An unambiguous characterisation of the polycrystalline-Si grain boundary behaviour has been obtained by studying thin film transistors, TFTs, realized on sequentially lateral solidified, SLS, material. The grain boundary, GB, carrier trapping has been distinguished from intra-grain trapping by analysing TFTs with channels aligned along two orthogonal directions in this anisotropic material: the thermal activation energy of the channel current was measured in the two orthogonal directions, and the difference in activation energy was related to carrier flow over perpendicular GBs. Two dimensional numerical simulations show that the experimental behaviour is correctly reproduced by a finite-width GB, in which carrier flow is controlled by transport across a resistive GB region, rather than by emission over a barrier.