Relationship between the ratio of the 2D (Eq. (9)) vs. 3D (Eq. (12)) aperture corrections SCO_2D/SCO_3D and the ratio of the molecular gas scale height of the disc to the beam size z_CO/θ for the HRS galaxies of the sample. Black open circles are for galaxies with an inclination ≤80 deg, red symbols for edge-on systems (i > 80 deg). The black solid line gives the relation determined for model edge–on galaxies.