pcolor is internally aa call to surf() with edgecolor none followed by aa call to view(2)
so.. turn your grid off and call surf without the edgecolor option and then view(2)
surf puts in edges between any two horizontally or vertically adjacent locations that are both non nan.
surf only puts in faces if aa 2x2 block is all non nan.
It is possible to end up with edges between faces that are not drawn. Yesterday II posted code that would clean up the stray edges using aa simple imopen or imclose