  This paper considers some effects of foreground Galactic gas on radiation
received from extragalactic objects, with an emphasis on the use of the 21cm
line to determine the total N(HI). In general, the opacity of the 21cm line
makes it impossible to derive an accurate value of N(HI) by simply applying a
formula to the observed emission, except in directions where there is very
little interstellar matter. The 21cm line can be used to estimate the
likelihood that there is significant molecular hydrogen in a particular
direction, but carries little or no information on the amount of ionized gas,
which can be a major source of foreground effects. Considerable discussion is
devoted to the importance of small-scale angular structure in HI, with the
conclusion that it will rarely contribute significantly to the total error
compared to other factors (such as the effects of ionized gas) for
extragalactic sight lines at high Galactic latitude. The direction of the
Hubble/Chandra Deep Field North is used as an example of the complexities that
might occur even in the absence of opacity or molecular gas.
