I have a 3D surface in $x$, $y$, and $z$. where $z$ is a function of $x$ and $y$ and my points are on a structured grid in $x$ and $y$. My function $z$ is highly oscillatory and irregular with multiscale phenomena. I'd like a fitting tool that will capture the large scale phenomena and provide a differentiable closed function without getting derailed by the small scale phenomena. Are there any good 3D data fitting toolboxes or methods?