| Re: POLY_FIT 5.0: inaccurate results. use 4.0 if possible. [message #11297] |
Fri, 27 March 1998 00:00 |
davidf
Messages: 2866
Registered: September 1996
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Senior Member |
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Russ Welti (on mapper) (rwelti@mapper.mbt.washington.edu) writes:
> The 5.0 POLY_FIT has accuracy problems.
>
> My app uses POLY_FIT extensively to solve a problem
> which can be expressed as "choosing the very best of
> many possible curve fittings".
>
> I noticed the app was producing some wrong results, after
> upgrading to IDL 5.0.
I submitted this problem to the RSI technical support staff
to see what they knew about it. Here is their response.
This is one of those "nice things to do" that cause confusion
because the change is not communicated carefully enough. I'm
not sure how to solve this "upgrade" problem, but I do know
it causes grief from time to time.
************
To obtain similar results for the POLY_FIT procedure from IDL 4.0.1 to
IDL 5.0, please set (/DOUBLE) the DOUBLE keyword when using POLY_FIT in
IDL 5.0.� For most of IDL 4.0.1 numerical analysis routines, the
internal computations are performed on double precision variables.
However, because of timing feature requests, these routines were changed
to perform single precision computations.� Fortunately, the DOUBLE
keyword was added to these routines to force double precision
computations.� Thus, to force IDL 5.0's POLY_FIT to perform like IDL
4.0.1's POLY_FIT, please use this DOUBLE keyword.� Sorry for any
inconvenience this change may have caused.
*************
Cheers,
David
-----------------------------------------------------------
David Fanning, Ph.D.
Fanning Software Consulting
E-Mail: davidf@dfanning.com
Phone: 970-221-0438
Coyote's Guide to IDL Programming: http://www.dfanning.com/
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