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Surface fitting error while using Fit

  • To: mathgroup at smc.vnet.net
  • Subject: [mg96517] Surface fitting error while using Fit
  • From: Srikanth <skt at xdtech.com>
  • Date: Sun, 15 Feb 2009 03:22:07 -0500 (EST)

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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