Single ¹²CO channel with a schematic of the quantities we measure. The disc has been rotated so that the semi-major axis is horizontal. The position of the central object is marked by a star. For a given offset δx along the disc major axis, two maxima N and F on the same vertical are located on the near and far sides of the disc’s upper surface. These two points are on the same inclined circular orbit (white ellipse), whose centre is marked by a diamond. Indicated are the geometrical parameters from which the cylindrical radius, height, and velocity of the points F and B can be measured. By repeating the procedure for each δx (green points, sampled every 0.08′′) and each velocity channel, a full mapping of the CO upper surface is performed. The cyan points mark the location of the lower surface (near side), while the magenta points represent the symmetric of the far upper surface relative to the disc major axis.