[PROJ] OGC blog post summarising Web-mapping misalignment problem

[Duncan Agnew]

I can only speak to ITRF, but the changes in this come mostly from there
being longer time series and more stations. Motion
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
loading by water changes, or by postglacial rebound. That said, if your
country is all on one plate, and your national authority
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
relevance of a value that doesn't include an epoch.

Duncan Agnew

On Thu, Aug 22, 2019 at 5:23 AM Martin Desruisseaux <
martin.desruisseaux at geomatys.com> wrote:

> Le 22/08/2019 à 10:58, Joaquim Luis a écrit :
>
> > Hello, with the risk of going a bit off topic I have another question
> > about the different WGS84, which is: why are they different? They all
> > use the same ellipsoid, right? So the difference is in the origin of
> > the referencing system? And if yes, why does it change in the several
> > WGS84 realizations?
> >
> The origin move slightly between realizations. But in addition of that
> translation, the axes may also have a slight rotation and a slight scale
> factor (I did not verified if it was the case for WGS84 realizations).
> Those changes exist both because of improvement in the accuracy of the
> measurements used for defining the WGS 84 reference frame, but also
> because datum defined by satellites have their origin at the center of
> mass of Earth and that center of mass moves continuously (because of
> plates tectonic, convection movements in Earth mantle, etc.).
>
> I think that users looking for stability should use the reference frame
> defined by their country instead than any satellite-based datum. For
> example for locating points in Australia, use the CRS defined by
> Australian mapping agency. National CRS are defined respective to the
> continental plate where the country is located. Even if they define a
> relationship between national CRS and satellites-based CRS, they take in
> account that their continental plate does not move in the same way than
> the Earth center of mass and they provide more accurate transformations
> between old and new CRS than if we were using the satellite-based CRS. I
> would suggest to reserve the use of satellite-based CRS like WGS84 to
> the cases where data are already in that CRS anyway (e.g. GPS data), or
> span a geographic area too wide for being expressed with a national CRS.
>
>     Martin
>
>
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