Example Hi spectra extracted towards dense clumps identified from the ATLASGAL survey. The velocity of the tangent position is determined from a fit to Hi data (e.g.  McClure-Griffiths & Dickey 2007) and is indicated by the blue line; the grey shaded region covers a velocity range of ±10  km s^-1. Sources in this region are placed at the distance of the tangent point. The dashed red vertical line shows the velocity of the source with the yellow shaded region showing the typical FWHM of the molecular lines. The top and bottom panels show examples of a source located at the near and far distances, respectively.