Re: FW: WGS84 / Web Mercator

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Re: FW: WGS84 / Web Mercator

Cameron Shorter

With permission, I'm forwarding through this email from ESRI, which provides some great clarifications to some of the questions being asked in this email list.

Thanks Gordon and Keith for this detailed response.

On 24/7/19 12:30 pm, Cameron Shorter wrote:

From: Gordon Sumerling [[hidden email]]
Sent: Wednesday, 24 July 2019 10:04 AM
Subject: RE: WGS84 / Web Mercator - next steps

Hello Michael and Cameron,

I have asked Kevin Kelly and Melita Kennedy from Esri on their thoughts on your proposal. The response from Kevin is as follows:

Geodetically, the proposed solution makes much more sense than equating GDA2020 with WGS84. It also maintains the equivalence of GDA94 and WGS84. Below are a few fine points of clarification. I am certain this will be confusing, but it’s the new normal in geodetic datum management.

GDA94 is aligned to ITRF92 at epoch 1994.0 and GDA2020 is aligned to ITRF2014 at epoch 2020.0. In the WGS84 series of realizations, GDA94 is most closely aligned with WGS84(G730), which is itself aligned to ITRF91 at epoch 1994.0.

The proposed solution, essentially defines a non-null WGS84-to-GDA2020 transformation that accounts for the roughly 1.8 meter difference between the two frames (GDA94/WGS84(G730) and GDA2020). This is good. More correctly though, the transformation should read: WGS84(G730)-to-GDA2020. It is understood that the dynamic part of WGS84 is neglected in this transformation, meaning that over time WGS84 and GDA2020 will drift apart.

Now, the current realization of WGS84 is WGS84(G1762) which is aligned to ITRF2008 at epoch 2005.0. And, I hear through the U.S. government grapevine that a new realization of WGS84 aligned to ITRF2014 is soon to be announced. Since GDA2020 is aligned with ITRF2014 at epoch 2020.0, there is a slight misalignment between GDA2020 and WGS84(G1762), which does not include plate motion between the two defining epochs.

The upshot of all this is that the proposed solution to get data into WGS84 from GDA2020 will actually yield GDA94/WGS84(G730) coordinates, it will not provide WGS84(G1762) coordinates, nor values in the forthcoming realization of WGS84, WGS84(G-Something). The reverse, getting data into GDA2020 from WGS84, is somewhat different. Users with data in any WGS84 realization after WGS84(G730), using the proposed solution, will neglect the misalignment between their realization and WGS84(G730), because the transformation parameters assume WGS84 coordinates are actually WGS84(G730) values.

Though confusing, just some facts to be aware of to better educate users.

Regards

Gordon Sumerling | Principal Consultant 

Esri Australia | Ground Floor, 15 Bentham Street | Adelaide, SA 5000 | Australia

-- 
Cameron Shorter
Technology Demystifier
Open Technologies and Geospatial Consultant

M +61 (0) 419 142 254

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