MATLAB: Surface fitting1

Question

Hi,

I'd like to ask for help regarding surface fitting and I have very little experience in Matlab.

I have a Z 77×77 matrix.

If I use surf(Z) I can plot it without problem.

I would like to interpolate Z but I couldn't get it right neither with the sftool nor with the gridfit.

If I call sftool I don't even have an option to select this Z matrix. I've read the manuals but it is still unclear how I could do this.

So the question is how to use this interactive sftool if I have a 77×77 large Z matrix [measurement data] and I want to interpolate or somehow fit it.

Thanks, Krivan

Answer 1

Why use gridfit, or for that matter, the surface fitting tool? It is already a surface, on a regular grid. Use interp2, a tool designed for exactly that purpose.

help interp2

Edit: If x and xi are identical, then why are you trying to interpolate anyway? You would get out the input array even if you had called interp2 properly! It does not sound as if you are trying to do interpolation at all.

As far as the surface goes, if it is too noisy and you just want to make it less so, then do some smoothing. A simple solution there is to use a convolution tool, although then you need to deal with edge effects. Easier is to use gridfit, with a larger than normal value for the smoothness parameter.

[xmesh,ymesh] = meshgrid(x,y); zgrid = gridfit(xmesh(:),ymesh(:),z(:),x,y,'smoothness',k);

By default, the smoothing parameter is 1, a value which will yield relatively little smoothing. Try 2 or 5 or 10. Pick a value that makes you happy.