Gemini OB1 field. Top: sky-projected map of the Gemini OB1 molecular cloud. The background image is the velocity-integrated ¹²CO data of Stacy & Thaddeus (1991), Dame et al. (2001), and black circles mark the locations of BGPS V2 sources. Stars mark the locations of the masers with trigonometric parallax measurements, and the enclosing circles mark the projected association volumes, computed as in Section 5.3. The white dashed line marks the approximate boundary of the BGPS observations, and the scale bars indicate the projected distances for the two subregions. Bottom: the longitude–velocity integration over −10 ⩽ b ⩽ 15 of the same field. Both panels share a common longitude axis.