[gov2001-l] Contour/Shaded Contour Plots

I am trying to produce a contour or shaded contour plot similar to those we produced in Gov 2001. However, unlike the examples we dealt with in Gov 2001, where we were evaluating likelihood functions, the set of points that I'm using does not fall neatly on a grid. Instead, I have an n x 3 table, each row of which contains a simulated observation point (x, y, z) that I would like to represent graphically. Can anyone recommend a good canned function that would allow me to produce smooth graphs from data of this type? The functions contour() and image() seem limited to evaluating functions over a grid. The function symbols() offers flexibility in terms of where points are located but doesn't produce smoothed graphs. Thanks, Clayton