Do the x,y values form an equally spaced regular grid or are the points randomly placed.
Do you need to interpolate in one direction, e.g. along each plane with constant x, or do you need to form a solid surface, potentially calculating Z for any fractional x and y. Do you know anything about the data? Is there a theoretical model that you can fit to it? This would be the most reasonable approach in general. If you simply need to interpolate without any theoretical model, you could do a 2D spline. Since a spline goes through all the data points, chisquare will be zero. Or are you looking for a solution that does some smoothing of your data?
