Radial distribution of CH column densities toward lines of sight probed in this study except SgrB2(M). The column density values for each distance interval are averages over the underlying distribution. The secondary y-axis display N(H₂) values computed using the Sheffer et al. (2008) relation. The dashed red vertical line indicates the galactocentric distance to the Sun. The spiral arm locations typical for a fourth quadrant source, are plotted with spiral arm widths of 0.85 kpc.