Scheme of the iterative fitting method used in this work. The first panel sketches the inner Milky Way divided into rings. The gas emission-line profile at longitude l is given by the sum of the contributions of all rings with 0 ≤ i ≤ i_max (second panel), the parameters of the last ring can be found by fitting the last Gaussian of the model profile (thick red curve) to the data (dashed curve). We iterate this procedure by decreasing l at each step (third panel).