Correlation of N(H₂) calculated from dust continuum and from ¹³CO(1−0) using a ¹³CO abundance of 10^-6. The circles mark the column densities derived from the excitation temperature estimates (where ¹²CO measurements were available). The vertical error bars show N(H₂)_gas lower and upper limits calculated using 8.5 and 19.5 K as excitation temperature and the horizontal error bars are from the uncertainty of the τ₂₅₀ values in the centre of the clumps. The colours indicate the dust colour temperature: blue is below 14 K and red is above 14 K. Where only lower and upper limits of N(H₂)_gas could be calculated, only error bars are shown with the colour code. The dashed line indicates N(H₂)_dust = N(H₂)_gas, the solid line is the linear fit to the plotted values and the shaded area shows the 1σuncertainty range of this fit.