  We perform a tomographic baryon acoustic oscillations analysis using the
two-point galaxy correlation function measured from the combined sample of BOSS
DR12, which covers the redshift range of $0.2<z<0.75$. Splitting the sample
into multiple overlapping redshift slices to extract the redshift information
of galaxy clustering, we obtain a measurement of $D_A(z)/r_d$ and $H(z)r_d$ at
nine effective redshifts with the full covariance matrix calibrated using
MultiDark-Patchy mock catalogues. Using the reconstructed galaxy catalogues, we
obtain the precision of $1.3\%-2.2\%$ for $D_A(z)/r_d$ and $2.1\%-6.0\%$ for
$H(z)r_d$. To quantify the gain from the tomographic information, we compare
the constraints on the cosmological parameters using our 9-bin BAO
measurements, the consensus 3-bin BAO and RSD measurements at three effective
redshifts in \citet{Alam2016}, and the non-tomographic (1-bin) BAO measurement
at a single effective redshift. Comparing the 9-bin with 1-bin constraint
result, it can improve the dark energy Figure of Merit by a factor of 1.24 for
the Chevallier-Polarski-Linder parametrisation for equation of state parameter
$w_{\rm DE}$. The errors of $w_0$ and $w_a$ from 9-bin constraints are slightly
improved when compared to the 3-bin constraint result.
