Top: grayscale GLIMPSE 8 μm image of the GMF 309.5-308.7. The blue contours show the ¹³CO integrated intensity of 1.5 K /km s^-1 and 3 K /km s^-1, integrated over the velocity range [–53, –35] km s^-1. The red contours show the ATLASGAL emission at a contour level of F_{870 μm} = 250 mJy/beam. The red boxes show regions with poor ThruMMS data or absence of it. The filled geometric objects show all the dense gas measurements from different surveys with v_LSR within the velocity range indicated in Table 2. Symbols as in Fig. A.1. Middle: position-velocity diagram of the ¹³CO line of the GMF 309.5–308.7, obtained from a slice following the extinction feature used to identify GMF 309.5–308.7. Bottom: PV diagram of the ¹²CO emission between | b | ≤ 1°. The yellow line shows GMF 309.5–308.7 in the PV space. The green solid line shows the Scutum-Centaurus arm as predicted by Reid et al. (2014) and the dashed green lines show ±10 km s^-1 of the velocity of the spiral arm.