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Using high-resolution transmission electron microscopy, we have studied the atomistic structure of 90° domain walls in ferroelectric PbTiO₃ thin films. Variations in the c and a lattice parameters across domain boundaries were determined by measuring the distance between specific image features associated with Pb column positions. These positions were located with high precision by cross-correlating a single-column template with the entire image. Image simulations were performed on the basis of the deduced structure and were used to verify the correctness of the method and to estimate errors. The experimental lattice parameter variations are compared with predictions based on Landau-Ginzburg theory, and the domain wall energy is estimated to be 50mJm⁻².