Difference between measured and modeled color terms (relating the natural and reference photometric system) as a function of radius. Solid points show the average value of the map in bins of radius. We plot separately the value average on all epochs of the winter field (gray dots) and summer field (black squares). The error bars are estimated from the dispersion between epochs. Data points are not independent, and the error is dominated by the uncertainty on the overall normalization of the curve due to the constraint . The synthetic color terms are computed on reddened spectra from the Pickles stellar library, to reflect the effect of the extinction measured in the two fields.