²Note that this method is universal, and may be used from any numerical data provided from a Grad Shafranov equation solver. However, for some codes that use finite elements techniques, it may be useful to use those directly instead of performing an additional interpolation procedure based on a spline-Fourier expansion. Indeed, finite elements contain all data needed for calculating magnetic field values at any radial and poloidal position including and first and second derivatives with an excellent accuracy. The choice to use the spline-Fourier expansion is here justified by the need to have a ray-tracing solver as general as possible.