Weighted rms of astrometric residuals as a function of apparent AB magnitude of the source for each component of the astrometric residual field. Four models are compared: no correction from a GP interpolation (red circles), correction from a GP interpolation with a Gaussian kernel (black crosses), correction from a GP interpolation with a von Kármán kernel (blue stars), and correction from a GP interpolation with a von Kármán kernel and the mean function computed as described in Sect. 5 (green triangles, hidden underneath the blue ones).