Question: contourplot from grid

Hi everyone!

I'm stuck with a problem, and I thought I might get some help here. Would be greatly appreciated. Anyway, here goes:

For a given function of two variables, say g(x,y), we can produce a 2d-contourplot (for example using the contourplot-command). No that's all swell as long as we actually have a closed expression for our function g (like, for example, sin(x^2 + y)).

What I need to do however is the following: I have a set of points (x,y) in the plane and for every single one of these points I have a value g(x,y) of my function, i.e. I have a discrete set of triples (x,y,g(x,y)), but NO closed form for this function.

 Of that situation I would like to make a contour plot.

Does anyone know how that can be done?

Thanks,

David