  We present a new calibration method based on cross-correlations with WMAP and
apply it to data from the Atacama Cosmology Telescope (ACT). ACT's observing
strategy and map making procedure allows an unbiased reconstruction of the
modes in the maps over a wide range of multipoles. By directly matching the ACT
maps to WMAP observations in the multipole range of 400 < ell < 1000, we
determine the absolute calibration with an uncertainty of 2% in temperature.
The precise measurement of the calibration error directly impacts the
uncertainties in the cosmological parameters estimated from the ACT power
spectra. We also present a combined map based on ACT and WMAP data that has
high signal-to-noise over a wide range of multipoles.
