Impact of observational noise on the PDF measured for the cloud from Fig. 1. The red solid curve shows the analytic description of the input PDF, the blue dotted line is the measured PDF of the fractal map with a finite pixel number. The other broken lines show the column density PDF that is measured if the map is affected by different levels of observational noise, characterized by a normal distribution with a standard deviation from 10% to 100% of the peak column density.