Histograms of the observed polarized angle at the full data resolution toward the Galactic plane (| b_II | < 5°) for the four Galactic quadrants. The various curves show data uncorrected for bandpass mismatch (red), and corrected using sky coupling coefficients derived either from ground (method A: green) or sky measurements (method B: dark blue). The vertical dashed lines show the peak value obtained from fitting the histograms with a Gaussian.