Upper panels: distributions of HII regions projected into the Galactic plane with (left) and without (right) position error bars. Lower left: HII region distribution for those only with position uncertainties better than 1 kpc. Lower right: HII region distribution overlaid with a spiral arm model (see the left panel of Fig. 11) to indicate the identified arm segments. The area of open squares is proportional to the excitation parameters (see Eq. (1)). The IAU standard R₀ = 8.5 kpc and Θ₀ = 220 km s^-1 and the standard solar motions together with a flat rotation curve are adopted in deriving the kinematic distances from velocities if no photometric or trigonometric distance is available. The coordinates originate from the GC, and the Sun is located at x = 0.0 kpc, y = 8.5 kpc. The open red ellipse indicates the Galactic bar (Churchwell et al. 2009), and the laurel-gray ellipse indicates the best-fitted Near 3 kpc Arm and Far 3 kpc Arm (Green et al. 2011).