Hi-GAL sources (red symbols), with unique velocity determination, overplotted on spiral arm models. For both models the Sun is at 8.5 kpc and its circular velocity 220 km s^-1. Top: model from Russeil et al. (2007) based on the Brand & Blitz (1993) rotation curve. The circles are the star-forming complexes. Bottom: model from Hou et al. (2009) based on a rotation curve similar to that of Brand & Blitz (1993). Star symbols locate the Sun and the Galactic Centre. The greyscale features represent the density distribution of H ii and massive molecular clouds.