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Next we need to model the gas between us and the galactic center. Let's ignore the Sgr B2 molecular cloud, at first. Making the assumption that the ISM composition is effectively isotropic on Galactic scales, we will adopt the characteristic ISM phase densities and filling factors from Draine's Table 1.3.  \begin{tabular}  Phase & $n_H$ ($\rm{cm}^{-3}$) & $f_V$ \\  HIM & 0.004 & 0.5 \\  HII/WNM & 0.5 & 0.5 \\  CNM & 30 & 0.01 \\  Diffuse H$_2$ & 100 & 0.001 \\  Dense H$_2$ & $5\times10^4$ & 0.0001  \end{tabular} The total column density is then $\rm{N_H}=(\sum n_H f_V)d$