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 employs multiple types of altitude measurements, including barometric, radar, and GPS, each with unique methods, applications, and inherent complexities.
Barometric altimeters, the primary instruments, infer altitude from air pressure, making them prone to errors influenced by local pressure variations (requiring altimeter settings) and temperature deviations from standard atmospheric models.
Radar altimeters provide precise height above ground (AGL) at lower altitudes, while GPS altitude is derived from absolute spatial coordinates and can differ significantly from barometric readings, potentially leading to discrepancies crucial for terrain avoidance.
Understanding the distinctions and potential errors of each altitude measurement system is critical for pilots to ensure safety, accurate navigation, and effective collision avoidance.

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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.