We improve on the model of the Galactic Magnetic Field (GMF) from Jannson\&Farrar (2012), which was constrained using all-sky rotation
measures of extragalactic sources and polarized and unpolarized synchrotron emission data from WMAP.
* developed several alternative functional forms for the coherent and random components
* used newer synchrotron products from Planck and WMAP
* used 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.