  We improve on the model of the Galactic Magnetic Field (GMF) from Jansson \&
Farrar (2012), which was constrained using all-sky rotation measures of
extragalactic sources and polarized and unpolarized synchrotron emission data
from WMAP. We have developed several alternative functional forms for the
coherent and random components, used newer synchrotron products from Planck and
WMAP and testes new models of the densities of thermal electrons and cosmic-ray
electrons. The differences in the resultant GMF models, depending on which
parameterization of the field, synchrotron product and electron densities are
used, provides a measure of the uncertainty in our inference of the GMF. We
discuss the impact of these uncertainties on charged-particle astronomy at
ultra-high energies.
