Peak observed optical depth τ_obs = −ln(T_MB/T_continuum) in the a) 1₁₀ − 1₁₁ and b) 2₁₁ − 2₁₂ lines and c) their ratio, 1₁₀ − 1₁₁/2₁₁ − 2₁₂. Figure d) shows the true optical depth ratio assuming T_ex(1₁₀ − 1₁₁) = 1.0 K and T_ex(2₁₁ − 2₁₂) = 1.5 K; these are reasonable and representative excitation temperatures but they are not fits to the data. The data are masked with a filter described in Sect. 3.2 and cover the range 75 >V_LSR> 40 km s^-1; see Figs. 4 and 5 for individual velocity components. In general, lower (redder) ratios in Figs. c) and d) indicate higher densities, however in the filament at 49.0−0.3, the low ratio is due to the geometry in which T_continuum is in the foreground of the molecular gas.