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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 mapTopographic maps also called contour maps topo maps or topo quads (for quadrangles), are maps that show topography, or land contours, by means of contour lines. Contour lines are curves that connect contiguous points of the same altitude. In other words,, variations in elevation are shown by contour lineA contour line (also isopleth level set isogram or isarithm for a function of two variables is a curve connecting points where the function has a particular value. A contour map is a map showing contour lines. The gradient of the function is always perpens. 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 heightHeight is a measurement of the distance from the bottom to the top of something which is upright. The measurement must be taken vertically, otherwise it is termed the length (or sometimes, the width). Like length and width, it is measured in units of leng shown in either metreFor other uses of "metre" and "meter", see Metre (disambiguation). The metre is the basic unit of length in the International System of Units (SI: Systeme International d'Unites). It is defined as the length of the path travelled by light in absolute vacus or feetA foot is a non- SI unit of distance or length. The popular belief is that original standard was the length of a man's foot. The average foot length is about 240 mm (9. 4 in) for current Europeans. About 996 of 1000 British men have a foot that is less th or both.
The height above average terrainHAAT is used extensively in radio, as it is actually much more important than power. For international coordination, it is officially measured in meters, even by the FCC in the USA. Stations that want to increase above a certain HAAT must reduce their pow (HAAT) for a stationA radio station is a site configured for broadcasting sound. Traditionally, radio stations have broadcast through the air via radio waves (a form of electromagnetic radiation), sent through a transmitter and antenna. Today, many if not most stations broad is determined from topographic maps by averaging the elevation AMSL at points along several radialSee also: radial tire A radial is a line drawn along a vector. A radial pattern is one that appears to radiate from a point. In radio, evenly- spaced points along evenly-spaced radials on a map are used to determine average elevation above mean sea levels 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 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.
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, effected 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 a 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.
For sample elevations, see 1 E3 m, list of mountains, peaks and passes of the Alps.
Geography
