  We report on initial results from a project to constrain the large-scale and
turbulent magnetic fields of the Milky Way galaxy, which eventually will
incorporate all of the relevant observational data. In this paper we fit
popular large scale magnetic field models to WMAP3 polarization maps. We find
that the polarization data can constrain certain model parameters but does not
uniquely determine the best-fit parameters. We also find that the polarization
data alone cannot distinguish between model symmetries, e.g., the existence of
field reversals. We show how future UHECR data can break this degeneracy.
