Re: Plotting 3-D Surface & Surface Fitting



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@xxxxxxxxxxxxxxxx> wrote in
message <g75o2h$oco$1@xxxxxxxxxxxxxxxxxx>...
"Sudha` N" <sudhanatarajan@xxxxxxxxx> wrote in message
<g75n6e$qf0$1@xxxxxxxxxxxxxxxxxx>...

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

.

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