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This reference can be the mean sea level pressure , the pressure at a nearby surface airport (QFE), or the "standard pressure level" of 1,013.25 hectopascals (29.92 inches of mercury) which gives pressure altitude and is used to maintain one of the standard flight levels.
The new altimeter used a series of high-pitched sounds like those made by a bat to measure the distance from the aircraft to the surface, which on return to the aircraft was converted to feet shown on a gauge inside the aircraft cockpit.