Polarisation fraction p_mas as a functionof column density, based on POL-2 data at 20′′, 40′′, and 80′′ resolution. The data are sampled at half-beam steps. The dashed lines correspond to least squares fits data N(H₂)> 7.5 × 10²¹ cm⁻². The error bars are shown only for the FWHM = 80′′ data points. The solid black line and the right-hand y-axis show the median S/N for polarisation fraction, p_mas∕σ_p,mas, at 40′′ resolution.