  We refine a classic relation between the hydrogen column density (NH) and
optical extinction (Av) by employing 39 Galactic Supernova Remnants (SNRs) with
X-rays, optical and/or infra-red data available. We find NH =
(1.69+/-0.07)*10^21 Av cm^(-2) mag^(-1) . Applying this relation to three
Galactic SNRs with good historical records allows us to further constrain
either their progenitor's distances or magnitudes, which is independent access
to their distances.
