Polarisation fraction of the G035.39-00.33 region based on Planck data. Panel a shows the 850 μm surface brightness, panel b the Herschel column density map convolved to 5′ resolution, and panel c a map of the polarisation fraction p_mas from Planck data. The white contour indicates the coverage of the SCUBA-2 map. Panel d shows the polarisation fraction as a function of column density, sampled with 2′ steps from maps at 5′ resolution, excluding the map boundaries (indicated by dashed lines in panel c), without a S/N cut-off. The red line is the running mean and the blue circles show values from the area covered by SCUBA-2 observations.