  We measure the acoustic scale from the angular power spectra of the Sloan
Digital Sky Survey III (SDSS-III) Data Release 8 imaging catalog that includes
872,921 galaxies over ~ 10,000 deg^2 between 0.45<z<0.65. The extensive
spectroscopic training set of the Baryon Oscillation Spectroscopic Survey
(BOSS) luminous galaxies allows precise estimates of the true redshift
distributions of galaxies in our imaging catalog. Utilizing the redshift
distribution information, we build templates and fit to the power spectra of
the data, which are measured in our companion paper, Ho et al. 2011, to derive
the location of Baryon acoustic oscillations (BAO) while marginalizing over
many free parameters to exclude nearly all of the non-BAO signal. We derive the
ratio of the angular diameter distance to the sound horizon scale D_A/r_s=
9.212 + 0.416 -0.404 at z=0.54, and therefore, D_A= 1411+- 65 Mpc at z=0.54;
the result is fairly independent of assumptions on the underlying cosmology.
Our measurement of angular diameter distance D_A is 1.4 \sigma higher than what
is expected for the concordance LCDM (Komatsu et al. 2011), in accordance to
the trend of other spectroscopic BAO measurements for z >~ 0.35. We report
constraints on cosmological parameters from our measurement in combination with
the WMAP7 data and the previous spectroscopic BAO measurements of SDSS
(Percival et al. 2010) and WiggleZ (Blake et al. 2011). We refer to our
companion papers (Ho et al. 2011; de Putter et al. 2011) for investigations on
information of the full power spectrum.
