Normal gravity value using Somigliana's formula

NGRAV computes the normal gravity value (in m/sec^2 unit) of a point with the geodetic latitude defined in GRS80 ellipsoid,which is the more up-to-date and internationally adopted reference ellipsoid.

The normal gravity value is used for the computation of the gravity anomaly which removes the effect caused by the Earth oblateness and centrifugal acceleration from the observed gravity value and which is used frequently in geophysics and geodesy for the physical interpretation of the gravity accelerations and for the solution of the boundary value problems.

The computation depends on a very simple equation;the Somigliana's formula. Despite the simplicity, it is the most accurate one among the others, e.g, the first and second Chebyshev approximations (commonly used ones).

References:
[1] Featherstone WE and Dentith MC, 1997, A geodetic approach to gravity data reduction for geophysics,Computers&Geosciences,Vol.2,No.10,pp:1063-1070
[2] Moritz H, 2000, Geodetic Reference System 1980, Journal of Geodesy, 74/1, pp:128-162
[3] Vanicek P and Krakiwsky EJ, 1986, Geodesy: The concepts, North-Holland, Amsterdam