Above mean sea level
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 leveldatum. AMSL is used extensively in radio(both in broadcastingand other telecommunicationsuses) by engineers to determine the coverage areaa station will be able to reach. It is also used in aviation, where all heights are recorded and reported with respect to AMSL (though also see flight level), and in the atmospheric sciences.
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 Surveyuses a height datum based on the measurements of mean sea level at a particular gauge at Newlyn, Cornwallfrom 1915 to 1921 [http://www.pol.ac.uk/ntslf/tgi/ntobs.html] for their maps of Great Britain, and this datum is actually some 80 cm 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 GPSdo. In aviation, the ellipsoid known as World Geodetic System84 is increasingly used to define mean sea level. Another alternative is to use a geoidbased datum such as NAVD88.
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 heightshown in either meters or feet or both.
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 toweritself and the ground it is on, to determine the difference. Negative numbers for HAAT sometimes result from this when the station or airportis 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 Amsterdam 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 failureof the electronic components within a transmitter.
Above ground level
* For sample AMSL elevations, see,
1 E3 m
list of mountains
** Ranges, peaks and passes of the Alps
Extreme points of the world
* [http://www.pol.ac.uk/ntslf/tgi/ntobs.html National Tidal & Sea Level facility]
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