Intensity-weighted mean line-of-sight HI velocity defined by Eq. (2) vs. the intensity-weighted mean position defined by Eq. (3). Each point represents a different horizontal strip in the moment images, at distance y from the major axis. In red we show data from the far side to the east of the major axis, and in blue we plot data from the near side, to the west of major axis. The solid line is the fitted regression, whose slope gives Ω_P × sin i.