Re: [Orekit Developers] Upgraded Interpolators FYI

[MAISONOBE Luc <luc.maisonobe@c-s.fr>]

Hi Hank,

Hank Grabowski <hank@applieddefense.com> a écrit :

> All,
>
> With the new Apache Math library came some upgrades to the 2D
> interpolators.  The original Bi-Cubic interpolator was returning numbers
> that grossly inacurate.  There is now a Piecewise and a modified Bi-Cubic
> interpolator that is based on the same principals as the original Bicubic
> interpolator but that is returning correct data.  I've changed the two
> instances where we used it, Troposopheric correction and Saastamoinen model
> to use the new interpolators.  The unit tests are relatively loose so I've
> been manually inspecting values by performing the same interpolation using
> a variety of interpolation methods in Octave.
>
> Good news is that the Tropospheric correction is working great.  The values
> are matching as you'd expect.  The Saastamoinen model is another matter.
> The zenith angle spacing for the grid is highly irregular.  This is
> throwing the interpolators for a loop, both in Apache Math and in Octave.
> The worst of all are the splines, so Piecewise Spline is out.  They are
> producing values 2-3x the actual table lookup values.  The Bicubic Apache
> Math and linear Octave are producing reasonable results.  Since there is no
> 2D linear interpolator in Apache Math and the Bicubic is more well behaved
> (it is only sampling four nearest neighbor points and derivatives thus not
> getting as tripped up by the irrelugar grid spacing), I have it using the
> Bicubic rather than Piecewise spline interpolator.  Ideally once Apache
> Math has a 2D linear interpolator we should probably use that.  I already
> double checked if there is a version of the table with regular gridding and
> I can't find any.  I am reluctant to create a new table that does have the
> regular spacing or attempt some other transformation to get around the
> interpolation problem.

Thanks for the analysis.
Another option we can use is to create a 2D linear interpolator as a private
class only for the sake of Saastamoinen model and contribute it to  
Apache Commons
Math, where it would become a public class belonging to the API. Once  
a new version
of Apache Commons Math including this class will be published, we will  
remove our
private class and use the official one from the math library. We  
already did this
for Hermite interpolation if I remember correctly.

best regards,
Luc

> If the above sounds good I can check this code in.