HI self-absorption spectrum at the position of l = 36.48 deg, b = −0.04. The spectrum is extracted from the THOR+VGPS data at 40′′ resolution. The black spectrum shows the original data with a red second-order polynom fit to the environmental gas. The red spectrum is then the resulting HISA feature used to determine the column density (Bihr et al., in prep.). The blue spectrum is the corresponding ¹³CO(1–0) emission from the GRS survey (Jackson et al. 2006).