Comparison of radial-velocity estimates from ideal C¹⁸O line observations. The frames show: (a) true column density of the model, (b) true mass-weighted average LOS velocity 〈υ_LOS〉 of the model, (c) mean radial velocity in non-LTE spectra, (d) difference between the radial velocity of the non-LTE spectra and 〈υ_LOS〉, (e) velocity difference between non-LTE and LTE spectra (including T_kin variations), and (f) velocity difference between non-LTE and LTE spectra, the latter assuming a constant kinetic temperature of T_kin = 15 K. All data are used at full resolution, without beam convolution.