  Mapping the expansion of the universe gives clues to the underlying physics
causing the recently discovered acceleration of the expansion, and enables
discrimination among cosmological models. We examine the utility of measuring
the rate of expansion, H(z), at various epochs, both alone and in combination
with distance measurements. Due to parameter degeneracies, it proves most
useful as a complement to precision distance-redshift data. Using the baryon
oscillations in the matter power spectrum as a standard rod allows
determination of H(z)/\sqrt{\Omega_mh^2} free of most major systematics, and
thus provides a window on dark energy properties. We discuss the addition of
this data from a next generation galaxy redshift survey such as KAOS to
precision distance information from a next generation supernova survey such as
SNAP. This can provide useful crosschecks as well as lead to improvement on
estimation of a time variation in the dark energy equation of state by factors
ranging from 15-50%.
