As we enter the high-precision era, it will play an increasingly important role in all areas of astronomy, astrophysics and cosmology. Key words: CCD astrometry catalogues reference f. The roles of astrometry from the ground, from Gaia and from a Gaia successor are discussed, but not small-field astrometry from space. The field of astrometry, the precise measurement of the positions, distances and motions of astronomical objects, has been revolutionized in recent years. The comparison with the radio positions shows good agreement with previous determinations for those sources. The final precision on the quasar positions was 0.05 arcsec on average. Here we present the optical position determinations for 18 quasars, all belonging to the ICRF, referred to TAC. The primary reference catalogue used is a Tycho-like catalogue, the Twin Astrographic Catalogue, TAC (Zacharias et al. Catalog and fainter stars, corrected towards more precise reference systems, to derive astrometric positions from CCD observations. We have employed a method that uses secondary reference frames, formed by Guide Star. Extragalactic radio source positions referred to Hipparcos or Tycho catalogues are useful for the determination and checking of the orientation between the reference frames defined by those catalogues and the International Celestial Reference Frame (ICRF), which represents the new astrometric fundamental system.
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