Large scale structure measurements require accurate and precise knowledge of the survey depth -- typically expressed in the form of a limiting magnitude - as a function of position on the sky. To date, most surveys only compute the point-source limiting magnitude measured within a fixed metric aperture. However, this quantity is ill suited to describe the limiting depth of galaxies, which depends on the detailed interplay of survey systematics with galaxy shapes and sizes. We describe an empirical method for directly estimating the limiting magnitude for large photometric surveys, and apply it to $\sim10,000\,\mathrm{deg}^{2}$ of SDSS DR8 data. Combined with deeper imaging from SDSS Stripe 82 and CFHTLens, we are able to use these depth maps to estimate the location-dependent galaxy detection completeness at any point within the full BOSS DR8 survey region. We show that these maps can be used to construct random points suitable for unbiased estimation of correlation functions for galaxies near the survey limiting magnitude. Finally, we provide limiting magnitude maps for galaxies in SDSS DR8 in \healpix{} format with NSIDE Show Partial Abstract