Examples of circular models (black dots) least square fitted with the Lundquist field (red curves). Three non-linear force-free models (n = 0.1,0.5,2) are selected to represent strong departure to the Lundquist field (n = 1). The true impact parameter, y/b, is either null (left) or large (right). B_z is the axial field component, and B_y is the azimuthal field component for y = 0. The field strength of the model on the axis, B_axis, is normalized to 1.