S/N of polarisation fraction. The panels correspond to Planck data at 5′ resolution (panel a) and to POL-2 data at 40′′ (panel b) and 20′′ (panel c) resolution. Panel a includes all data over a 2° × 2° area. In panels b and c, histograms show POL-2 data above column density thresholds of 1, 2, and 3 times N(H₂) = 10²² cm⁻² (blue, orange, and green histograms, respectively). Vertical lines at 2 indicate an approximate limit above which the p_mas estimates are reliable. All data are sampled at steps FWHM/2.