Atas min aras laut: Perbezaan antara semakan

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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 (geodesy)|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<ref>[http://www.pol.ac.uk/ntslf/tgi/ntobs.html Newlyn Tidal Observatory<!-- Bot generated title -->]</ref> for their maps of Great Britain, and this datum is actually some 80&nbsp;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 [[reference ellipsoid|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 (geodesy)|datum]] such as [[NAVD88]].
 
==UsagePenggunaan==
When referring to [[geography|geographic]] features such as [[mountain]]s on a [[topographic map]], variations in elevation are shown by [[contour line]]s. 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 [[Foot (length)|feet]] or both.