Part of the galactic plane of Fig. 1. The radial velocity for circular rotation is shown by the shaded regions. Thecontour lines indicate the density of a simulated log-normal density field with a power-law power spectrum. The solar circle, the tangent point circle, and the lines of constant longitude are indicated. The dark green contours delineate a high-density feature at the near distance embedded in a large complex, shown by the light green contours. The red contours at the far distance mark a feature that would look similar in gas-line survey data, but would be penalised by the correlation structure assumed in the Bayesian inference approach. See text for a discussion of how the correlation structure of the density helps resolving the near-far ambiguity.