  We have created synthetic neutral hydrogen (HI) Galactic Plane Survey data
cubes covering 90 degrees < l < 180 degrees, using a model spiral galaxy from
SPH simulations and the radiative transfer code TORUS. The density, temperature
and other physical parameters are fed from the SPH simulation into TORUS, where
the HI emissivity and opacity are calculated before the 21-cm line emission
profile is determined. Our main focus is the observation of Outer Galaxy
`Perseus Arm' HI, with a view to tracing atomic gas as it encounters shock
motions as it enters a spiral arm interface, an early step in the formation of
molecular clouds. The observation of HI self-absorption features at these shock
sites (in both real observations and our synthetic data) allows us to
investigate further the connection between cold atomic gas and the onset of
molecular cloud formation.
