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Elevation correction (Free air correction). This correction considers бgh only distance of observation point above mean sea level. It is considered that between physical surface and sea level surface there is no attractive masses, that is, all spaces between them as if it is filled with air. Due to that, this correction is frequently called free-air correction.
With consideration of
free-air correction will be
(1)
where М – Earth’s mass; R – medium radius of Earth; at calculation (1) it is considered that height h ~ R.
Considering 1'0=980000 mGal, R=6371.103 m, we will receive
(2)
As it is seen from (2), alteration of gravitational force to 1 m height, it accounts approximately for 0,3 mGal. Free air correction should be accepted with “+” sign, if observation point is below sea level, and with “-“ sign, if observation point is above sea level.
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