Top: magnetic field and total intensity measured by BLASTPol towards two regions in the Vela molecular ridge. The colours represent I₅₀₀, the total intensity observed in the 500-μm wavelength-band of BLASTPol. The “drapery” pattern, produced using the line integral convolution method (LIC, Cabral & Leedom 1993), indicates the orientation of magnetic field lines, orthogonal to the orientation of the submillimetre polarization observed by BLASTPol at 250 (left), 350 (centre), and 500 μm (right). Bottom: histogram of the relative orientations (HRO) between the iso-I₅₀₀ contours and the magnetic field orientation inferred from the BLASTPol observations at 250 (left), 350 (centre), and 500 μm (right). The figures present the HROs for the lowest, an intermediate, and the highest I₅₀₀ bin (blue, orange, and dark red, respectively). The bins have equal numbers of selected pixels within the I₅₀₀-ranges labelled. The horizontal dashed line corresponds to the average per angle bin of 15deg. The widths of the shaded areas for each histogram correspond to the 1σ uncertainties related to the histogram binning operation. Histograms peaking at 0deg correspond to predominantly aligned with iso-I₅₀₀ contours. Histograms peaking at 90deg correspond to predominantly perpendicular to iso-I₅₀₀ contours.