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 involves many distinct altitude measurements (e.g., barometric, radar, GPS), each with unique principles, reference points, and potential for discrepancies, making accurate altitude awareness crucial for safety.
Barometric altimeters, the most common type, measure air pressure relative to a mean sea level datum and the International Standard Atmosphere, but are prone to errors caused by variations in local pressure and cold temperatures.
Other altitude sources include radar altimeters, which provide accurate height above ground (AGL) at low altitudes, and GPS altitude, which is based on an absolute position in space and can differ significantly from barometric readings.

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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.