       Re: Surface fitting error while using Fit

• To: mathgroup at smc.vnet.net
• Subject: [mg96561] Re: Surface fitting error while using Fit
• From: dh <dh at metrohm.com>
• Date: Mon, 16 Feb 2009 06:59:10 -0500 (EST)
• References: <gn8jb8\$7oa\$1@smc.vnet.net>

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Hi Srikanth,

you should give an full example. Otherwise I can not tell what you are

doing wrong. Here is a working example:

d = Flatten[Table[{x, y, x + 2 y + 3 x y + Random[]}, {x, 1.1, 1.9,

0.1}, {y,7.5, 10.5, 0.5}], 1];

Fit[d, {1, x, y, x y}, {x, y}]

hope this helps, Daniel

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?