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Subject: Re: Plotting 3-D Surface & Surface Fitting
Date: Mon, 4 Aug 2008 03:40:04 +0000 (UTC)
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Got my answers here:
http://www.mathworks.de/matlabcentral/newsreader/view_thread/163107

"Sudha` N" <sudhanatarajan@gmail.com> wrote in message
<g75pui$90r$1@fred.mathworks.com>...
> 
> 
> > If your true goal was that equation,
> > then you should fit it to your original data.
> 
> John - Thanks. Yes my true goal is the equation, but I
> needed a nice graphical representation of my data too. Using
> gridfit was very quick and easy to do it. 
> 
> What is the best and fastest way to model an equation 
> for my data using matlab? 
> 
> Please provide me some pointers and I can read up from
> there.
> 
> Thanks.
> 
> 
> 
> 
> "John D'Errico" <woodchips@rochester.rr.com> wrote in
> message <g75o2h$oco$1@fred.mathworks.com>...
> > "Sudha` N" <sudhanatarajan@gmail.com> wrote in message 
> > <g75n6e$qf0$1@fred.mathworks.com>...
> > > 
> > > Hi - How do you generate the equation for z=f(x,y) from a
> > > surface fitting?
> > > I am using gridfit and have surface fitted my data
> > > successfully. But the key purpose of my experiment is to
> > > generate the equation for z as a function of x and y.
> > 
> > Sorry, but the surface that gridfit generates
> > has no simple model. There is no "equation",
> > besides a local, piecewise linear interpolant.
> > In effect, gridfit produces a variant of a low
> > order spline, in two dimensions.
> > 
> > Can you generate an equation for the surface?
> > Yes, you can choose to fit a model to that,
> > but that will require that you choose a model
> > form, as well as a fitting method.
> > 
> > If your goal was really to generate a model
> > expression for this surface, then gridfit was
> > the wrong choice for your fit. You are now
> > fitting a model to the approximation that
> > gridfit chose. So you essentially have two sets
> > of residuals, and two sources of lack of fit to
> > deal with. If your true goal was that equation,
> > then you should fit it to your original data.
> > 
> > KNOW YOUR GOALS!!! Know what it is you
> > need to gain from any such effort, before
> > you begin the process of modeling. Otherwise
> > you are just wasting time.
> > 
> > John
>