  We present a combined photometric calibration of the SNLS and the SDSS
supernova survey, which results from a joint effort of the SDSS and the SNLS
collaborations. We deliver fluxes calibrated to the HST spectrophotometric star
network for large sets of tertiary stars that cover the science fields of both
surveys in all photometric bands. We also cross-calibrate directly the two
surveys and demonstrate their consistency. For each survey the flat-fielding is
revised based on the analysis of dithered star observations. The calibration
transfer from the HST spectrophotometric standard stars to the multi-epoch
tertiary standard star catalogs in the science fields follows three different
paths: observations of primary standard stars with the SDSS PT telescope;
observations of Landolt secondary standard stars with SNLS MegaCam instrument
at CFHT; and direct observation of faint HST standard stars with MegaCam. In
addition, the tertiary stars for the two surveys are cross-calibrated using
dedicated MegaCam observations of stripe 82. This overlap enables the
comparison of these three calibration paths and justifies using their
combination to improve the calibration accuracy. Flat-field corrections have
improved the uniformity of each survey as demonstrated by the comparison of
photometry in overlapping fields: the rms of the difference between the two
surveys is 3 mmag in gri, 4 mmag in z and 8 mmag in u. We also find a
remarkable agreement (better than 1%) between the SDSS and the SNLS calibration
in griz. The cross-calibration and the introduction of direct calibration
observations bring redundancy and strengthen the confidence in the resulting
calibration. We conclude that the surveys are calibrated to the HST with a
precision of about 0.4% in griz. This precision is comparable to the external
uncertainty affecting the color of the HST primary standard stars.
