Written by Richard Stanaway, director of Quickclose and chair of the IAG working group on transformations between reference frames.
Much has been said in recent times about Australia moving towards a dynamic datum. GDA2020 is a tentative step in this direction and most likely will be the final plate-fixed geodetic datum for Australia.
The coordinate shift between GDA94 and GDA2020 will be between 1.5 metres in south-east Australia and 1.8 metres in Western Australia, which largely accounts for tectonic rotation of the Australian plate from 1994 projected to 2020. There will also be a step in ellipsoid heights of between 6 cm in south-west Western Australia and 11 cm in north-east Queensland.
It is important to note that the term ‘dynamic datum’ is in fact a misnomer. The coordinates of “apparently fixed” points on the dynamic surface of the Earth do change continuously due to geodynamical processes such as plate tectonics. An Earth-fixed frame or datum is fixed to the geocentre and the mantle co-rotating with the Earth beneath the mobile tectonic plates on the surface. Earth surface coordinates in an Earth-fixed frame are time dependent or kinematic.
“The term ‘dynamic datum’ is in fact a misnomer”
In 2020, a fully time-dependent Earth-fixed datum will be realised called the Australian Terrestrial Reference Frame (ATRF) which is expected to be aligned with the International Terrestrial Reference Frame (ITRF), with coordinates in Australia changing continuously by between 6 and 7 cm per year to account for movement of the Australian plate.
From a geodetic and positioning perspective these datum improvements are fundamentally necessary because positioning is now largely achieved by GNSS techniques (including GPS, Glonass, Galileo and BeiDou). These systems rely on stable orbit models which sense the centre of mass of the Earth, the geocentre.
The Earth’s surface is highly dynamic although most of the motion—plate tectonics and Earth tides— is imperceptible to most people (unless you are experiencing an earthquake of course). So, it seems logical that a geodetic datum actually represents the dynamics of the real world that we live in.
Smartphones and other personal positioning devices which natively use GNSS are on the cusp of providing users 20 cm or better ITRF coordinates in real-time as a result of multi-GNSS interoperability, improvements to predicted orbits, real-time orbits and augmentation. These major technological advances are driving the need for datum modernisation. The United Nations has recently endorsed the Global Geodetic Reference Frame (GGRF) concept which will be a major impetus for adoption by UN member states in years to come. GGRF is currently realised by the ITRF.
An inconvenient truth
So, why not end the discussion there? An inconvenient truth is that most spatial data is a snapshot of the dynamic Earth at the time it was captured. Imagery, cadastral surveys, feature surveys, as-built surveys and laser scanned point clouds are all intrinsically 2D or 3D and are the primary products of surveyors and remote sensing techniques. Vectors are measured at a fixed point in time (or epoch) relative to a datum with coordinates fixed at the epoch of the survey. The difficulty arises when GNSS positioning is used in conjunction with these ‘static’ spatial data products. With 7 cm a year tectonic movement, survey data on GDA94 for example (a snapshot of the global frame in 1994) appears to be 1.6 metres in error when checked with precise GNSS today.
This of course is not a desirable situation and can lead to some serious positioning errors, costly mistakes and assumptions of inaccuracy of spatial data by the general population. Either the spatial data has to be transformed to today’s global position, or today’s global position has to be transformed to the date of the survey in order for the position to line up exactly with the original data. So what’s it to be?…
This is the first half of Richard Stanaway’s article on Earth-fixed datums, which first appeared in the February/March issue of Position magazine. In the second half, Stanaway explores further implications of Earth-fixed datums, including potential solutions and an important opportunity for surveyors.
About the author
Richard Stanaway is the director of Quickclose, a geodetic consultancy specialising in geodetic datum analysis. Richard is currently undertaking a PhD at UNSW investigating time-dependent geodetic transformations and deformation modelling. He is the chair of the IAG working group on transformations between reference frames and an active member of the FIG working group on Reference Frames in Practice.Stanaway acknowledges the input and support of Dr Craig Roberts at UNSW for preparing this article.