... AAS link ] . ... Our internal perspective makes it difficult to determine the spiral structure of our own galaxy. ... H I absorption, on the other hand, traces colder, denser gas that should be more confined in discrete clouds and perhaps discrete spiral arms. In addition, H I self-absorption (HISA) requires background H I emission at the same velocity as the foreground absorbing gas, thus introducing a much-needed constraint on the large-scale sightline geometry and kinematics. ...