How accurate are SuperCOSMOS positions?

Optical positions from the SuperCOSMOS Sky Survey have been compared in detail with accurate radio positions that define the second realisation of the International Celestial Reference Frame (ICRF2). The comparison was limited to the IIIaJ plates from the UK/AAO and Oschin (Palomar) Schmidt telescopes. A total of 1373 ICRF2 sources was used, with the sample restricted to stellar objects brighter than $B_J=20$ and Galactic latitudes $\mathrm{\mid }b\mathrm{\mid }>{10}^{\circ }$. Position differences showed an rms scatter of ${0.16}^{\mathrm{\prime }\mathrm{\prime }}$ in right ascension and declination. While overall systematic offsets were $<{0.1}^{\mathrm{\prime }\mathrm{\prime }}$ in each hemisphere, both the systematics and scatter were greater in the north.