could a non linear surface be fitted to a scattered data?

Hi, i have a scattered data [x, y, z], not gridded. i would like to fit a surface to that data, although i dont what that surface to be linear. maybe some sort of a curved surface. is that possible?

i use those comand now:

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ft = fittype( 'poly11' );

opts = fitoptions( 'Method', 'LinearLeastSquares' );

opts.Normalize = 'on';

opts.Robust = 'LAR';

[fitresult, gof] = fit( [Rdata(:,1), Rdata(:,2)], Rdata(:,3), ft, opts );

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but they only allows me to create some sort of linear surface.

thank you for your help.