(a.1, b.1): Purple data points show the surface height obtained with disksurf, applied to the data presented in 2. Solid blue and red lines show the best-fitted model for CO and [CI], respectively. (a.2, b.2): Residuals as a function of radial distance. (a.3, b.3): Normalized histogram of the residuals. The solid orange line shows a normal distribution with mean and standard deviations measured from residuals.