Top: magnetic field and total intensity measured by BLASTPol towards two regions in the Vela molecular ridge. The colours represent N_H, the total gas column density inferred from the Herschel observations. 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-N_H 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 bin, an intermediate bin, and the highest N_H bin (blue, orange, and dark red, respectively). These bins have equal numbers of selected pixels within the indicated N_H ranges. The horizontal dashed line corresponds to the average. The widths of the shaded areas for each histogram correspond to the 1σ uncertainties related to the histogram binning operation. Histograms peaking at 0deg would correspond to predominantly aligned with iso-N_H contours, while histograms peaking at 90deg would correspond to predominantly perpendicular to iso-N_H contours.