Earth-fixed vs plate-fixed and GDA2020

The march towards the adoption of GDA2020 continues, and one of the great things to have come out of this is the move away from the term “dynamic datum” to “earth-fixed datum”.

Not sure what the difference is?
Why it’s super interesting and you should care?

Ah ha, Dear Reader, read On!

GDA94 is a “plate-fixed datum”, and it is defined by (from the ICSM GDA FAQ):

the coordinates realised on the Australian Fiducial Network (AFN) geodetic reference stations, referred to as the Geodetic Reference System 1980 ellipsoid (GRS80), determined within the International Terrestrial Reference Frame 1992, at the 1 January 1994 epoch.

The AFN stations, sit on the tectonic plate, so they move when it does.
The datum is, therefore, ‘fixed’ to the plate.

GDA2020 will also be a “fixed-plate datum” as it will be based on

a selection of Australian geodetic reference stations referred to the International Terrestrial Reference Frame 2014, at the 1 January 2020 epoch, using the Geodetic Reference System 1980 ellipsoid (GRS80).     (ICSM GDA FAQ)

Note the change in reference frame though?

We are still on a plate-fixed datum, but now referencing an international framework. Hrmmm…

So, what is “earth-fixed” and how do we get from here to there?

Earth-fixed datums have axes that appear to co-rotate with the Earth as it spins in space. They are ‘fixed’ to the earth as a whole, not to a tectonic plate.

So, imagine a globe with a coordinate system based on that datum drawn on it.

Now zoom in on Australia in your mind…

In an Earth-fixed datum, the tectonic plate moves, but the axes don’t.

In a plate-fixed datum, the tectonic plate moves AND the axes over that plate move with it.

(If you need/want a more technical explanation, there is a whole lot more information available from the  Intergovernmental Committee on Surveying and Mapping’s website)

Back to our datum journey here in Australia:
We have been using GDA94 for a while, are about to move to GDA2020 now, a plate-fixed datum, referencing an International Framework… and then… it gets a bit confusing for many people.

Why not go from GDA94 straight to an Earth-fixed datum?
Is GDA2020 going to become redundant, like AGD84 and GDA94, etc?

Again, from the ever useful ICSM GDA FAQ!

The adoption of the ATRF […] will partly depend upon the development of international standards and the subsequent applications of these standards by software and hardware providers. For example, spatial referencing which incorporates the date of measurement as part of the coordinate reference system. Standards organisations and some international software providers are already considering this issue.

Wait!
Where did “ATRF” come from?

The ATRF is the Australian Terrestrial Reference Frame and is known as “Stage Two” of the  “Datum Modernisation in Australia” Progam. From Geoscience Australia:

The second stage will commence in January 2020, when users will be able to transition to a time-dependent reference frame, that is, a datum/reference frame in which coordinates will change with time. This datum will be called the Australian Terrestrial Reference Frame (ATRF) and will provide the Australian community with a sustainable, traceable, high-precision geodetic reference system capable of meeting the most demanding positioning requirements. This datum will be fixed to the earth and will model the locations of reference points and their movement over time by maintaining full alignment to the International Terrestrial Reference Frame (ITRF).

NOTE: GDA2020 will still be available for users who do not need a time-dependent reference frame.

GDA2020 is a necessary and useful step on our way to an Earth-fixed datum, and it didn’t really make sense to go from GDA94 to ATRF, even if the software and hardware vendors were already all set to go.  Getting the change from GDA94 to GDA2020 across the industry is a massive task, trying to bring in both GDA2020 and ATRF at the same time sounds to me like a bad deal 😮

The two-stage approach may seem complicated at first, but once you dig into it, it makes a lot of sense. This also means that it is a great time to be paying attention to datums in Australia if you care about geospatial data  🙂

And I didn’t lie about it being interesting either!

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