  We use a galaxy sample derived from the DECaLS DR9 to measure the Baryonic
Acoustic Oscillations (BAO). The magnitude-limited sample consists of 10.6
million galaxies in an area of 4974 deg$^2$ over the redshift range of [0.6,
1]. A key novelty of this work is that the true redshift distribution of the
photo-$z$ sample is derived from the self calibration method, which determines
the true redshift distribution using the clustering information of the
photometric data alone. Through the angular correlation function in four
tomographic bins, we constrain the BAO scale dilation parameter $\alpha$ to be
$1.025\pm 0.033 $, consistent with the fiducial Planck cosmology.
Alternatively, the ratio between the comoving angular diameter distance and the
sound horizon, $D_{\rm M} / r_{\rm s}$ is constrained to be $18.94 \pm 0.61 $
at the effective redshift of 0.749. We corroborate our results with the true
redshift distribution obtained from a weighted spectroscopic sample, finding
very good agreement. We have conducted a series of tests to demonstrate the
robustness of the measurement. Our work demonstrates that the self calibration
method can effectively constrain the true redshift distribution in cosmological
applications, especially in the context of photometric BAO measurement.
