There exist the differences between the radii of the earth ellipsoid in high-precision navigation and positioningcalculations. By using the computer algebraic system, the change and difference values between earth radius vector and radius ofcurvature of normal section are analyzed. The results show that as the latitude of the earth B increases, the difference values betweenthe radius of curvature of the normal section continue to decrease and are equal at the poles; In short distance plane calculation, theradius of curvature at A = 45°can be approximated regarded as the average radius of curvature Ra; The extreme values of thedifference between the earth radius vector ? and the commonly used radius of curvature are located at the equator or at the pole.