The Schmidt telescope surveys of the northern and southern sky form an
invaluable resource for seeking counterparts at other wavelengths and for
transferring coordinates onto CCD images. But how reliable are the
positions from the Schmidt surveys, given the complex distortions
introduced by the bending of the glass plates to the telescope's focal
surface? SuperCOSMOS provides the best image sampling of the various
digitized versions of the UK and Palomar Schmidt surveys, and in this
paper we compare SuperCOSMOS optical positions with milliarcsecond radio
positions that define the International Celestial Reference Frame (ICRF).
The comparison sample has been confined to stellar objects brighter that
magnitude 20, and Galactic latitude > 10 deg. Mean position offsets in
both hemispheres are <0.1 arcsec in both coordinates, with rms scatter
<0.2 arcsec. However, there are significant systematic errors in both
hemispheres, particularly in the north.