  Using a set of compilations of measurements for extragalactic radio sources
we construct all-sky maps of the Faraday Rotation produced by the Galactic
magnetic field. In order to generate the maps we treat the radio source
positions as a kind of "mask" and construct combinations of spherical harmonic
modes that are orthogonal on the masked sky. As long as relatively small
multipoles are used the resulting maps are quite stable to changes in selection
criteria for the sources, and show clearly the structure of the local Galactic
magnetic field. We also suggest the use of these maps as templates for CMB
foreground analysis, illustrating the idea with a cross-correlation analysis
between the Wilkinson Microwave Anisotropy Probe (WMAP) data and our maps. We
find a significant cross-correlation, indicating the presence of significant
residual contamination.
