Average astrometric residuals along y (expressed in pixels) as a function of the y coordinate of measurements, for measurements in CCD 12 of stars brighter than 20.5. The average runs over the 4 SNLS science fields, i.e. ~2000 r images and ~3000 i images. The shape is very similar in r- and i-bands, suggesting some instrumental source. The jumps of residuals happen at y values close to multiples of 512.