(1) One of a set of three coordinates (u, Ψ, λ) of a point in a spheroidal coordinate system of given distance 2k between foci of the rotational ellipsoids of the system. Spheroidal coordinates are related to the rectangular Cartesian coordinates (x, y, z) in a system whose axes coincide with those of the spheroidal coordinate system, by the equations x = (√(u² +k²)) cos Ψ cos λ , y = (√(u² +k²)) cos Ψ sinλ and z = u sin Ψ , in which u is the length of the semi major axis of the rotational ellipsoid through the point, Ψ is the reduced latitude of the point with respect to the same ellipsoid and λ is the angle from a reference plane through the minor axis to the plane through the minor axis and the point. Called an elliptic coordinate or ellipsoidal coordinate by some geodesists. Other version of the above set of coordinates and of the transformation equations are in use. e.g., the form x = rcosh Γ cos Ψ cosλ, y = rcosh Γ cos Ψ sinλ and z = rsinh Γ sin Ψ has been used in satellite geodesy. (2) A coordinate in a coordinate system on a rotational ellipsoid (spheroid). (3) A coordinate in a coordinate on a surface which is approximately spherical.