Sky distribution of the median difference between the Gaia G magnitude and a G-band magnitude estimated from the SDSS r and i photometry using the photometric transformations listed in Appendix A. The comparison is limited to sources with 13 < G < 21, 0.6 < r − i < 1.2, Gaia photometric errors lower than 0.03, and SDSS photometric errors lower than 0.05. The median value for each HEALPix pixel is computed over all sources falling in the same pixel of level 8.