Predictions of the toy magnetic field model with constant grain alignment. The polarisation vectors are shown on the column density map in panel a (central part of the full map) and the p vs. N(H₂) relation is plotted in panel b at Planck resolution. The lower panels are the same for synthetic POL-2 observations, at a resolution of 20′′, assuming high-pass filtering with θ = 200′′. The scaling of the absolute p values is arbitrary but identical between the panels.