a) Examples of [C ii] and H i spectra at two longitudes. The downward arrows over the H i velocity profiles represent the H i absorption features used to resolve the distance ambiguity (see text). The temperature scales for H i and CO are not indicated. b) The V_lsr distance relation is shown for the longitudes of the spectra in panel a). The dotted line represents the solution for a standard Galactic rotation model. The solid line represents that for standard Galactic rotation supplemented by the gas-flow model (see text). The vertical dashed line indicates the tangent distance.