Re: [Orekit Users] geoid undulation

On Thu, Mar 5, 2015 at 2:02 PM, MAISONOBE Luc <luc.maisonobe@c-s.fr> wrote:

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.

I can confirm that the provided altitude has to be as height above geoid.

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!

To compute the geoid height according to EGM96 does not look too difficult, so we could quite easily add this if there is a need for it.

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 ?

the format to store the geomagnetic field coefficients is a derived from the original WMM.COF file and contains more data as it is intended to support also other models, e.g. IGRF.

I can not recall where the min/max altitudes for WMM came from, but for now they are ignored anyway.

Thomas