a) Ratio of the magnetogram signal registered by HMI and by MDI, ⟨B_l⟩_HMI/ ⟨B_l⟩_MDI as a function of ⟨B_l⟩_MDI within 68 partially overlapping intervals of μ (the boundary of ten of which are annotated). Also illustrated is the same from the comparison between b) KP_SPM and rescaled MDI magnetograms, and c) KP₅₁₂ and rescaled KP_SPM magnetograms. The vertical dashed lines at 100 G mark the threshold below which the points are excluded from the surface fitting to these data due to the influence of magnetogram noise (see text).