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 >