Masers in high-mass star-forming regions (Xu et al. 2016) plotted on top of our inferred dust reddening density. The masers are colored according to the spiral arm to which they are assigned by Xu et al. (2016): the Perseus Arm in orange, the Local Arm in yellow, the Sagittarius Arm in purple, the Scutum Arm in turquoise, and all others in black. The distances are calculated naively as the inverse of parallax.