Problem with double integral for a 3d surface

[gcase44]

The attached file has a 3d curve fit of pressure data on a disc. The curve fitting routine could only do so in x,y,z coordinates and the this is best handled in r,z theta. I replaced x with r*cos(theta) and y with r*sin(theta). The integral will not work. I believe that it is having a problem because it is evaluating one variable at a time. Does anyone have a method to integrate this surface.

[RichardJ]

cos(theta) is the cosine function applied to variable theta. costheta is a variable name. Same for sin.

I deleted the huge bitmap, which does nothing good for the file size.