A method, including generating a magnetic field in a region from a first magnetic field radiator located at a first position and a second magnetic field radiator located at a second position. A volume having a multiplicity of vertices is delineated within the region, and respective values of the magnetic field at the multiplicity of vertices are measured. In response to the respective values, respective first dipole moments to the first magnetic field radiator and respective second dipole moments to the second magnetic field radiator are assigned. A value of the magnetic field within the volume is calculated in terms of the first dipole moments and the second dipole moments.
