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Newbie's question [message #45924]

Wed, 19 October 2005 15:06

ChiChiRuiz@gmail.com is currently offline

ChiChiRuiz@gmail.com
Messages: 32
Registered: October 2005

Member

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?

Besides, I've thought about using "polyfit", but if I remember
correctly, polyfit only takes in one x value vs. one y value. Scatter
plot has one x value vs. several y values. I don't think it'll
work in my case, but I may be wrong...

TIA (thanks in advance)

Angie

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[Message index]

		Newbie's question By: ChiChiRuiz@gmail.com on Wed, 19 October 2005 15:06
		Re: Newbie's question By: James Kuyper on Fri, 21 October 2005 15:06
		Re: Newbie's question By: ChiChiRuiz@gmail.com on Fri, 21 October 2005 11:22
		Re: Newbie's question By: James Kuyper on Fri, 21 October 2005 10:29
		Re: Newbie's question By: James Kuyper on Fri, 21 October 2005 10:19
		Re: Newbie's question By: JD Smith on Fri, 21 October 2005 09:35
		Re: newbie's question By: geonline714 on Sat, 14 April 2007 11:08
		Re: newbie's question By: David Fanning on Sat, 14 April 2007 10:04

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