Left: ∫ τdv versus N_HI(HI4PI) is plotted on a double logarithmic scale. The dashed and dash-dotted lines mark the range of uncertainty by fitting a linear function to the data values. The dashed red line represents the average of both least-square approximations. From this we calculated the correlation coefficient of 0.89 and a scaling relation of $\int \tau dv\propto N_{\mathrm{H}\mathrm{I}}^{2.10\pm 0.22}$. Right: Same as left but instead of N_HI, the dependence of ∫ τdv from A_V is displayed. Here, the correlation coefficient is 0.85 and the scaling relation is $\int \tau dv\propto A_{\mathrm{V}}^{1.50\pm 0.13}$.