  We use chiral perturbation theory to study the extrapolations necessary to
make physical predictions from lattice QCD data for the electromagnetic form
factors of pseudoscalar mesons. We focus on the quark mass, momentum, lattice
spacing, and volume dependence and apply our results to simulations employing
mixed actions of Ginsparg-Wilson valence quarks and staggered sea quarks. To
determine charge radii at quark masses on the lattices currently used, we find
that all extrapolations except the one to infinite volume make significant
contributions to the systematic error.
