\begin{table}%t1
\caption{\label{NH_AV_ratio}Methods for determining $N_{\rm H}$/$A_{V}$ ratios.}
%\centerline
{
\begin{tabular}{ccccc}
\hline
\hline
Method & $A_{V}$ max & Medium & Length scale & Ref.\\
\hline
B stars (UV)     & $\sim$2 mag & diffuse local & $\sim$few 100~pc & \citet{BOH78} \\
B stars (UV)     & $\sim$2 mag & diffuse local & $\sim$pc & \citet{WHI81} \\
SNR (X-rays)     & $\sim$30 mag & diffuse galactic & $\sim$kpc & \citet{GOR75} \\
SNR (X-rays)     & $\sim$30 mag & diffuse galactic & $\sim$kpc & \citet{RYT75} \\
SNR + compact objects (X-rays)$^*$  & $\sim$20 mag & diffuse galactic & $\sim$kpc & \citet{PRE95}\\
{\it Diskless T~Tauri stars (X-rays)$^*$} & {\it $\sim$50 mag} & {\it dense clouds} & $\sim${\it sub$-$pc} & {\it This work} \\
\hline
\end{tabular}}
\smallskip
$^*$ X-ray spectroscopy with imaging X-ray satellites.
\end{table}