Panel a1: rotation curves for the model of a galaxy with an inclination angle i = 30° obtained for the case A. The solid line is the model rotation curve. The rotation curve derived from the modelled map of the line-of-sight velocities without errors is shown by plus signs. Six rotation curves derived from the modelled maps of the line-of-sight velocities with random errors are indicated by points. Panel a2: same as panel a1 but for the case B. Panels b1 and b2: same as panels a1 and a2 but for an inclination angle of i = 50°.