The dependence of the extracted Plummer parameters ρ_c, p, and M_line on the noise level in the case of model A and the near cloud distance. The level of instrumental noise is either 0.5, 1.0, 2.0, or 4.0 times the default value (see Sect. 2). The plot shows for all the twelve filaments the median values and, as vertical bars, the inter-quartile intervals of the estimated parameters that are obtained from 50 noise realisations of the surface brightness maps. The coloured lines connect the median values. The vertical bars plotted for the individual filaments are shifted slightly along the x axis to avoid overlap.