT of minus-35°C is ISA minus-10°C.

Substituting into the above equation:

True Altitude = 20,000 + ( - 10 × 4/1,000 × 20,000)

True Altitude = 20,000 + ( - 800 ) = 19,200 feet

Being able to safely pilot an aircraft requires both a lot of knowledge and experience. From the very early days of flight training, pilots learn of the five different types of altitudes and how understanding them can be critical for safe and efficient flying.

Indicated altitude is the altitude shown on the altimeter. It is what the altimeter reads when the local pressure (altimeter setting) is set in the Kollsman window.

True altitude is the height above msl. It is the distance between an airplane at altitude and the average height of the oceans worldwide, which is determined by using still or calm water measurements to avoid fluctuations caused by waves or wind.

Absolute altitude is the height agl. It is similar to true altitude; however, instead of being measured from the mean sea level, absolute altitude is measured from the ground level, which is always changing.

Pressure altitude is the indicated altitude when an altimeter is set to 29.92 inHg (1013 hPa in other parts of the world). In other words, it is the height above a Standard Datum Plane (SDP), which is a theoretical level where the air pressure equals 29.92 inHg. It is primarily used in aircraft performance calculations and in high-altitude flight, such as Class A airspace.

Density altitude is pressure altitude corrected for non-standard temperature variations. As temperature and altitude increase, air density decreases. In a sense, it is the altitude at which the airplane "feels" its flying.

While indicated altitude is used to calculate true altitude, it is also true the two altitudes can be the same in a rare scenario. If the local barometric pressure is 29.92 inHg, then indicated and true altitudes should be the same. Simply setting the airplane’s altimeter to the proper setting helps pilots avoid many issues, but ensuring they are also prepared to make corrections for temperature variations is key to safe and successful operations. 

Wherever you plan to fly, knowing the difference in the types of altitudes and how to accurately calculate them is a necessary piece to being a great pilot. No matter the temperature, with the right information, you can be sure of your true altitude in any environment.

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Is flight level true altitude?

No, the main difference between true altitude and flight level altitude is the reference point used to measure them. True altitude is the vertical distance of the aircraft above mean sea level (msl), while flight level altitude is measured in 100-foot increments above the standard datum plane and uses QNE or pressure altitude as reference.

What affects true altitude?

When the ambient (at altitude) temperature is colder than standard, the aircraft's true altitude is lower than the indicated barometric altitude. On hot days, when the ambient temperature is warmer than the standard day, the aircraft's true altitude is higher than the indicated barometric altitude.

Is GPS altitude true altitude?

No, true altitude is the vertical distance of an aircraft above mean sea level (msl), while GPS altitude values are measured based on a different reference called an ellipsoid. The ellipsoid can be offset from true altitude by up to 100 meters, depending on the location.