Face-on view of the Galactic plane. The black solid lines show constant projected v_LSR values of − 5 and 20 km s⁻¹. The orange-shaded area indicates the coverage of the GRS. Inferred positions and estimated widths for the Perseus and Outer spiral arm from Reid et al. (2019) are shown with the shaded green and blue areas, respectively. The central positions of the spiral arms are marked with solid and dashed lines depending on whether the majority of the arm is visible in the selected velocity range of − 5 < v_LSR < 20 km s⁻¹. The position of the Galactic centre and the sun are indicated with a black dot and the sun symbol, respectively. Dotted grey lines indicate distances to the sun in 2.5 kpc intervals. The black dash-dotted line shows the distances used to estimate the FWHM extent of the molecular gas disk. See Sects. 6.1 and 6.2 for more details.