# [Proj] "Double ellipsoid" case?

Clifford J Mugnier cjmce at lsu.edu
Sat Nov 29 09:15:23 EST 2008

```Frank:

Ah, there's a BIG difference between the true coordinate system relations of geodesy used by national governments and one cooked up by an ignoramus at Google Maps that did not know what they were doing ... I guess there's a lot of that going around, too.

I suppose even twits help contribute to keep knowledgeable consultants in business.

Cliff Mugnier
LSU

________________________________

From: proj-bounces at lists.maptools.org on behalf of Frank Warmerdam
Sent: Fri 28-Nov-08 10:52
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] "Double ellipsoid" case?

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,
--
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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