Astrometric RMS values given by SCAMP for the singleexposure plates (panels a and b) and triple-exposure plates (panels c and d), separately along RA and Dec. For the single-exposure plates, the red histograms are for the stars that have S/N > 10, and the green histograms are for the stars that have S/N > 70. For the triple-exposure plates, the red histograms are for the stars that have S/N > 5, and the green histograms are for the stars that have S/N > 50.