By definition, the altimeter setting is the pressure value to which an aircraft scale is set, so it will indicate the altitude above mean sea level of the aircraft on the ground at the location for which the pressure value was determined.

It is calculated using th field pressure ( Pa) and field elevation of the pressure sensor ( Ha) using the following equation:

AS  =  [Pa0.1903 + (1.313 x 10-5) x Ha]5.255

If the field pressure is missing, average pressure ( P) and elevation of the pressure sensor (Hz) are used.

AS  =  [P0.1903 + (1.313 x 10-5) x Hz]5.255