What Is Altimetry?

FUNDAMENTAL: The altimeter measures your height above the datum which you set on the sub scale.

There are 3 datum you can set:

QFE: Barometric pressure at aerodrome elevation.

Reference datum is aerodrome.
Altimeter gives height above aerodrome.
(QFE = height AGL)

QNH: Barometric pressure at mean sea level.

Reference datum is sea level.
Altimeter gives height above sea level (ALTITUDE)
[Remember, regional QNH is given by ATC and spot QNH by the tower.]
(QNH = height AMSL)

QNE: Standard Pressure - 1013.25 hPa

Reference datum is the isobaric surface.
Altimeter gives pressure altitude (FLIGHT LEVEL)
(QNE = height above 1013.25 hPa)