They give you either QNH or QFE.
QNH is the airfield pressure reduced to sea level via ISA (International Standard Atmosphere) figures i.e. 1mb per 27ft. This gives the pilot their altitude
above mean sea level
QFE is the airfield pressure. Therefore, when you select the QFE and start flying (from the same aerodrome) you will have a height
above the airfield
As an aside, QFF is similar to QNH but it is aerodrome pressure reduced to sea level via the actual conditions rather than ISA conditions
QNE is (strictly speaking) a height that is read on the altimeter when landing with 1013.25mb set on the altimeter. A common misconception is that QNE = 1013.25mb = standard pressure but it's actually a height.
Does that make sense?
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