The spheroid now still has one `degree of freedom`; it can rotate about the normal BN,and this is settled by the obviously desirable condition that the geometrical axis of the spheroid should be parallel to the Earth`s axis of rotation.This condition will be referred to again. The condition will be satisfied if the geometrical meridian on the spheroid through B lies in the direction of true north, that is the astronomical and spheroid meridians at B coincide.
The nest stage is to calculate the spheroid coordinates of A.C.D. Let us suppose that the spheroid latitude of C. is found to be 60 27`19 205" ; this is the angle made by the normal CS with the spheroid equatorial plane. The astronomical latitude d C is determined by the direction of the gravity vertical at that point,and if it is