Above mean sea level
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The term above mean sea level (AMSL) 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).
When referring to geographic features such as mountains on a topographic map, variations in elevation are shown by contour lines. The elevation of a mountain denotes the highest point or summit and is typically illustrated as a small circle on a topo map with the AMSL height shown in either metres or feet or both.
The height above average terrain (HAAT) for a station is determined from topographic maps by averaging the elevation AMSL at points along several radials or radii. 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 or airport is in a valley, which is significantly lower AMSL than the surrounding mountains. In the rare case that a location is below sea level, AMSL itself is a negative number. For one such case see Schiphol Airport.
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.
It should be noted that the concept of a "mean sea level" is in itself rather artificial, because it is not possible to determine a figure for mean sea level for the entire planet, and it varies quite a lot even on a much smaller scale. This is because the sea is in constant motion, affected by the high and low pressure zones above it, the tides, local gravitational differences, and so forth. The best one can do is to pick a spot and calculate the mean sea level at that point and use it as a datum. For example, the Ordnance Survey uses a height datum based on the mean sea level of a particular gauge at Newlyn, Cornwall for their maps of Great Britain, and this datum is actually some 80cm different from the mean sea level reading obtained on the other side of the country. An alternative is to base height measurements on an ellipsoid of the entire earth, which is what systems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 is increasingly used to define mean sea level.
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