       Re: Surface fitting error while using Fit

• To: mathgroup at smc.vnet.net
• Subject: [mg96547] Re: Surface fitting error while using Fit
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Mon, 16 Feb 2009 06:56:36 -0500 (EST)
• References: <gn8jb8\$7oa\$1@smc.vnet.net>

```Hi,

xy is not x*y and not x y
and

data = Flatten[Table[{x, y, 1 + x*y}, {x, 0, 1, 0.1}, {y, 0, 1, 0.1}],
1];
Fit[data, {1, x, y, x*y}, {x, y}]

gives

1. + 5.11805*10^-16 x + 2.04722*10^-16 y + 1. x y

Regards
Jens

Srikanth wrote:
> Hi
>
> I am using mathematica to fit data to a surface. My data is in a
> matrix form:
> Data1={{1.1, 7, 8.765}, {1.1, 7.5, 10.6481},.....,{1.9, 10.5,
> 26.1048}}
>
> I want to get a form z=f(x,y) that fits this data, with x and y being
> my first and second element of each vector. I expect the solution to
> be a linear combination of x, y, xy and some constant term.
>
> So I tried
> Fit[Data1, {1, x, y, xy}, {x, y}]
>
> and I get an error message:
> Fit::fitm: Unable to solve for the fit parameters; the design matrix
> is nonrectangular, non-numerical, or could not be inverted.
>
> I'm not sure what it meant by non-numerical (for example Data1[[1,1]]
> gives 1.1 as expected). The data is rectangular and well defined (by
> which I mean I never have cases like {x, , z} with y missing or
> anything absurd like that). It cannot be inverted of course, but that
> didn't seem to be a problem from the example 9 given here:
> http://reference.wolfram.com/mathematica/tutorial/CurveFitting.html
>
> Any suggestions on what I need to do?
> Thanks for your help
> Srikanth
>

```

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