The one formula I need I cannot find. I have an Oblate Ellipsoid which I can describe as a=b=2c. The values of a, b, and c are known to me. I've encountered multiple representations of symbols here, so... let Phi = angle from positive z-axis: 0<= Phi <=180. let Theta = angle from positive x-axis: 0<= Theta <=360 Phi and Theta are also known to me. How can I determine the radius where the vector described by Phi and Theta pass through the surface of the Ellipsoid?