Fit function doesn't fit custom arctan function

Hi everyone,

I have come across a rather weird issue, which may be very simple but I cannot figure it out myself. Basically I'm attempting to use the 'fit' function to fit a function to a surface. The function plots a surface with a rectangular hole using arctan. I define the fittype as below:

myfittype = fittype('(-a*atan(x-b)+a*atan(x-c)).*(-a*atan(y-b)+a*atan(y-c))',...
      'independent',{'x', 'y'},'dependent','z')

This also works. Now I'm attempting fitting this to my surface using:

[f, gof] = fit( [x, y], z, myfittype)

This executes but the estimated coefficients (a, b, c) aren't of any use and not even close to the surface.

Hence, my question: did I misunderstand something? Am I doing something horrible wrong?

Thanks to everyone who read my post.

[Siddharth Sundar]

So the z variable contains the data to fit right? Is there a reason you are using the custom fittype that you use? The model you are using could be a reason that the fit isn't as good as you expected it to be.