Position-velocity diagram of H I. The colored background shows the H I emission averaged over − 1.25° < b < +1.25°. The black and gray curve indicate the intensity-weighted average velocity of Maggie and the second fit component at each longitude as given by the GAUSSPY+ decomposition, respectively. The white line segments give the predicted spiral arm centroids of the Outer arm and Outer-Scutum-Centaurus (OSC) arm along with their uncertainties (dashed lines) in velocity (Reid et al. 2019).