Measurement precision for Obs16 after GD correction. Top: standard deviation (σ) of the transformation residuals after GD1 correction as a function of Gmag by a six-parameter linear transformation to the standard coordinate. The left shows the RA direction and the right shows the DE direction. Bottom: standard deviation of the transformation residuals after GD2 correction as a function of Gmag by a six-parameter linear transformation to the master frame. The left shows the X direction and the right shows the Y direction. The red dashed line marks the median (σ) for the stars brighter than 18 Gmag.