Steps to statistically derive an approximate volume density from the column density under the assumptions of isotropy and of nested distributions of increasing density. Panel a: spatial distribution of the column density of gas as deduced from the spectral energy distribution of the dust continuum emission. Panel b: surface area of the observed field of view that has a column density larger than a lower limit. Panel c: typical depth (square root of the previous surface) associated with a given column density. Panel d: typical volume density associated with a given column density. The contour levels in panel a and the dashed vertical lines in panels b to d take the column density values corresponding to a visual extinction of 2, 6, 18, 54, and 162 mag.