[Proj] "Double ellipsoid" case?

[Frank Warmerdam]

support.mn at elisanet.fi wrote:
> 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

Janne,

PROJ.4 does not currently handle this situation conveniently.  One special
case is where the projection uses a particular ellipsoid, but datum shifts
should treat the corresponding lat/long values as being WGS84. In that
special case you can use +nadgrids=@null to effectively say the datum shift
to WGS84 is a no-op.

I have at times contemplated having a way of having a datum/ellipsoid
definition used for datum shifting purposes that is independent of the
normal ellipsoid used by the projection functions but I have not pursued it.

Clifford J Mugnier wrote:
 > 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.

Cliff,

The very common case we see a lot these days is the google maps mercator
projection.  The mercator calculations are done based on a particular
spherical earth model, but the resulting lat/long coordinates are intended
to be treated as WGS84 after that.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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