A method is presented for calculating the Lie point symmetries of a scalar
difference equation on a two-dimensional lattice. The symmetry transformations
act on the equations and on the lattice. They take solutions into solutions and
can be used to perform symmetry reduction. The method generalizes one presented
in a recent publication for the case of ordinary difference equations. In turn,
it can easily be generalized to difference systems involving an arbitrary
number of dependent and independent variables
