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Re: Piecewise curve fitting in idl [message #61788 is a reply to message #61785]

Thu, 31 July 2008 05:39

jameskuyper
Messages: 79
Registered: October 2007

Member

d.poreh@gmail.com wrote:
> On Jul 31, 1:21 pm, Wox <nom...@hotmail.com> wrote:
>> On Thu, 31 Jul 2008 03:30:22 -0700 (PDT), d.po...@gmail.com wrote:
>>> Folks
>>> How we can do the piecewise curve fitting in idl. Say we have an array
>>> that this array has got 2 or 3 trends in data and we want to fit a
>>> liner curve for each trends. In MATLAB curve fitting tool, we can
>>> easily exclude or include a part of data and then fit a curve. How we
>>> can do this in IDL
>>> Cheers
>>> Dave
>> Euhm, just do the fitting on the different parts? Or do you mean
>> fitting with a piecewise polynomial (i.e. spline: see e.g. IMSL_BSLSQ
>> or IMSL_CONLSQ)
>
> just doing the fitting on the difrent part. how we can select this
> parts and how we can fit a curve to these parts separatly?
> Cheers

Identifying the different parts is up to you. How do you know that there
are 2-3 different trends? Whatever method you use to reach that
conclusion will have to be adequate to identify where the different
trends start and end. However, once you have identified the different
parts you want fit separately, fitting each one separately is trivial:
pass x[trend_start[i]:trend_end[i]] and y[trend_start[i]:trend_end[i]]
to the curve-fitting routine.

If you want a curve fitting routine that automatically figures out where
each trend starts and ends, then it gets a LOT more complicated. You
could do that by using a non-linear curve fitting routine, and make the
transition point between the two trends be one of the parameters of your
fitting curve. However, I would strongly recommend trying to understand
why you see 2 or 3 different trends, and then try to come up with a
single mathematical model for the entire curve that reflects that
reason. Then fit that model to your data.

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[Message index]

		Re: Piecewise curve fitting in idl By: Wox on Fri, 01 August 2008 01:27
		Re: Piecewise curve fitting in idl By: jameskuyper on Thu, 31 July 2008 07:58
		Re: Piecewise curve fitting in idl By: pgrigis on Thu, 31 July 2008 07:41
		Re: Piecewise curve fitting in idl By: Paul Van Delst[1] on Thu, 31 July 2008 07:31
		Re: Piecewise curve fitting in idl By: David Fanning on Thu, 31 July 2008 07:03
		Re: Piecewise curve fitting in idl By: d.poreh on Thu, 31 July 2008 07:00
		Re: Piecewise curve fitting in idl By: jameskuyper on Thu, 31 July 2008 05:39
		Re: Piecewise curve fitting in idl By: d.poreh on Thu, 31 July 2008 04:54
		Re: Piecewise curve fitting in idl By: Wox on Thu, 31 July 2008 04:21
		Re: Piecewise curve fitting in idl By: Craig Markwardt on Sun, 03 August 2008 21:58

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