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

[OvV_HN]

Van: proj-bounces at lists.maptools.org namens Jan Hartmann
Verzonden: wo 1-9-2010 20:06
Aan: PROJ.4 and general Projections Discussions
CC: Hermann Peifer
Onderwerp: Re: [Proj] How to convert a sphere to ellipsoid with correct 
datum?


........


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.

This should also work for converting spherical coordinates to ellipsoidal or 
the other way around, but then the geodetic/cartesian conversion routines 
must have a support for the sphere and I don't believe the PROJ routines 
have.

Oscar van Vlijmen