Simply put, true altitude is the elevation about mean sea level (MSL).
Aeronautical charts, airspace altitudes, airways, obstacles, and terrain are all commonly indicated in true altitude.
True altitude can also be referred to as actual altitude and is often expressed in feet.
How to Determine True Altitude
True altitude can be determined in a couple of ways:
1. Obtain the indicated true altitude (ITA) by setting the local altimeter setting on the barometric scale of the pressure altimeter. Then use a DR computer to convert the indicated true altitude to true altitude.
2. Use an absolute altimeter to measure altitude over water.
Altitude Errors and True Altitude
In a perfect world (i.e. in perfect conditions that match International Standard Atmospheric (ISA) values), you will always have complete confidence that other altitude readings, like indicated altitude, are identical to the true altitude. However, in practice, this is rarely the case.
While true altitude will equal indicated altitude when an altimeter is set to the airport elevation if you are on the ground…
1. Flying in cold temperatures can cause true altitude to be different from indicated altitude. The reading will be more inaccurate the lower the temperature. This can result in inadequate obstacle clearance.
In extremely cold conditions, to ensure terrain and obstacle clearance, a pilot may need to add an appropriate temperature correction.
As a general rule, for every drop of 10 degrees Celsius compared to standard, expect true altitude to be about 4% lower. Another way of putting this is that this amounts to an error of 500 feet at 12,000 feet MSL.
2. In warmer conditions, the true altitude will be higher than the indicated altitude reading on an altimeter.
3. When flying in an isolated low-pressure area, the difference between true altitude and indicated altitude can be significant – as much as 1,000 feet.
4. Incorrectly applying or failing to apply pressure and altitude correction can result in more significant errors.
Related: How High Do Planes Fly?