Re: [Orekit Users] geoid undulation

[Thomas Neidhart <thomas.neidhart@gmail.com>]

On 03/05/2015 03:58 PM, Thomas Neidhart wrote:
> 
> 
> On Thu, Mar 5, 2015 at 2:02 PM, MAISONOBE Luc <luc.maisonobe@c-s.fr
> <mailto:luc.maisonobe@c-s.fr>> wrote:
> 
>     Hi Odi,
> 
>     First, sorry for this late answer.
> 
>     Odenstein Manfred <odix@odiisi.net <mailto: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.

I am sorry, but I have to correct myself.
There was a misunderstanding while porting the respective code portion
from the original WMM code.

The provided altitude is in fact expected to be in km above WGS 84
ellipsoid. I did verify this with the latest models and the online
calculator available here:

http://www.ngdc.noaa.gov/geomag-web/#igrfwmm

Then the results are identical.

I will update the javadoc and the unit tests in the next few days.

Sorry for the wrong statement.

Thomas

>     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.

I could now remember where these came from, it's from the geomag
software, available from here:

http://www.ngdc.noaa.gov/IAGA/vmod/igrf.html

Thomas