Above mean sea level

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The term above mean sea level (AMSL) is the elevation or altitude of any object, relative to the average sea level. AMSL is used 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, where most heights are recorded and reported in AMSL (see flight level), and in the atmospheric sciences. It tells how high a point is above the average level of the oceans. (For example, an ocean beach would be at 0 AMSL.)

Definition[change | change source]

A "mean sea level" is imaginary. It is not possible to determine the average sea level for the entire planet. The sea level also changes over time when measured at just one location. 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 other reasons. People can only pick a spot and calculate the mean sea level at that point and use it as a datum. For example, the British 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[1] 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. GPS and other satellite systems do this. 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.

Once surveyors know (or define) the AMSL at one point, they can measure it at other points.

Usage[change | change source]

Topographic maps show the AMSL with contour lines connecting all points with the same AMSL. This suggests the shape of features such as mountains. The elevation of a mountain denotes the highest point or summit and is typically illustrated as a small circle on a topographic 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 drawn from an antenna site. 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 Amsterdam Airport Schiphol.

Engineers also watch AMSL in high-elevation areas. People who design radio stations or other electronics worry about keeping equipment cool. Overheating the equipmen can damage and cuase failure of the electronic components within a transmitter. At high AMSL, the air is thinner. Some equipement will not have enough airflow for sufficient cooling at high AMSL.

People call the AMSL of a point on the ground "elevation". The AMSL of a point in the air is called "altitutde".

Related pages[change | change source]

References[change | change source]

  1. Newlyn Tidal Observatory National Oceanography Centre - NERC