So Many Altitudes

The one you should be concerned with depends on how you measure it, and how hot or cold conditions might be.

Jim Wolper

Friday, January 10, 2025 at 08:00 AM ET

Verified

Edited By: Omar Filipovic

Key Takeaways:

Aviation uses multiple types of altitude (barometric, radar, GPS), each based on different measurement principles and with specific applications.
Barometric altimetry, the primary method, measures air pressure and relies on the International Standard Atmosphere, making it susceptible to errors caused by non-standard temperatures or local pressure variations.
Other methods like radar altimeters provide accurate height above ground (AGL) at lower altitudes, while GPS offers a geometric altitude that can significantly differ from barometric readings.
Understanding the unique characteristics, advantages, and potential inaccuracies of each altitude measurement system is crucial for pilots to ensure safety and prevent collisions with terrain or other aircraft.

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I recently overheard a couple of pilots talking about various kinds of altitude. “How many altitudes are there?” one asked. I don’t recall the details of any answer he got but, alas, the answer is that there are many. If you use the wrong one, you might get into an uncomfortable situation.

Aviation Safety has discussed altimetry errors in numerous articles over the years. See November 2015’s “Your Altimeter Is Lying” for an example. Meanwhile, let’s stick to the more conceptual errors.