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Re: Speaking of curve fitting... [message #58466 is a reply to message #58465]

Thu, 31 January 2008 08:42

Vince Hradil
Messages: 574
Registered: December 1999

Senior Member

On Jan 31, 10:17 am, Paul van Delst <Paul.vanDe...@noaa.gov> wrote:
> Lasse Clausen wrote:
>> ... run the following code, spot the difference and explain, s'il vous
>> plait.
>
>> nn = 1000
>> xx1 = dindgen(nn)
>> xx2 = timegen(nn, start=julday(5,25,1980,11,23))
>
>> yy1 = sin(2.*2.*!pi*xx1/(nn-1.))
>
>> d = poly_fit(xx1, yy1, 6, yfit=yfit1, /double)
>> d = poly_fit(xx2, yy1, 6, yfit=yfit2, /double)
>
> Try
> d = poly_fit(xx2-xx2[0], yy1, 6, yfit=yfit2, /double)
>
>> !p.multi = [0,1,2]
>> plot, xx1, yy1, /xstyle
>> oplot, xx1, yfit1, linestyle=1
>> plot, xx2, yy1,/xstyle
>> oplot, xx2, yfit2, linestyle=1
>
>> end
>
>> I had a quick look at POLY_FIT.PRO but I can spot nothing which could
>> explain the above behaviour. I run 32bit IDL 6.4 on some Linux.
>
>> Cheers
>> Lasse Clausen

Sure that works, but the underlying issue is still there - why should
it matter?
My guess: propagation of (roundoff) errors when poly_fit.pro
calculates the b-matrix.

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[Message index]

		Re: Speaking of curve fitting... By: David Fanning on Thu, 31 January 2008 09:13
		Re: Speaking of curve fitting... By: Paul Van Delst[1] on Thu, 31 January 2008 09:04
		Re: Speaking of curve fitting... By: Vince Hradil on Thu, 31 January 2008 08:42
		Re: Speaking of curve fitting... By: pgrigis on Thu, 31 January 2008 08:34
		Re: Speaking of curve fitting... By: lasse on Thu, 31 January 2008 08:33
		Re: Speaking of curve fitting... By: Paul Van Delst[1] on Thu, 31 January 2008 08:17

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