Plan view of Galactic spiral structure. The spiral structure estimated by Reid et al. (2014) is shown as black lines for reference. Maser emission from 11 HMSFRs has been used to estimate the position of the outer arm. The different outer arm descriptions discussed in Sect. 4.3 are highlighted in color curves. The Galactic center is located at (0,0), and the yellow star corresponds to the Solar position (Reid et al. 2014), while the orange line demarcates the latitude of the outer arm kink suggested (i.e., 140°).