[Proj] How to convert a sphere to ellipsoid with correct datum?

[Jan Hartmann]

On 09/02/10 12:27, OvV_HN wrote:
> ........
>
>
> The only way I can get from the old latlon value to the new is using
> intermediate projections: from old latlon to old projected coordinates,
> from old projected coordinates to new projected coordinates using the
> old ellipsoid, and from new projected coordinates back to latlon on the
> new ellipsoid. The results look OK (within 10 meters), but I don't
> understand what I am doing here. I certainly dislike the faking of an
> old ellipsoid on a new projection.
>
>
> REPLY:
>
> Please elaborate on that! How do you calculate the new grid coordinates on
> the new ellipsoid starting just from grid coordinates on the old ellipsoid?
>
> In my opinion it would be simpler or even better to do a conversion from
> lat-lon to cartesian XYZ starting with the old ellipsoid and to convert back
> from cartesian to lat-lon with the parameters for the new ellipsoid. Regular
> datum transformation routines do that, but a Helmert transform is done in
> between (in cartesian space). So inactivate the Helmert transformation by
> giving dx=dy=dz=0, rx=ry=rz=0, ds=1.0.
>
Yes, that's logical but it just doesn't work. Doing a transformation 
with Helmert transformation parameters set to 0 and 1 just gives results 
that are hundreds of meters off.

Jan