Correlations of column density estimates for the externally heated, lower column density model. The x-axis is the true column density of the model cloud at the resolution of 40′′ (panel a) or 18′′ (panels b and c). On the y-axes are the results from direct MBB fits at 40′′ resolution (panel a) and from methods A and B at 18′′ resolution (panels b and c, respectively). The dashed blue lines show the one-to-one relations and the dashed red lines the least squaresfits. The slopes k of the least squares lines as well as the estimated cloud masses (in solar masses) are given in the panels. The true mass of the cloud is quoted in panel c.