What epoch is GCS_WGS_1984 in 10.4?

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03-30-2016 02:47 PM
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JoeBryant2
Occasional Contributor II

What epoch does ArcMap 10.4 reference when you choose GCS_WGS_1984 as the datum?
Is it the current epoch of 2005.0 equivalent with ITRF08, or the previous epoch
of 1997.0 equivalent with ITRF00?

I am finding that much of the reference material online is outdated
as it lists the current realization of WGS84 to be equivalent to
ITRF00, Epoch 1997.0. This article by NGS (http://www.ngs.noaa.gov/CORS/Articles/SolerWGS84(G1674)-to-NAD83(2011).pdf)
explains that GPS satellites actually report their X,Y & Z location values
in IGS08, which is also equivalent to WGS84(G1674)/ITRF08. However, the epoch
of the recorded points is the time they were transmitted from the satellites;
they need to be reduced to a common epoch before they can be compared with
other coordinates (because of tectonic movement). The article also mentions
your “software” may already do this for you. I am collecting points with
Trimble Geo6000 receivers; I don’t know if the TerraSync software does this
during collections or if the Pathfinder Office software does this during post
processing. And if it does, I don’t know what the epoch it uses for WGS1984 is.
I suspect it is epoch 1997.0 as the base station reference positions are listed
in ITRF00.

Once I figure out the correct conversion process from GPS points in Trimble’s
TerraSync to our NAD1983(epoch 1986) GIS, I will need to revisit this for using
the Collector App or possibly the Trimble Positions plug-in: Where is the epoch
conversion made and what is that epoch?

Thanks.

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MelitaKennedy
Esri Notable Contributor

There's no set epoch/realization on the WGS 1984 definition. So far we've been following the EPSG / IOGP registry and have not defined the separate WGS84 realizations. We've been working on supporting time-based transformations and as part of that project, will likely add the various realizations.

As far as I know, if you're RTK or post-processing against base stations, your data's going to end up in the same 'realization' as the base stations. For details, you'll have to contact Trimble. In Esri software, we are not doing any velocity/tectonic plate/subsidence or other time-based movements of data yet.

Melita

Disclosure: I'm on the subcommittee that maintains the EPSG registry.

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ChrisDonohue__GISP
MVP Frequent Contributor

Melita Kennedy

Chris Donohue, GISP

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MelitaKennedy
Esri Notable Contributor

There's no set epoch/realization on the WGS 1984 definition. So far we've been following the EPSG / IOGP registry and have not defined the separate WGS84 realizations. We've been working on supporting time-based transformations and as part of that project, will likely add the various realizations.

As far as I know, if you're RTK or post-processing against base stations, your data's going to end up in the same 'realization' as the base stations. For details, you'll have to contact Trimble. In Esri software, we are not doing any velocity/tectonic plate/subsidence or other time-based movements of data yet.

Melita

Disclosure: I'm on the subcommittee that maintains the EPSG registry.

JoeBryant2
Occasional Contributor II

Thanks for your response, Melita. That's interesting that ESRI will support time/velocity based transformations in the future.

My Trimble rep says a new "universal" coordinate system is coming in the next few years that will automatically account for tectonic movement so that we will no longer have to take this into account. I'm not sure how this would work, but I'm guessing our whole GIS would need to be transformed to support it.

Is it correct to assume in 10.4 that if my collected data (RTK or post processed) is referenced to ITRF08 (epoch 2005.00), that it would not need to be transformed (read = a shift in position) to WGS1984, other than for projection bookkeeping? But if it was referenced to ITRF00 (epoch 1997.00), it would need to be transformed to WGS84 (shifted)?

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MelitaKennedy
Esri Notable Contributor

The Trimble rep may have been talking about NAD2022 and NAVD2022. NGS is planning on replacing NAD83 and NAVD88 around 2022. From what I understand, they'll be 'dynamic datums' and update as needed rather than have a static re-adjustment every few years.

It depends on what you're planning to do with the data. If you're merging it into a database that's using an older WGS84/ITRF or NAD83 realization, then you might want to transform it to match the database.

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JaimieChhu1
New Contributor II

Hi Melita.  When you said "There's no set epoch/realization on the WGS 1984 definition", do you mean that if I bring in data that is WGS84 (G1762) epoch 2005.0 equivalent with ITRF08 and WGS84 (G1150) epoch 2001.0 equivalent to ITRF 00 into the same ArcGIS data frame with the frame projection as GCS_WGS_1984 (EPSG 4326), I would need no transformation between the two data sets?  Is the difference between the two epochs (2001.0 and 2005.0) insignificant enough we can consider them the same?  

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MelitaKennedy
Esri Notable Contributor

Hi Jaimie, 

We would not be able to provide you with any transformations, so yes, there would be no transformations. We only have one "known" definition for WGS84. Even if you created custom geographic coordinate systems that included the realization and epoch date the software has no way to handle it correctly. I don't know if the difference between your two epochs and realizations are insignificant or not. It will depend on your accuracy needs. If the data's in the US, you could try the HTDP software from the National Geodetic Survey and calculate what the differences are.

Melita

NicoTripcevich
New Contributor II

Hello Melita, thank you for shedding light on this.  This is an old thread and we're approaching 2022 do you know if there is a path forward for ITRF provided by epsg? What will become of precision of the web mapping standards like GeoJSON that currently point to WGS1984 without Epoch? Maybe pointing to GD2022 along with Epoch?
I just found Jim Baumann's 2019 ArcUser piece. The urgency of the situation for Web mapping (especially tile maps) based on the ensemble datum of WGS 1984 is pretty well laid out here

http://cameronshorter.blogspot.com/2019/08/time-dependent-datum-problems.html

 

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