Radial distribution of the azimuthally averaged ¹²CO and ¹³CO emissivities observed in the Galactic plane for the case where ¹²CO is detected in individual spaxels but ¹³CO is not (mask CO1; upper panel), and for the case where both ¹²CO and ¹³CO are detected in individual spaxels (mask CO2; lower panel). Typical uncertainties of the ¹²CO and ¹³CO emissivities are 0.1 and 0.02 K km s^-1 kpc^-1, respectively.