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