Schematic view of the decomposition method proposed by JT11. The f₁ and f₂ column density ratios observed along a given line of sight (top panel) are supposed to result from the linear combination of a fraction, g_low, of neutral carbon at low pressure, and a fraction, g_high, of neutral carbon at high pressure. The thermal pressure of the high-pressure components is supposed to be known (10⁶ K cm⁻³ in this example). g_low and g_high are derived from the geometric position of the observed point along the line that connects the low-pressure and high-pressure components (dashed black line in the top panel). This decomposition procedure is applied to all observations (bottom panel) to derive the distribution of thermal pressure of the low-pressure component.