Transformation from FK4 system to FK5 system

Transformation procedures from the FK4 reference system of B1950.0 to the FK5 reference system of J2000.0 are reviewed and their differences are discussed. It is noted that the proper motions in the FK4 are distorted because the precession formulae have been applied to a frame located on the FK4 equinox instead of the dynamical equinox. It is shown that the transformed results do not depend on the epoch of the transfer from the FK4 to the FK5 if systematic and individual corrections are applied to the FK4 having higher-order terms.