The aim of this study is to predict the strains induced by welding-brazing of automotive thin metal sheets using finite elements simulations. In order to validate the simulations, experimental tests on industrial parts are carried out. One goal of these tests is to determine 3D surface displacement fields during welding. The measurement technique chosen for these tests is 3D surface DIC (stereocorrelation). To take into account the specificity of these tests (object size about one square meter) a new calibration method of the stereovision system has been developed. Currently, calibration methods are based on the knowledge of the calibration target geometry and its projections onto both right and left camera frames. In the approach proposed herein, the calibration is performed directly on the measured part, which is modeled by a Bézier patch. A first step consists in finding the best 3D (CAD surface) to 2D (camera picture) transformation.