The article deals with the measurement of heterogeneous deformations during the plastic flow of metals. To encode de surface of the test-pieces, transferable carbon grids were used. They proved a reliable technique in the conditions of the experiments (channel-die, step of 100 mu m), even for highly localized deformations. To calculate the displacement field, the grid method was chosen. Up to now it has been developed for elastic structures. It is extended here to large displacement situations. Results are analysed from a metrological point of view and the resolution is kept at 1/100(th) fringe, the final spatial resolution being at two fringes. The mean deformation is 15% between each loading step. Displacement maps, or better, principle shear strain maps, derived from the previous ones, show the presence of a preferential direction even at low deformation levels. They put forward the decisive role played in the development of heterogeneities by the corners of the test-piece, sensitive in channel-die compression because two of their faces are submitted to friction while the third is free. This first attempt of using the grid method for large displacements will be optimized in the future by automating the iterative procedure and reducing the calculation time.