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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.
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)
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 ...
Since NGVD 29 used a simple model of gravity based on latitude to calculate the geoid and did not take into account other variations, [citation needed] elevation difference between points in a local area in it and NAVD 88 will show negligible change from one datum to the other, even though the elevation of both does change between datums.
In aviation, atmospheric sciences and broadcasting, a height above ground level (AGL [1] or HAGL) is a height measured with respect to the underlying ground surface.This is as opposed to height above mean sea level (AMSL or HAMSL), height above ellipsoid (HAE, as reported by a GPS receiver), or height above average terrain (AAT or HAAT, in broadcast engineering).
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The most common and convenient datum which is internationally accepted is mean sea level which is a universal measure and based upon a common base line in the whole world determined by earth's gravitational model (see geoid) that gives the standard to measure elevation of a place above or below mean sea level.
This is as opposed to altitude/elevation above mean sea level (AMSL), or (in broadcast engineering) height above average terrain (HAAT). In other words, these expressions (AGL, AMSL, HAAT) indicate where the "zero level" or "reference altitude" is located.