Maximal residual between chip corners within a single pointing, assuming all chips can be shifted to a common RA and Dec centroid. Cases where the astrometric distortion parameters are poorly constrained (and so result in unphysical chip distortions) manifest as large residuals. The distribution in the u-band when using third−order distortion polynomials is clearly systematically larger than the other bands, with some chips having significant biases (greater than 10″), due to the lower number of stars creating instability in the polynomial fits. To minimise the effect of this bias, we reprocessed all u-band tiles with maximal residuals greater than 1 using a linear polynomial distortion order.