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Height above mean sea level (AMSL) is the elevation (on the ground) or altitude (in the air) of an object, relative to a reference datum for mean sea level (MSL). It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (contrast with flight level ), and in the atmospheric sciences , and in land ...
Long term changes in the mean sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term. [14] During the glacial-interglacial cycles over the past few million years, the mean sea level has varied by somewhat more than a hundred metres. This is primarily due to the growth and decay ...
A chart datum is the water level surface serving as origin of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. [1] Common chart datums are lowest astronomical tide (LAT) [1] and mean lower low water (MLLW).
Height above mean sea level is a measure of a location's vertical distance (height, elevation or altitude) in reference to a vertical datum based on a historic mean sea level. In geodesy, it is formalized as orthometric height. The zero level varies in different countries due to different reference points and historic measurement periods.
Since the Sea Level Datum of 1929 was a hybrid model, it was not a pure model of mean sea level, the geoid, or any other equipotential surface. Therefore, it was renamed the National Geodetic Vertical Datum of 1929 (NGVD 29) May 10, 1973, by the National Geodetic Survey , a part of the National Oceanic and Atmospheric Administration .
Tunnel datum is a datum based on an ordnance datum and used in designing tunnels which pass below sea level. for the London Underground, a tunnel datum of ODN −100 m is used; [7] thus a depth of −60 m AOD is 40 m ATD (above tunnel datum) for the Channel Tunnel, a tunnel datum of ODN −200 m is used; [8] thus a depth of −60 m is 140 m ATD
In cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. [3] A contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes. [4]
In general, the trend is towards using lowest astronomical tide (LAT), the lowest tide predicted in the full tidal cycle, but in non-tidal areas and some tidal areas Mean Sea Level (MSL) is used. Heights, e.g. a lighthouse, are generally given relative to mean high water spring (MHWS).