These types of questions will come up in many subjects (Met, Gnav, Comms and possibly Ops and Law) so it is essential you understand them.
An altimeter measures a difference in pressure between the actual static pressure and a subscale setting that you give it. By making certain assumptions, this pressure difference can be converted into a vertical distance. The subscale settings you give it - and hence, the vertical distances on the altimeter - are as follows.
- QFEQFE —Static pressure measured at an aerodrome. Pressure measured at an aerodrome. If set, the altimeter reads height.
- QNHQNH —Static pressure at MSL calculated from QFE using ISA temperature lapse rates. Pressure at mean sea level calculated from QFEQFE —Static pressure measured at an aerodrome using ISAISA —International Standard Atmosphere lapse rates. If set, the altimeter reads altitude.
- SPSSPS —1) Pressure: Standard Pressure Setting (1013.25hPa)2) GNSS: Space Segment. 1013 hPahPa —Hectopascal. If set, the altimeter reads pressure altitude (and above the transition altitude, every 500' is a flight level.)