[PROJ] GRS80 ellipsoid off-center

[Lesparre, Jochem]

Hi Pierre,

It is normal that ellipsoids with the same shape but different centre have the same name. An ellipsoid is defined by the semi-major axis (a) and the flattening (f). The location of the centre, orientation and scale are part of the geodetic datum, not of the ellipsoid.

To make it even more complicated, slight changes in the datum transformation parameters due no new measurements are often considered a new realisation. But not always a new name is used for the datum.

Often, the distinction between reference system, ellipsoid and datum is not clearly made and they get the same name or people consider (wrongly according to me) that the ellipsoid is part of the datum. Differences between reference frames (=realisation) and the reference system are seldomly made. ITRS being the most important exception.

So the solution is not to use a different names for the same ellipsoid, but to introduce clear unique names for the different datums and realisations.

Kind regards, Jochem

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________________________________
From: PROJ <proj-bounces at lists.osgeo.org> on behalf of Pierre Abbat <phma at bezitopo.org>
Sent: Friday, February 7, 2020 8:30:10 PM
To: proj at lists.osgeo.org <proj at lists.osgeo.org>
Subject: [PROJ] GRS80 ellipsoid off-center

I'm at the North Carolina surveyors' conference, and yesterday I attended the
talk about the new coordinate systems coming in 2022. The projection will
still be conformal conic, with some changes. There will be one parallel and a
scale factor instead of two parallels. (The representations are equivalent,
and I've written code that converts two parallels into one parallel and a
scale factor.) The false easting will be a round number in metric rather than
feet converted to meters and rounded to the centimeter. And the ellipsoid will
be centered at the center of mass of the earth, rather than a few meters off.

I asked the speaker what name to use to distinguish the two ellipsoids. He
didn't know, said they're both GRS80. I mentioned that the Greek ellipsoid
(HGRS87, but I didn't remember the name at the moment) has the same shape, but
is offset to fit Greece better, and has its own name. Is there a retronym to
distinguish the previous GRS80, which turned out to be slightly offset, from
GRS80 centered at the center of the earth?

Pierre
--
La sal en el mar es más que en la sangre.
Le sel dans la mer est plus que dans le sang.



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