We have presented simulations and experiments for 2-dimensional di�raction distributions of a transmission volume hologram. We sample the input pattern with a few elementary points and calculate the di�raction of the corresponding elementary grating. The simulations can describe well the di�raction pattern obtained in the experiment and can be used to predict the tolerance for angular deviation of the reading beam. The tolerance for angular deviation depends on its direction. It is a�ected by the Bragg degeneracy. The simulation results provide a clear picture of the spatial tolerance for reading a trans-mission volume hologram.