Median photometric metallicity of all M31 stars in each field, along with the linear fit to the median field values (solid line), calculated using the nominal 10 Gyr, [α/Fe] =0.0 VandenBerg et al. (2006) isochrones. Error bars show the error in the median value of each field. The dashed and dotted lines show the linear least-squares fits to the median metallicity estimates in each field calculated using alternative methods: 10 Gyr, [α/Fe] =0.3 isochrones (red dashed line), 5 Gyr, [α/Fe] =0.0 isochrones (blue dotted line), and spectroscopic metallicity estimates based on the Ca II triplet EW (purple dot-dashed line).