Many relevant contributions on this matter can be found in Digitized Optical Sky Surveys (MacGillivray and Thomson 1992), in the newsletters of the IAU Working Group on Wide-field Imaging (contact the editor, H. T. MacGillivray at hmg@star.roe.ac.uk for copies), and in the proceedings of the IAU Symposium 161 (MacGillivray et al. 1994).