In one dimension, the concept of a mesh is a bit difficult,
however in two and three dimensions
it is
vital to
finite difference methods.
The one dimensional method used a uniform step **dx**, and in the two
dimensional method we will again use a uniform step **h=dx=dz** in
both directions as in Figure 10.

Figure 10: Two Dimensional Mesh.

The continuous velocity function is
discretized into an average value to each square of the mesh and
is assumed to be an appropriate approximation.
This assumption is valid if **h** is small compared to the
wavelengths of propagation.