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MathGroup Archive 2005

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Re: Function Fit to 3D data set

  • To: mathgroup at smc.vnet.net
  • Subject: [mg57792] Re: [mg57748] Function Fit to 3D data set
  • From: "David Annetts" <davidannetts at aapt.net.au>
  • Date: Wed, 8 Jun 2005 03:21:42 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

 Hi Adrienne 

> I have been trying to do several analyses with an [x,y,z] 
> data set, where z is dependent on x and y.  First, I tried a 
> 3D plot.  However, the only way I was able to get Mathematica 
> to plot my data was to assume a grid form for x and y, and 
> use ListPlot3D.  First question: can Mathematica make a 3D 
> plot from stated x,y,z points?

Yes.

See documentation, particularly the functions in Graphics`Graphics3D.

> Secondly, I am trying to fit a function to the data.  I have 
> determined a function to which to fit parameters.  However, 
> Mathematica does not seem to want to accept y as a variable.  
> Here is a simple suggestion of what I was trying.  Note, my 
> function is much more complicated.  Please also ignore if the 
> given does not work as stated, as I'm making it up.
> 
> points = {{1, 1, 1}, {1, 2, 4}, {1, 4, 16}}
> f1 = FindFit[points, a + b*x^0 * y^1 + c*x^0 *y^2 +k*x^2 *y^0 
> + l*x^2 * y^1, {a, b, c, k, l}, x, y]
> 
> The error I receive is \!\(\*
>   RowBox[{\(FindFit::"nonopt
>   "\), \(\(:\)\(\ \)\), "\<\"Options expected (
>     instead of \\!\\(y\\)) beyond 
>       position \\!\\(4\\) in \\!\\(\[LeftSkeleton] 1 
> \[RightSkeleton]\\). An \ option must be a rule or a list of 
> rules. \\!\\(\\*ButtonBox[\\\"Moreb

The error (and documentation) tells you that the last argument to FindFit
(in this form) must be a list.  This list must have the same length as the
number of independent variables -- in your case, two.

As an example, load the package, then generate some data & plot them:-

	Needs["Graphics`Graphics3D`"]
	pts = Table[{x, y, .1 * (3 x + 2 y + x y)}, {x, -2, 2, .25}, {y, -2,
2, .25}];
	ListSurfacePlot3D[pts, Axes -> {Automatic, Automatic, Automatic}];

Perturb the z-variable slightly & show the new data set as a series of
points:-

	prt = Table[{x, y, .1 * (3 x + 2 y + x y) + Random[Real, {-.01,
.01}]}, {x, -2, 2, .25}, {y, -2, 2, .25}];
	dplt = Show[Graphics3D[Point[#] & /@ Partition[Flatten[prt], 3]],
Axes -> {Automatic, Automatic, Automatic}];

See if we can find a function that fits our perturbed data set:-
	eqn = FindFit[Partition[Flatten[prt], 3], a * (b x + c y + d x y),
{a, b, c, d}, {x, y}]
	DisplayTogether[{
	      Plot3D[a * (b x + c y + d x y) /. eqn, {x, -2, 2}, {y, -2,
2}],
      	dplt}];

Visual inspection shows we did pretty well, but if you like looking at
numbers:-
	a * (b x + c y + d x y) /. eqn // ExpandAll
	.1 * (3 x + 2 y + x y) // ExpandAll

Compares our fitted equation with the unperturbed equation.

Alternatively, use NonlinearRegress (from "Statistics`NonlinearFit`") to get
a better idea of the fit.  

Regards,

Dave.


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