A primary output of Global Positoning System (GPS) post-processed data is a set of non-trivial (independent) vector components and their full covariance information referred to a specific local Cartesian terrestrial frame (e. g., ITRF, WGS84) and epoch. It is important to recognize that when GPS-determined vector components are simultaneously combined into 3D geodetic network adjustments, they should always refer to a common coordinate frame and epoch. This paper uses geometric concepts to formulate rigorous matrix transformations to correct vector components for changes in coordinate systems, secular displacements due to plate rotations, and antenna centering and/or height measuring errors. Finally, the associated variance-covariance matrix of the transformed vector components is derived. © 2001 John Wiley & Sons, Inc.