Example of cross-check between the positions of the traces B1a, B1b, and B2 in the transverse PV_⊥ diagrams and in the velocity channel maps. In the map at V_LSR=16.2 km s⁻¹ (rotated clockwise by 48°as in Fig. 9) the horizontal dashed lines indicate the offset from the source of the diagrams in Figs. B.2 to B.4. The crosses indicate the (x, y) positions of the traces B1a, B1b, and B2 in the PV_⊥ diagrams formed at each offset y and at the selected velocity. The three ellipses are drawn to show the good correspondence with the observed arcs in the channel maps.