  We present a one per cent measurement of the cosmic distance scale from the
detections of the baryon acoustic oscillations in the clustering of galaxies
from the Baryon Oscillation Spectroscopic Survey (BOSS), which is part of the
Sloan Digital Sky Survey III (SDSS-III). Our results come from the Data Release
11 (DR11) sample, containing nearly one million galaxies and covering
approximately $8\,500$ square degrees and the redshift range $0.2<z<0.7$. We
also compare these results with those from the publicly released DR9 and DR10
samples. Assuming a concordance $\Lambda$CDM cosmological model, the DR11
sample covers a volume of 13\,Gpc${}^3$ and is the largest region of the
Universe ever surveyed at this density. We measure the correlation function and
power spectrum, including density-field reconstruction of the baryon acoustic
oscillation (BAO) feature. The acoustic features are detected at a significance
of over $7\,\sigma$ in both the correlation function and power spectrum.
Fitting for the position of the acoustic features measures the distance
relative to the sound horizon at the drag epoch, $r_d$, which has a value of
$r_{d,{\rm fid}}=149.28\,$Mpc in our fiducial cosmology. We find
$D_V=(1264\pm25\,{\rm Mpc})(r_d/r_{d,{\rm fid}})$ at $z=0.32$ and
$D_V=(2056\pm20\,{\rm Mpc})(r_d/r_{d,{\rm fid}})$ at $z=0.57$. At 1.0 per cent,
this latter measure is the most precise distance constraint ever obtained from
a galaxy survey. Separating the clustering along and transverse to the
line-of-sight yields measurements at $z=0.57$ of $D_A=(1421\pm20\,{\rm
Mpc})(r_d/r_{d,{\rm fid}})$ and $H=(96.8\pm3.4\,{\rm km/s/Mpc})(r_{d,{\rm
fid}}/r_d)$. Our measurements of the distance scale are in good agreement with
previous BAO measurements and with the predictions from cosmic microwave
background data for a spatially flat cold dark matter model with a cosmological
constant.
