Two bidimensional spline functions are applied to the interpolation and automatic contouring of data from a ground magnetic survey of the Rhinegraben, between Karlsruhe and Strasbourg. These bidimensional spline functions can be applied in the general case, i.e., where a set of nonequispaced data is given. These splines define a surface passing through the original points. The most promising spline functions are the thin plate ("plaque mince") and the pseudocubic spline, studied by Atteia (1966), Thomann (1970), and Duchon (1975, 1976). A contouring method based on them has been implemented.