Re: [Orekit Users] geoid undulation

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

Hi Odi,

First, sorry for this late answer.

Odenstein Manfred <odix@odiisi.net> a écrit :

> Hi Luc,
>
> thank you for the information, I’ve another question regarding geoid  
> undulation, please correct me if I’m wrong:
>
> In the orekit WMM implementation (GeoMagneticField), it is stated  
> that the given hight in calculateField is "the altitude in  
> kilometers above mean sea level“, but the WMM.COF file with the  
> mentioned NGA WMM_SubLibrary.c only supports hight above ellipsoid  
> (wgs84), is there documentation error ? The NOAA software accepts  
> both ellipsoid hight and MSL by doing an undulation calculation with  
> EGM96 15min grid interpolation, but I miss this in the orekit  
> implementation.

I am not sure (I am not the one who did the implementation), but I think
the documentation is correct and needs a height with respect to geoid.  
However,
as you already noticed, we do not provide this computation yet in Orekit, so
it was up to the caller to provide the height.

We clearly miss this and we should have this transform available. I even
think it would be good to use interpolation from a published geoid model
and direct computation from a gravity field which may be different from EGM
(assuming we can get the correction factors to apply to the pure gravity-only
field). Could you open a feature request in our issue tracker on the  
forge wo we
don't forget this? Also if you can provide an implementation it would  
be great!

Concerning the min/max altitude in the file, they are not used by  
Orekit for now
(see comment in readModel method from GeoMagneticModelLoader).

best regards,
Luc

> In the orekit WMM.COF file format there exists both min. and max.  
> altitude, from where are these values from  ?
>
> thank you and best regards
> odi
>
> > Am 23.02.2015 um 09:20 schrieb Luc Maisonobe <Luc.Maisonobe@c-s.fr>:
> >
> > Hi Odi,
> >
> > Le 22/02/2015 16:30, Odenstein Manfred a écrit :
> > > Hi, sorry, I'm not the math expert, but I'm using orekit for the
> > > magnetic field calculations, as I now need also some gravity
> > > calculations, especially undulation, I've found the implementation of
> > > a model reader for EGM96, EGM2008, etc but found no method which
> > > takes lat/lon and date to retrieve the information. I'm just oversee
> > > something or is nothing there ?
> >
> > You did not oversee anything. There are no undulation computation yet in
> > Orekit. We will probably add one in the future, as it is important for
> > example for some Digital Elevation Models that are referenced to geoid
> > and not to ellipsoid.
> >
> > The model readers you found are dedicated to look at the gravity field
> > from a dynamical point of view, not from the undulation perspective.
> > This means they are used to compute at any point in space the gravity
> > vector. Undulation are more seen as an iso-surface on the combination of
> > this gravity field plus some other parts like inertial forces due
> > to Earth rotation. So they share some data, but we did not implement yet
> > the second perspective.
> >
> > Of course, contributions are welcome so if you decided to come up with
> > the missing parts, we will look at how to include it in Orekit.
> >
> > > any help would be appreciated thank you and best regards odi
> >
> > best regards,
> > Luc