Datum

A datum in this context is simply a known point from which relative measurements are made. Each measurement MAY use a different datum for a type of measurement.

Altitude

Some possible data used for altitude measurements.

Datum Name Description
AGL Above ground level The altitude from ground level at a known lat/long.
MSL Mean sea level The mean sea level for a known lat/long or region (e.g. US).
SFC Surface The surface, typically 0 AGL.
SPS Standard pressure setting The flight level in meters as measured at standard pressure setting (1013.2 mb). Note: This is a moving datum.
WGS84 WGS84 (GPS) The WGS84 datum used by GPS.