Rotational diagrams of each species towards GMC 6. Parameters E_u, N_u, and g_u are the level energy (with respect to the ground state), column density, and degeneracy of the upper level, respectively. The error bars on ln(N_u/g_u) include a calibration error of 15% (see Sect. 2). The blue (and orange if a second component is fitted) solid lines represent the best fits and the dashed grey lines are the extrapolations of the fit for the full range of E_u covered. The source size assumed is indicated for each component (see Sect. 4.1).