Planck 850 μm (353 GHz) surface brightness with vectors showing the POS magnetic field orientation (panel a) and polarisation angle dispersion function S(δ = 2.5′) calculated from Planck data (panel b). The maps are at the original 5′ resolution. The white contours indicate the area covered by SCUBA-2 observations. The length of the polarisation vectors is proportional to the polarisation fraction (90′′ for 1% of polarisation).