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

[Nick Mein]

Hi Joaquim,

A non-geodesist, layman's answer from me: New realizations include new
data, and maintain/improve coincidence with the ITRF. See, eg,
http://www.unoosa.org/documents/pdf/icg/2018/icg13/wgd/wgd_12.pdf,
ftp://ftp.nga.mil/pub2/gps/sat_out/SteveM/NGA_ICG11_2Nov.pdf

Regards,
Nick.

On Thu, 22 Aug 2019 at 20:58, Joaquim Luis <jmfluis at gmail.com> wrote:

> 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?
>
> Thanks
>
> J Luis
>
> Sent from my iDedo
>
> No dia 22/08/2019, às 14:01, Nick Mein <nick_mein at trimble.com> escreveu:
>
> Hi Roger,
>
> The answer is ... it depends.
>
> If you are using autonomous positions then the positions will be in the
> reference frame of the satellite orbits. Currently WGS84(G1762) for GPS.
> But the precision that you are going to get is a couple of meters at best,
> so the difference between different realizations of WGS84 is fairly much
> irrelevant. The epoch may be relevant , if the data is still going to be of
> interest in a couple of decades time.
>
> If you are using a correction service, then you will need to check with
> your service provider to find out the reference frame of the positions that
> your receiver is giving you. It could be ITRFxxxx, it could be a local
> reference frame such as GDA2020 or NAD83. (For what it is worth - there are
> proposals to add reference frame information to NTRIP and/or RTCM, but
> currently there is no way to tell without checking with your provider.)
>
> Regards,
> Nick.
>
> On Thu, 22 Aug 2019 at 18:30, Roger Oberholtzer <
> roger.oberholtzer at gmail.com> wrote:
>
>> On Thu, Aug 22, 2019 at 12:51 AM Cameron Shorter
>> <cameron.shorter at gmail.com> wrote:
>> >
>> > Proj folks,
>> > A bit of an update. A few of us have been refining our thinking and
>> description of the web-mapping misalignment problem. Scott from the OGC has
>> noted the importance of this topic and invited us to raise the topics at
>> the next OGC Technical Committee meeting at Banff (9 Sept). (Getting
>> approval to travel to the event is becoming trickier than we'd expected.)
>> >
>> > The OGC has published a blog post from us summarising the issues:
>> > https://www.opengeospatial.org/blog/3045
>>
>> I have a question about the WGS84 datum ensemble in regards to GNSS
>> receivers. When providing WGS84 locations, how would I know which
>> WGS84 variant is being supplied? Is the receiver involved in this? Or
>> is it the satellites that control this? Is it enough to know the date
>> of the measurement? Or must one track more to know which WGS84 variant
>> is provided?
>>
>>
>>
>>
>>
>> --
>> Roger Oberholtzer
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