Thanks for the clarification. There are several other ways to approach that; but without knowledge of the application there is no way to choose.
On Mon, Nov 30, 2015, 8:49 PM Patrick Alken <[email protected]> wrote: > He's interpolating, not least-squares fitting. Therefore to interpolate N > points he needs a polynomial of degree N-1. A least squares approach might > be better since he could interpolate his 1000 points with a much lower > degree polynomial, say 6 or 7. > > > On 11/30/2015 05:22 PM, Raymond Rogers wrote: > > Are you sure you aren't fitting noise? This results in oscillation due > instantaneous fluctuations shoving high frequency into the model. > > On Mon, Nov 30, 2015, 6:58 PM Patrick Alken <[email protected]> wrote: > >> Hello, >> >> I confirmed that your program is failing for the polynomial >> interpolation, but polynomial interpolation is known to be difficult for >> large datasets. From the manual: >> >> ==== >> Interpolation Type: gsl_interp_polynomial >> >> Polynomial interpolation. This method should only be used for >> interpolating small numbers of points because polynomial interpolation >> introduces large oscillations, even for well-behaved datasets. The >> number of terms in the interpolating polynomial is equal to the number >> of points. >> ==== >> >> So basically you are trying to fit a degree 1000 polynomial to your >> dataset, which will not be numerically stable - even though your dataset >> is well behaved. >> >> Cubic splines are probably the way to go here, but if you insist on a >> degree 1000 polynomial (even though you shouldn't) I might be able to >> give you some further ideas. >> >> You mentioned that previous versions of GSL worked for you. Can you >> verify if you used exactly this same dataset successfully with a >> previous version of GSL? If so please tell me the version number. >> >> Thanks, >> Patrick >> >> On 11/30/2015 10:25 AM, Petrus, Adam (UK) wrote: >> > I am attempting to use the polynomial interpolation method with GSL. >> However I am getting very strange results. >> > >> > If I run the attached program I get the below result. If I change the >> method to linear or cspline the interpolation works fine. >> > >> > Not in previous gsl versions the polynomial interpolation has worked! >> > >> > [cid:[email protected]] >> > >> > Is there something I am missing? >> > >> > Yours aye >> > >> > Adam >> > >> > >> > >> > ******************************************************************** >> > This email and any attachments are confidential to the intended >> > recipient and may also be privileged. If you are not the intended >> > recipient please delete it from your system and notify the sender. >> > You should not copy it or use it for any purpose nor disclose or >> > distribute its contents to any other person. >> > ******************************************************************** >> >> >
