  The Beijing-Arizona Sky Survey (BASS) is an imaging survey and uses the 2.3 m
Bok telescope at Kitt Peak. In order to tap the astrometric potential of the
Bok telescope and improve the astrometry of BASS, a distortion solution for the
Bok telescope is made. In the past, we used a single lookup table to correct
all the positional errors. However, this method can not be applied to the
reduction of the observation for the Bok telescope where four CCD chips are
equipped with. Then quite different from our previous method, two third-order
polynomials were used to fit the lookup table. By using the polynomial
Geometric Distortion (called GD hereafter) correction the astrometry of BASS is
improved greatly. Moreover, an additional lookup table correction is found to
be more effective to obtain a final GD. The results show that the positional
measurement precision of the appropriate bright stars is estimated at about 20
mas and even better in each direction. Besides, the relative positions of the
chips are measured. The change of the inter-chip gaps in horizontal or vertical
is no more than 5 pixel between 2016 and 2017 and the change of the roll angle
is no more than 0.1 degree.
