Empirical uncertainty estimators for astrometry from digital databases

In order to understand the positional uncertainties of arbitrary objects in several of the current major databases containing astrometric information, a sample of extragalactic radio sources with precise positions in the International Celestial Reference Frame is compared with the available positions of their optical counterparts. The discrepancies between the radio and various optical positions are used to derive empirical uncertainty estimators for the USNO-A2.0 and USNO-A 1.0 catalogs, the Guide Star Selection System images, and the first and second Digitized Sky Surveys. In addition, an estimate of the uncertainty when the USNO-A2.0 catalog is transferred to different image data is provided. These optical astrometric frame uncertainties can in some cases be the dominant error term when cross-identifying sources at different wavelengths.