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

OvV_HN ovv at hetnet.nl
Thu Sep 2 05:27:43 EST 2010

```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.

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

```