Model prediction for the relative centroid positions of the H30α emission (solid and dashed thick lines for the cases with rotating and non-rotating outflow) in the velocity range between − 64 km s^-1 and 58 km s^-1 superimposed on the observed data as blue and red thin lines for MWC349A. The positions of the line velocity channels are shown as crosses labeled by their LSR velocities. The blueshifted and redshifted maser spikes (− 16 and 32 km s^-1, respectively) peak at the blue and red circles. The associated errors are shown by horizontal and vertical bars. The offsets are in milliarcseconds (mas) relative to the continuum centroid (, δ_J2000 = 40°39′36.8′′).