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R = radius of the earth
Earth is considered as
sphere and hence
radius is constant.
Earth is not sphere and hence radius
is not constant.
Hence correction must be applied
Because many factors affect the
value of g. As an example:
• anomalous masses inside the
What is geoid?
It is equipotential surface where the g
is always constant. The equipotential
surface is usually taken as the sea level.
However, due to some heterogeneity in
the earth Geoid may differ slightly from
Reduction is carried out to correct for:
The earth is flattened,
i.e the radius at the
equator is longer than
at the poles.
gn = 978031.85 (1.0 + 0.005278895 sin2(lat) + 0.000023462 sin4(lat)) (mGal)