Impact of line optical depth. The optical depths shown on the left are calculated for the fiducial case (see Sect. 4.2), while those on the right are for an increased column density of 5 × 10¹⁴ cm^-2. The red lines indicate the observed line ratios of 0.25 (top panel) and 0.35 (bottom panel) at 218 and 291 GHz, respectively, in the 8−14 km s^-1 slice of the 20 km s^-1 cloud. The blue lines indicate an intensity of 15 K km s^-1 at 218 GHz (top panel) and of 4 K km s^-1 at 291 GHz (bottom panel), respectively, in the same slice. Black lines give optical depth contours. The lines come in pairs since we allow for ± 20% variation in drawing. Simultaneous fits to line ratios and intensities are found in locations where the blue and red lines intersect. At 218 GHz (top panel) these simultaneous matches require either relatively high densities (left panel) or high optical depths (right panel).