[Proj] "Double Ellipsoid" error, reproduction

[support.mn at elisanet.fi]

Hello,

Let's try to reproduce the double ellipsoid case somehow. The candidate for that might
be such a situation where several projections use the same datum, but each projection
is using some different ellipsoid. Still the same datum parameters would be used. There
have to rise a situation where the conversion uses wrong ellipsoid either for the datum
conversion or the projection, if they have to always share the ellipsoid?!!

And that was the reason to introduce different names for different ellipsoids. One for the
projection and one for the datum conversion.

For example someone wants to inverse project some old map, which was calculated with
different ellipsoid than what is used in the normal "modern" datum conversion. Actually
exactly the Dutch case, which somebody was talking about. They got the usual 650 m
"north jump" due to the difference in ellipsoids... Either using wrongly same ellipsoids
for projection and datum or vice versa. (?)

Regards: Janne.