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Unless otherwise stated this page contains Version 1.0 content (Read more about versions) 2.7.5 GravityThe gravity field of the Earth The potential of the external gravity field of the Earth is usually expressed as a series of spherical harmonic terms:
The values of
J_{n} are determined from the behaviour of artificial
satellites about the Earth. where is the geographical latitude, H
is the height above sea level (in metres) and g_{e} is the
value of gravity at the equator. The above formula gives
the best simple method of calculating g at a place where it has not
been measured. It will almost always give results within 10^{−3}
and usually within The standard acceleration of gravity is defined as 9.80665 m s^{−2} exactly. Absolute value of the acceleration due to gravity, g. Values of the acceleration due to gravity in terms of the fundamental units of length and time were, until recently, measured at very few sites and values elsewhere were found from measurements of differences. In recent years absolute apparatus using laser interferometer measurements of positions of falling reflectors has become highly developed and easy to use. The constant term in the gravity above is now derived from a number of absolute measurements and it is no longer necessary to give an absolute value at one or two or three preferred sites. References Geodetic Reference System (1980), IUGG. Sir Alan Cook 
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