Iron abundance for the far disk CCs classified in this work as a function of their Galactocentric distance, plotted using filled blue diamonds. Open symbols are used for the same stars as in Fig. 8. The distances have been computed assuming A_Ks/E(J − K_s) = 0.465 and A_Ks/E(H − K_s) = 1.308. The error bars in the distances represent the total uncertainties calculated taking into account all the sources of uncertainty discussed in Sect. 2.5. The metallicity error bar is shown in the bottom left corner of the plot (σ_[Fe/H] = 0.2 dex, see Sect. 7.2). The metallicity gradient measured by Genovali et al. (2014), based only on abundances provided by their group (red light circles), is shown as a light green dashed line; the orange dashed line is their fit including iron abundances for CCs available in the literature, which are plotted with red light triangles. The purple and light blue lines show the metallicity gradient on the far side of the Galactic disk as derived in this work, when using CCs in the whole range of R_GC and for R_GC ⪅ 17 kpc, respectively.