[Openstreetmap] Re: Landsat update

[Nick Whitelegg]

On Saturday 30 Apr 2005 16:00, Gabriel Ebner wrote:
> On Sat, Apr 30, 2005 at 01:03:00PM +0100, Nick Whitelegg wrote:
> > I'm wondering if it's to do with the inaccuracy of WGS-84 at the UK's
> > latitude and longitude?
>
> WGS84 is just a reference system, the only thing that can be accurate about
> it is how well it fits the geoid.
>
> > However, if I correct the code so that it converts *WGS84* track and
> > waypoints to screen display, the error becomes worse!
>
> Did you do a datum transformation?  From what you describe, it looks as if
> the coordinates are in the wrong datum and you just adjusted the ellipsoid
> parameters.
>

I used some code from the "jeeps" library for the transformations. Have to 
admit my knowledge in this area is rather patchy but if it helps here is the 
documentation for the function which does the transformation - it uses 
something called Molodensky.

/* @func GPS_Math_Known_Datum_To_Known_Datum_M *************************
**
** Transform WGS84 to other datum using Molodensky
**
** @param [r] Sphi [double] source latitude (deg)
** @param [r] Slam [double] source longitude (deg)
** @param [r] SH   [double] source height  (metres)
** @param [w] Dphi [double *] dest latitude (deg)
** @param [w] Dlam [double *] dest longitude (deg)
** @param [w] DH   [double *] dest height  (metres)
** @param [r] n1   [int32] source datum number from GPS_Datum structure
** @param [r] n2   [int32] dest   datum number from GPS_Datum structure
**
** @return [void]
************************************************************************/

Nick