Locations of HMSFRs (blue dots) determined by trigonometric parallaxes in the Sagittarius arm. The green squares and red triangles represent maser parallaxes in the Local arm (Xu et al. 2013) and Scutum arm (Sato et al., in prep.). Error bars indicate 1σ distance uncertainties. The Galactic center is at (0, 0) and the Sun at (0, 8.33). Galactocentric azimuth, β, increases in clockwise direction, with β = 0 defined as the direction from the Galactic center toward the Sun. The background gray disks provide the scale, with radii corresponding to the Galactic bar region (≈4 kpc) and the solar circle (≈8 kpc). The short COBE boxy-bar and the long bar (Fux 1998; Blitz & Spergel 1991; Hammersley et al. 2000; Benjamin 2008) are indicated with shaded ellipses. The solid curved line traces the center (and the dotted lines the ± 1σ width) of the Sagittarius arm from the log-periodic spiral fitting (see Sect. 4.2). For this view of the Milky Way from the north Galactic pole, Galactic rotation is clockwise.