Measurements of the Crab nebula polarisation angles from Table 1 for WMAP (blue diamonds), XPOL (green square), Planck-LFI (purple circles), Planck-HFI (red triangles), and NIKA (yellow crosses). Statistical error bars are coloured, and the systematic error bars (ground systematics for Planck-HFI, corresponding to the cst-PlanckGround case of Sect. 3) are plotted in black. The solid and dashed black horizontal lines indicate the weighted mean polarisation angle and its ±1σ uncertainty, .