Everything about Amsl totally explained
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 datum. 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), and in the
atmospheric sciences.
Definition
The concept of a "mean sea level" is in itself rather artificial, because it isn't 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 measurements of mean sea level at a particular gauge at
Newlyn,
Cornwall from 1915 to 1921
(External Link
) 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
GPS do. In aviation, the ellipsoid known as
World Geodetic System 84 is increasingly used to define mean sea level. Another alternative is to use a
geoid based
datum such as
NAVD88.
Usage
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
meters 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's 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 isn't 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.
Further Information
Get more info on 'Amsl'.
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