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Re: Newbie's question [message #45916 is a reply to message #45915]

Thu, 20 October 2005 06:44

Paul Van Delst[1]
Messages: 1157
Registered: April 2002

Senior Member

ChiChiRuiz@gmail.com wrote:
> Hi there,
>
> I have a scatter plot which has the shape of a parabola, like y=x^2.
> I want to find the best curve fit to the scatter plot, so I used the
> function "curvefit" with no weights and with initial guesses (1.0, 2.0)
> i.e. y = 1.*x^(2.). So, here's the problem...when I use only the right
> half of the data points (i.e. x and y values are positive), I get the
> curvefit returns parameter (0.5, 1.5), which means, the best fit curse
> is y=.5*x^(1.75). I know the fit should be symmetric, so the same curve
> SHOULD fit the other half. Now unto the left half side of the data
> set, curvefit does not work anymore, and here's why, x^(1.5)=x^(3/2)
> and when x is a negative number, IDL returns "NaN" because it can't
> take the square root of a negative number, hence the entire procedure
> will not work. I ended up having to throw away half of my data points,
> and I'm not very comfortable with that. Any idea how to go around it
> or suggest another function to do the same thing?

Try Craig Markwardt's MPFIT suite (google will find it). It is a much more robust curve
fitter than IDL's CURVEFIT.

cheers,

paulv

--
Paul van Delst
CIMSS @ NOAA/NCEP/EMC

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[Message index]

		Re: Newbie's question By: JD Smith on Thu, 20 October 2005 17:07
		Re: Newbie's question By: James Kuyper on Thu, 20 October 2005 15:08
		Re: Newbie's question By: ChiChiRuiz@gmail.com on Thu, 20 October 2005 13:18
		Re: Newbie's question By: James Kuyper on Thu, 20 October 2005 11:41
		Re: Newbie's question By: Paolo Grigis on Thu, 20 October 2005 08:10
		Re: Newbie's question By: Paul Van Delst[1] on Thu, 20 October 2005 07:10
		Re: Newbie's question By: ChiChiRuiz@gmail.com on Thu, 20 October 2005 06:53
		Re: Newbie's question By: Paul Van Delst[1] on Thu, 20 October 2005 06:44
		Re: Newbie's question By: peter.albert@gmx.de on Thu, 20 October 2005 04:02
		Re: Newbie's question By: Paolo Grigis on Thu, 20 October 2005 00:33
		Re: Newbie's question By: ChiChiRuiz@gmail.com on Fri, 21 October 2005 09:58
		Re: Newbie's question By: James Kuyper on Fri, 21 October 2005 05:55

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