Geodetic Reference System 1980
Due to some unfortunate error this article appeared wrongly in The Geodesists Handbook 1992 (Bulletin Geodesique, 66, 2, 1992). Among several errors a q (polar distance) was interchanged with a M (geographical latitude) affecting the formulas for normal gravity. It is advised that you use the formulas here or in the Geodesists handbook from Bulletin Geodesique, Vol. 62 , no. 3, 1988.