<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body>
<div dir="auto" style="direction: ltr; margin: 0; padding: 0; font-family: sans-serif; font-size: 11pt; color: black; ">
Hi Pierre,<br>
<br>
</div>
<div dir="auto" style="direction: ltr; margin: 0; padding: 0; font-family: sans-serif; font-size: 11pt; color: black; ">
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. <br>
<br>
</div>
<div dir="auto" style="direction: ltr; margin: 0; padding: 0; font-family: sans-serif; font-size: 11pt; color: black; ">
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.<br>
<br>
</div>
<div dir="auto" style="direction: ltr; margin: 0; padding: 0; font-family: sans-serif; font-size: 11pt; color: black; ">
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.<br>
<br>
</div>
<div dir="auto" style="direction: ltr; margin: 0; padding: 0; font-family: sans-serif; font-size: 11pt; color: black; ">
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.<br>
<br>
</div>
<div dir="auto" style="direction: ltr; margin: 0; padding: 0; font-family: sans-serif; font-size: 11pt; color: black; ">
Kind regards, Jochem<br>
<br>
</div>
<div dir="auto" style="direction: ltr; margin: 0; padding: 0; font-family: sans-serif; font-size: 11pt; color: black; ">
<span id="OutlookSignature">
<div dir="auto" style="direction: ltr; margin: 0; padding: 0; font-family: sans-serif; font-size: 11pt; color: black; ">
Get <a href="https://aka.ms/ghei36">Outlook for Android</a></div>
</span><br>
</div>
<hr style="display:inline-block;width:98%" tabindex="-1">
<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> PROJ <proj-bounces@lists.osgeo.org> on behalf of Pierre Abbat <phma@bezitopo.org><br>
<b>Sent:</b> Friday, February 7, 2020 8:30:10 PM<br>
<b>To:</b> proj@lists.osgeo.org <proj@lists.osgeo.org><br>
<b>Subject:</b> [PROJ] GRS80 ellipsoid off-center</font>
<div> </div>
</div>
<div class="BodyFragment"><font size="2"><span style="font-size:11pt;">
<div class="PlainText">I'm at the North Carolina surveyors' conference, and yesterday I attended the
<br>
talk about the new coordinate systems coming in 2022. The projection will <br>
still be conformal conic, with some changes. There will be one parallel and a <br>
scale factor instead of two parallels. (The representations are equivalent, <br>
and I've written code that converts two parallels into one parallel and a <br>
scale factor.) The false easting will be a round number in metric rather than <br>
feet converted to meters and rounded to the centimeter. And the ellipsoid will <br>
be centered at the center of mass of the earth, rather than a few meters off.<br>
<br>
I asked the speaker what name to use to distinguish the two ellipsoids. He <br>
didn't know, said they're both GRS80. I mentioned that the Greek ellipsoid <br>
(HGRS87, but I didn't remember the name at the moment) has the same shape, but <br>
is offset to fit Greece better, and has its own name. Is there a retronym to <br>
distinguish the previous GRS80, which turned out to be slightly offset, from <br>
GRS80 centered at the center of the earth?<br>
<br>
Pierre<br>
-- <br>
La sal en el mar es más que en la sangre.<br>
Le sel dans la mer est plus que dans le sang.<br>
<br>
<br>
<br>
_______________________________________________<br>
PROJ mailing list<br>
PROJ@lists.osgeo.org<br>
<a href="https://lists.osgeo.org/mailman/listinfo/proj">https://lists.osgeo.org/mailman/listinfo/proj</a><br>
</div>
</span></font></div>
<BR>
<BR>
<FONT SIZE=2>
Disclaimer:<BR>
De inhoud van dit bericht is uitsluitend bestemd voor geadresseerde.<BR>
Gebruik van de inhoud van dit bericht door anderen zonder toestemming van het Kadaster<BR>
is onrechtmatig. Mocht dit bericht ten onrechte bij u terecht komen, dan verzoeken wij u<BR>
dit direct te melden aan de verzender en het bericht te vernietigen.<BR>
Aan de inhoud van dit bericht kunnen geen rechten worden ontleend.<BR>
<BR>
Disclaimer:<BR>
The content of this message is meant to be received by the addressee only.<BR>
Use of the content of this message by anyone other than the addressee without the consent<BR>
of the Kadaster is unlawful. If you have received this message, but are not the addressee,<BR>
please contact the sender immediately and destroy the message.<BR>
No rights can be derived from the content of this message.<BR></body>
</html>