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We describe the methodology required for estimation of photometric estimatesof metallicity based on the SDSS gri passbands, which can be used to probe theproperties of main-sequence stars beyond ~ 10 kpc, complementing studies ofnearby stars from more metallicity-sensitive color indices that involve the upassband. As a first application of this approach, we determine photometricmetal abundance estimates for individual main-sequence stars in the VirgoOverdensity, which covers almost 1000 square degrees on the sky, based on acalibration of the metallicity sensitivity of stellar isochrones in the grifilter passbands using field stars with well-determined spectroscopic metalabundances. Despite the low precision of the method for individual stars,internal errors of in [Fe/H] ~ +/- 0.1 dex can be achieved for bulk stellarpopulations. The global metal abundance of the Virgo Overdensity determined inthis way is <[Fe/H]> = -2.0 +/- 0.1 (internal) +/- 0.5 (systematic), fromphotometric measurements of 0.7 million stars with heliocentric distances from~ 10 kpc to ~ 20 kpc. A preliminary metallicity map, based on results for 2.9million stars in the northern SDSS DR-7 footprint, exhibits a shift to lowermetallicities as one proceeds from the inner- to the outer-halo population,consistent with recent interpretation of the kinematics of local samples ofstars with spectroscopically available metallicity estimates and full spacemotions.

Metallicity Mapping with gri Photometry: The Virgo Overdensity and the Halos of the Galaxy