  The Gaia DR2 catalog released in 2018 gives information about more than one
billion stars, including their extremely precise positions that are not
affected by the atmosphere, as well as the magnitudes in the G, RP, and BP
passbands. This information provides great potential for the improvement of the
ground-based astrometry. Based on Gaia DR2, we present a convenient method to
calibrate the differential color refraction (DCR). This method only requires
observations with dozens of stars taken through a selected filter. Applying
this method to the reduction of observations captured through different filters
by the 1-m and 2.4-m telescopes at Yunnan Observatory, the results show that
the mean of the residuals between observed and computed positions (O-C) after
DCR correction is significantly reduced. For our observations taken through an
N (null) filter, the median of the mean (O-C) for well-exposed stars (about 15
G-mag) decreases from 19 mas to 3 mas, thus achieving better accuracy, i.e.
mean (O-C). Another issue correlated is a systematic error caused by the poor
charge transfer efficiency (CTE) when a CCD frame is read out. This systematic
error is significant for some of the observations captured by the 1-m telescope
at Yunnan Observatory. Using a sigmoidal function to fit and correct the mean
(O-C), a systematic error up to 30 mas can be eliminated.
