Median [Fe/H] values, as a function of Galactocentric distance, r, shown as a Hess diagram that displays the relative density of stars for each bin of width 0.25 dex in [Fe/H] and 2.5 kpc in distance. The orange points with their error bars summarize the median values of the analyzed set of data with distance. The left panel corresponds to the BOSS F-type spectrophotometric calibration stars, and the right panel to the full SDSS/SEGUE sample (limited to the range described in Sect. 3.1, considering only stars with |Z| > 5 kpc). A clear gradient in [Fe/H], decreasing with distance, is seen for both samples.