  We use flux-transmission correlations in \Lya forests to measure the imprint
of baryon acoustic oscillations (BAO). The study uses spectra of 157,783
quasars in the redshift range $2.1\le z \le 3.5$ from the Sloan Digital Sky
Survey (SDSS) Data Release 12 (DR12). Besides the statistical improvements on
our previous studies using SDSS DR9 and DR11, we have implemented numerous
improvements in the analysis procedure, allowing us to construct a physical
model of the correlation function and to investigate potential systematic
errors in the determination of the BAO peak position. The Hubble distance,
$\DHub=c/H(z)$, relative to the sound horizon is $\DHub(z=2.33)/r_d=9.07 \pm
0.31$. The best-determined combination of comoving angular-diameter distance,
$\DM$, and the Hubble distance is found to be
$\DHub^{0.7}\DM^{0.3}/r_d=13.94\pm0.35$. This value is $1.028\pm0.026$ times
the prediction of the flat-\lcdm model consistent with the cosmic microwave
background (CMB) anisotropy spectrum. The errors include marginalization over
the effects of unidentified high-density absorption systems and fluctuations in
ultraviolet ionizing radiation. Independently of the CMB measurements, the
combination of our results and other BAO observations determine the open-\lcdm
density parameters to be $\om=0.296 \pm 0.029$, $\ol=0.699 \pm 0.100$ and
$\Omega_k = -0.002 \pm 0.119$.
