Surfaces: evaluating z for given (x, y)

I interpolated/approximated a surface with available (x, y, z) data points. I would now like to evaluate the z value for given arbitrary (x, y) coordinates. Can anyone help me with this? All I can see is how to evaluate the surface for given (u, v), and I need to do that for given (x, y) instead. Thanks in advance, if anyone can help. 