Positions and peculiar motion vectors of the Scutum arm masers (blue: this paper, white: Li et al., in prep., gray: literature) for sources with uncertainties on the vector components of less than 20 km s⁻¹. A representative velocity vector of 20 km s⁻¹ is shown in the upper right corner. The red shaded area around this vector displays the median uncertainty on the velocity vectors (ΔU_S  =  10 km s⁻¹, ΔV_S  =  5 km s⁻¹). The 22 GHz water, 6.7 GHz methanol, and 12 GHz methanol masers are indicated with circles, squares, and diamonds, respectively. The positions of the Sun and the Galactic center are indicated. The dotted lines represent the distance uncertainty on each source. The black and gray dashed lines represent the position and width of the Scutum spiral arm, as described in Sato et al. (2014).