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

[Joaquim Luis]

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. 

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No dia 22/08/2019, às 20:16, Duncan Agnew <dagnew at ucsd.edu> escreveu:

> 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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