Optical Reddening, Integrated HI Optical Depth, Total Hydrogen Column
  Density
  Despite the vastly different angular scales on which they are measured, the
integrated $\lambda$21 cm \HI\ optical depth measured interferometrically,
\WHI, is a good proxy for the optical reddening derived from IR dust emission,
with %\WHI\ $\equiv \int\tau({\rm \HI}) ~{\rm dv} =
(13.8\pm0.7)$\EBV$^{(1.10\pm0.03)}$ \kms\ \WHI\ $\propto$ \EBV$^{1.10}$ for
0.04 mag $\la$ \EBV\ $\la$ 4 mag. For \EBV\ $\la 0.04$ mag or \WHI\ $< 0.7$
\kms, less-absorbent warm and ionized gases assert themselves and $\tau$(HI) is
a less reliable tracer of \EBV. The \WHI-\EBV\ relationship can be inverted to
give a broken power-law relationship between the total hydrogen column density
N(H) and \WHI\ such that knowledge of \WHI\ alone predicts N(H) with an
accuracy of a factor 1.5 ($\pm 0.18$ dex) across two orders of magnitude
variation of \WHI. The \WHI-N(H) relation is invariant under a linear rescaling
of the reddening measure used in the analysis and does not depend on knowing
properties of the HI such as the spin temperature.
