An approximation for the average/mean radius of an ellipse's circumference, , is the elliptical quadratic mean: (where is the central, horizontal, transverse radius/semi-major axis and is the central, vertical, conjugate radius/semi-minor axis) As a meridian of an ellipsoid is an ellipse with the same circumference for a given set of values, its average/mean radius, , is also the same as for an ellipse, as is its quadratic mean approximation: This would be appropriate if the arc paths of an ellipsoid were only north-south: They are not.