Polarisation angle dispersion function S and polarisation fraction p_mas from Planck data in a 2° × 2° area centred on the POL-2 field. Panels a and b show these as a function of dust optical depth and panel c shows their mutual correlation. The colours of the plot symbols correspond to the data resolution, as indicated in panel c. The solid lines show the corresponding moving averages. The points inside the area mapped with POL-2 are plotted with large symbols. The data are sampled at steps of FWHM/2 and S is calculated for lag δ = FWHM/2. In panel c the upper solid black line shows the relation log₁₀(S) = − 0.834 × log₁₀(p_mas) − 0.504 from Planck Collaboration Int. XIX (2015) and the lower black line the relation S = 0.1∕p_mas. The red dashed line is the relation log p_mas = − 0.670 log S − 0.97 from Fissel et al. (2016). The blue dashed line corresponds to the fit to Gould Belt cloud data, S × p = 0.31° (FWHM/160′)^0.18 of Planck Collaboration XII (2018), calculated with FWHM = 9′.