<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto">I learned recently that Earth center of mass changes seasonally due to water accumulation in plants of northern hemisphere at spring. This change is detectable by altimeter satellites. <br><br><div id="AppleMailSignature" dir="ltr"><div>Sent from my iDedo</div></div><div dir="ltr"><br>No dia 22/08/2019, às 20:16, Duncan Agnew <<a href="mailto:dagnew@ucsd.edu">dagnew@ucsd.edu</a>> escreveu:<br><br></div><blockquote type="cite"><div dir="ltr"><div dir="ltr"><div>I can only speak to ITRF, but the changes in this come mostly from there being longer time series and more stations. Motion</div><div>of the Earth's center of mass is at the sub-cm level--and at this level there are actual motions of the ground that enter in, from</div><div>loading by water changes, or by postglacial rebound. That said, if your country is all on one plate, and your national authority</div><div>has defined a CRS moving with that plate (a plate-fixed system) then that is probably the one to use, since it maximizes the future<br></div><div>relevance of a value that doesn't include an epoch.</div><div><br></div><div>Duncan Agnew<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Aug 22, 2019 at 5:23 AM Martin Desruisseaux <<a href="mailto:martin.desruisseaux@geomatys.com">martin.desruisseaux@geomatys.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Le 22/08/2019 à 10:58, Joaquim Luis a écrit :<br>
<br>
> Hello, with the risk of going a bit off topic I have another question<br>
> about the different WGS84, which is: why are they different? They all<br>
> use the same ellipsoid, right? So the difference is in the origin of<br>
> the referencing system? And if yes, why does it change in the several<br>
> WGS84 realizations?<br>
><br>
The origin move slightly between realizations. But in addition of that<br>
translation, the axes may also have a slight rotation and a slight scale<br>
factor (I did not verified if it was the case for WGS84 realizations).<br>
Those changes exist both because of improvement in the accuracy of the<br>
measurements used for defining the WGS 84 reference frame, but also<br>
because datum defined by satellites have their origin at the center of<br>
mass of Earth and that center of mass moves continuously (because of<br>
plates tectonic, convection movements in Earth mantle, etc.).<br>
<br>
I think that users looking for stability should use the reference frame<br>
defined by their country instead than any satellite-based datum. For<br>
example for locating points in Australia, use the CRS defined by<br>
Australian mapping agency. National CRS are defined respective to the<br>
continental plate where the country is located. Even if they define a<br>
relationship between national CRS and satellites-based CRS, they take in<br>
account that their continental plate does not move in the same way than<br>
the Earth center of mass and they provide more accurate transformations<br>
between old and new CRS than if we were using the satellite-based CRS. I<br>
would suggest to reserve the use of satellite-based CRS like WGS84 to<br>
the cases where data are already in that CRS anyway (e.g. GPS data), or<br>
span a geographic area too wide for being expressed with a national CRS.<br>
<br>
Martin<br>
<br>
<br>
_______________________________________________<br>
PROJ mailing list<br>
<a href="mailto:PROJ@lists.osgeo.org" target="_blank">PROJ@lists.osgeo.org</a><br>
<a href="https://lists.osgeo.org/mailman/listinfo/proj" rel="noreferrer" target="_blank">https://lists.osgeo.org/mailman/listinfo/proj</a><br>
</blockquote></div>
</div></blockquote><blockquote type="cite"><div dir="ltr"><span>_______________________________________________</span><br><span>PROJ mailing list</span><br><span><a href="mailto:PROJ@lists.osgeo.org">PROJ@lists.osgeo.org</a></span><br><span><a href="https://lists.osgeo.org/mailman/listinfo/proj">https://lists.osgeo.org/mailman/listinfo/proj</a></span><br></div></blockquote></body></html>