Gaseous disc model at two different inclinations. We show 2D projections of a Miyamoto–Nagai density profile as described in Eq. (1) at an inclination of i = 30° (top panel) and i = 60° (bottom panel). Both discs are turned by a position angle of 20°. The colour-bar indicates the surface density. With black lines, we indicate iso-velocity curves, with velocity values separated by 30 km s⁻¹ (with a value of 0 km s⁻¹ on the minor axes). With an orange (resp. green) circle, we show the area observed by a 3″ spectroscopic fibre at a redshift of z = 0.05 (resp. z = 0.17). The more inclined the disc, the smaller the distance of equidistant velocity lines and thus the steeper the velocity gradient probed by the spectroscopic fibre.