The term above mean sea level refers to the elevation (on the ground) or altitude (in the air) of any object, relative to the average sea level. AMSL is used extensively in radio (both in broadcasting and other telecommunications uses) by engineers to determine the coverage area a station will be able to reach. It is also used in aviation, all heights are recorded and reported with respect to AMSL (though also see flight level).
The height above average terrain or HAAT for a station is determined from topographic maps by averaging the elevation AMSL at points along several radials or radii ("radiuses"). This is subtracted from the elevation AMSL of the antenna, including both the tower itself and the ground it is on, to determine the difference.
Negative numbers for HAAT sometimes result from this when the station is in a valley, which is significantly lower AMSL than the surrounding mountains. In the extremely rare case that a location is below sea level, AMSL itself is a negative number.
AMSL is also important to engineers in high-elevation areas because some equipment is not designed with enough airflow for sufficient cooling in the thin air, which can cause overheating, damage, and failure of the electronic components within a transmitter.
For sample elevations, see 1 E3 m, list of mountains, peaks and passes of the Alps.
