Re: [android-developers] Barometer reading changes throughout the day while on same altitude.

As a pilot I can tell you that we get a MSL (Mean Sea Level) pressure reading and key that into the altimeter which compares it with current pressure and gives altitude to +50ft/-75ft accuracy.

The MSL is provided by the local airport, is adjusted to ensure any regional error puts us higher than any obstacles, and it varies so the airport will alert us if and when it changes.

Above what's known as the Transitional Layer we are high enough to avoid all ground obstacles to we use standard pressure of 1013.25 mb so that aircraft's altitude is computed relative to each other.

We do heed GPS altitude warnings that we're about to bust airspace but not for accurate altitude measurement.

Hope that helps.

Cheers
Steve

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