  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=2048.
