Schematic representation of the volume density estimation. The indicated numbers correspond to the column (respectively volume) density values, in arbitrary units. Top half: original column density map with contours that define the following structures: a 9 pc² diffuse background a, a main cloud with a 4 pc² intermediate density envelope b and a 1 pc² dense core c, and a small isolated 0.25 pc² dense clump d. The column density map is then decomposed into column density increments. Bottom half: column density increments are converted into volume density increments by ascribing to them depths equal to the square root of their area (3 pc, 2 pc, 1 pc and 0.5 pc for the diffuse background, the envelope, the core, and the clump, respectively). The final volume density structure is obtained by summing the volume density increments, assuming that each successive increment is nested within the volume underlying the previous contour. The nested volumes are represented in the mid-plane along the line of sight only by convention.