Astrometric accuracy has two components; the accuracy with which an image can be centred on a CCD or photographic plate and the accuracy with which two image centres can be mapped on to standard co-ordinates. Photographic plates cover a sufficient area of sky that several standard stars can be measured with the satellite images and the mapping to standard coordinates can be done in one operation. CCDs have much smaller areas and the number of stars with sufficiently accurate positions may not be sufficient. The surface densities of the best available catalogues are shown in Table I.