HISA Radiative Transfer as a Probe of Galactic Structure

H I Self-Absorption (HISA) arises from cold H I gas in front of warmer H I emission at the same velocity. HISA is thus a unique probe of the velocity and temperature structure of the Galactic ISM, from sub-parsec scales within clouds up to multi-kiloparsec scales across the disk. Our location inside the Milky Way gives us simultaneously a better view of its ISM than any other galaxy's and a worse view of its overall structure, especially spiral arms. HISA longitude-velocity distributions extracted from the IGPS might assist with this problem by tracing cold H I in spiral density waves. But how clear is this tracer likely to be, and how unique? To address this question, a series of simple H I radiative transfer models of the Galaxy will be presented.