The following program can be used to excamine the error of the spline tool. This could be taken as a measure for the smoothnes of a surface.
gr_spline -f test.dat -n 1000 | gr_lift -f test.dat -pipe
In the same way the error of the bspline or the nurbs tool can be measured. This is useful cause the control points are often used for the representaion of the curve itself. Therfore one would like the error between curve and controlpoint polygon to be small.