Re: [Gnuplot-info] 3-d-fitting

[Thomas S. <t.s...@fz...>]

Spinora wrote:
> 
> I have data with 3 values: x,y and z and I would like to fit f(x,y) to it.
> I read in the help that the usual fit-function could do the job but when i
> actually try it the fitted function in nowhere near my data.
> 
> I've tried differnt functions and they all seemed to fail in different
> ways: one kept quitting after 2 iterations, giving back the starting
> values, one did about 15 Iterations and gave back something far from the
> starting values but even further from the data, one kept giving back a
> nearly flat plane (which is pretty wrong) but at least it seemed to be at
> the average of the data.
> From this variety of errors I was unable to find out what went wrong, hope
> someone can help me here.
> 

the first question you have to answer is:
which function would describe your data best?

then use this function and set the parameters by hand, so that the function
is already in the region where the datapoints are, and then start the fit.

if you start a fit with parameters set to values which cause the function to
be far off the datapoints, then this fit will not work. due to the
inadequate stepsize chosen automatically it may find some minimum but not
the real one (for comparison: with 1m steps you may explore the bumpiness of
a floor but will not find a hole which has a diameter of 1mm and is 10.5m
away).

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