Altimeter Scale Ring, 57 mm
Scale ring for use with 4 FGH 40 altimeters
Prior to cross-country flight, the altimeter is usually set to field elevation.  Atmospheric pressure reduced to pressure at sea level (QNH pressure) is then indicated from the Kollsman window.  Throughout the flight, the altimeter reads height above sea level.
In many instances, it is also desirable to know one's relative altitude above a defined landmark, as well as height above sea level.  One a final approach, for example, it is essential to know one's height above the landing field at any given time.  Once the arrow on the rotatable scale ring has been set to the elevation of the landing field, altitude above the field can be read off the graduations at any time until touchdown (reading corresponds to the QFE setting).
Further applications include the safe crossing of ground obstacles (mountain ranges) or control zones, cases in which the difference in altitude can be read directly once the ring has been correctly adjusted.  The scale ring has also proved particularly useful for destination approach calculations in performance gliding.