  The calibration of ground based optical imaging data to photometric accuracy
of 10 mmag over the full survey area and to color uniformity better than 5 mmag
on the scale of the VIS focal plane is a key science requirement for the Euclid
mission. These accuracies enable stable photometric redshifts of galaxies and
modeling of the color dependent VIS PSF for weak lensing studies. We use the
Gaia photometry to calibrate the $g/r/i/z$ magnitudes of Dark Energy Survey
(DES) data to meet the stringent Euclid requirements. The Gaia G band magnitude
along with the BP-RP color information of stars observed in the DES single
epoch (SE) exposures are used to derive the transformation from Gaia to DES
photometry for individual CCDs and to characterize persistent photometric
errors across the DECam focal plane. We use the color dependence of these
persistent errors to characterize the $g/r/i/z$ bandpass variations across the
DECam focal plane.
