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Re: Help Wanted: IDL Math Expert [message #9809 is a reply to message #9807]

Tue, 19 August 1997 00:00

John Votaw
Messages: 6
Registered: June 1997

Junior Member

David Fanning wrote:
>
> Hi Folks,
>
> I had a friend ask a question about IDL that I didn't know how
> to answer. (Uh, math is not my strength, you understand. :-))
>
> Suppose I have a set of raw data that is described
> theoretically by two parametric equations. And suppose
> I need to fit these two parametric equations to the data
> since there is no mathematical way to convert the x(t)
> and y(t) equations into a y(x) form. Is there a
> curve fitting routine in IDL that can handle parametric
> equations? Or, failing that, has anyone handled something
> like this in IDL and would be willing to give us a little
> help?
>> Many thanks,

The real question here is what do you mean by a good fit? You must
write an equation in terms of the parameters in your parametric
equations that is minimal when the desired fit is achieved.

For example, suppose your data is (x,y) pairs and you would like to fit
it to a generalized ellipse. Do you want to minimize the squared
distance along a vector from the center of the ellipse between the data
and the fit? Perhaps you want to minimize the distance along a line
normal to the fit curve.

The difficult part is writting this 'figure of merit' equation. Once
you have it, you can use any of the function minimization routines. I
suggest you start by looking at Powell.

Good Luck,

John R. Votaw
votaw@commander.eushc.org

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[Message index]

		Re: Help Wanted: IDL Math Expert By: David Ritscher on Wed, 20 August 1997 00:00
		Re: Help Wanted: IDL Math Expert By: John Votaw on Tue, 19 August 1997 00:00
		Re: Help Wanted: IDL Math Expert By: Jack Saba on Tue, 19 August 1997 00:00
		Re: Help Wanted: IDL Math Expert By: J.D. Smith on Tue, 19 August 1997 00:00
		Re: Help Wanted: IDL Math Expert By: Brad Gom on Tue, 19 August 1997 00:00
		Re: Help Wanted: IDL Math Expert By: Marty Ryba on Tue, 19 August 1997 00:00

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