Illustrating the impact of using an irregular (top) and a smooth parametric (bottom) emission surface on the peak intensity and velocity residuals extracted for the ¹²CO disc of MWC 480. Unphysical quadruple-like patterns observed at R = 300, 400 au and at R > 400 au in the velocity residuals resulting from the non-parametric surface suggest that a smooth (and less tapered) layer provides a better representation of the ¹²CO emission surface of this disc.