[Proj] "Double ellipsoid" case?

[support.mn at elisanet.fi]

Hello,

I just started to think about a situation where there might be a double
ellipsoid case.

1) projection uses ellipsoid A independently
2) datum shift uses ellipsoid B

Is this possible to be handled with proj.4? Since there is only one ellipsoid
definition available

"ellps=something"

The user might want to have:

"ellps_proj=ellps1"
"ellps_datum=ellps2"

This would mean (?) that the projection should be calculated using
ellps1 and the datum shift using the ellps2. And if the user writes:

"ellps=ellpsX"

that might be interpreted as both ellipsoids beeing the same:

"ellps_proj=ellps_datum=ellpsX"

So to do a full datum conversion there might be maximum four
ellipsoids involved. Two on both ends for projection and additional
two for datum shifts to wgs84 on both ends. All together four
ellipsoids. This might finally tame the "north jump" in all cases?

Regards: Janne.