Coverage of the FQS survey with the observed 30′ × 30′ tiles (squares) overplotted on the H₂ column density maps derived from Herschel data. Top panel: range of Galactic longitude 219°–230°, the coverage of ¹²CO (1–0) and ¹³CO (1–0) data (black squares) is the same. Bottom panel: range 230°–240°, where the coverage of the ¹³CO (1–0) data (yellow squares) is smaller than the coverage of the ¹²CO (1–0) data (composed by yellow squares plus black squares). The intensity bar of the H₂ column density at the bottom is in units of cm⁻². The two contours correspond to N(H₂) = 2.8 × 10²¹ cm⁻² and 3.8 × 10²¹ cm⁻², equivalent to a visual extinction level of 3 and 4 mag, respectively. The sensitivity level of our observations allows detection of signal only from regions inside the contour at about 3 mag for ¹²CO (1–0) and at about 4 mag for ¹³CO (1–0).