In this paper, GPS-synchronized harmonic measurements performed on the transmission network of the Danish TSO Energinet.dk are presented. The purpose of the measurements was to verify a computer model of the network. The selected verification method was based on determination of transfer harmonic impedance by switching a series component that contains a shunt branch , . Harmonic currents and voltages were simultaneously measured at three 400 kV substations with OMICRON CMC256 units with EnerLyzer function and GPS synchronization units. Time domain "snapshot" measurements of three-phase voltages and currents were synchronously taken, while a power line was switched out and in, and the results were post-processed in Matlab. Harmonic voltages were measured using capacitive taps of the 400 kV/150 kV autotransformer bushings. Harmonic currents were measured as voltage drop on low-inductance resistors inserted in series with the metering equipment. Due to largely unbalanced harmonic currents, the computer network model was verified by a modified verification method, based on injection of measured harmonic currents and comparison of resultant voltage increments. The method requires precise synchronization of the units and sufficient level of harmonic distortion in the network.