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Published online by Cambridge University Press: 03 August 2017
It was pointed out already in the early stage of the development of the Mark-III VLBI that the structure of the quasars could affect VLBI results for geodesy or astrometry (Cotton, 1980; Robertson, 1981). As errors from other sources are gradually reduced, this effect can not be ignored any longer. We should monitor the structure variations of the sources and study their internal kinematics closely. 8GHz maps made from geodetic VLBI observations may give us important information.