1. A method to determine distortion correction data for distortion correction of a magnetic resonance image acquired by a magnetic resonance system, using a gradient coil, formed by a coil conductor, in said magnetic resonance system, comprising the steps of: (a) in a processor, calculating a starting spherical function representing a magnetic field produced by said gradient coil, based on conductor geometry of the coil conductor of the gradient coil; (b) in said processor, determining a three-dimensional parameter map that represents a magnetic field generated by the gradient coil using a defined measurement subject, with a defined gradient strength set for the gradient coil; (c) in said processor, generating a deviation parameter map, representing deviations between the parameter map and a reference parameter map provided to the processor, obtained with the gradient coil deactivated; (d) in said processor, determining spherical function coefficients of the gradient magnetic field by fitting a spherical function representing the magnetic field of the gradient coil to the deviation parameter map, using the starting spherical function; and (e) in said processor, determining distortion correction data from the spherical function coefficients of the fitted spherical function, and making said distortion correction data available in electronic form in a format for correcting said magnetic resonance image.