# [Proj] "Double ellipsoid" case?

Clifford J Mugnier cjmce at lsu.edu
Fri Nov 28 10:43:54 EST 2008

```I've seen a few projections and datums in my day, and I've never come accross that.  For there to be a different ellipsoid of reference for a projection than there is for a datum is a contradiction in terms.  I'll believe that when I see it.  The closest to that I have ever seen is the coordinate systems of Hong Kong.  From one datum adjustment to another, they went to an older ellipsoid (Clarke 1880 to Clarke 1858) with a corresponding change in the defining parameters for their projection.  I think it was Cassini-Soldner, it's been so long since I published on that place.

Cliff Mugnier
LOUISIANA STATE UNIVERSITY

________________________________

From: proj-bounces at lists.maptools.org on behalf of support.mn at elisanet.fi
Sent: Fri 28-Nov-08 06:00
To: proj at lists.maptools.org
Subject: [Proj] "Double ellipsoid" case?

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.

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