Comparison of column densities derived from the λ149 μm and λ560 μm lines, assuming a 5% error in the continuum level (top panel) and a 10% error (center panel). The gray-shaded area covers all data points within the 1σ limits of the weighted linear regression (derived slopes in the bottom right corners). The column densities refer to 3 km s⁻¹ wide velocity intervals in the spectral distribution of N_v (cf. Fig. 2). Only velocities above 30 km s⁻¹ were considered. The dotted blue line indicates the bisector for identical column densities derived from both lines. Bottom panel: normalized statistical weights to convert the total column densities to opacities.