Home' Position : Position 90 Aug-Spt 2017 Contents a solid foundation provided by geodetic
control, so they actually fit together and
can facilitate meaningful results.
The main drivers for datum
modernisation in Australia include:
• Including up-to-date geodetic
observations and increased precision
• Removing known distortions
• Providing seamless coordinates across
• Accounting for tectonic plate motion
• Accounting for tectonic plate rotation
• Introducing a truly 3D datum defined
by ellipsoidal heights
A plate-fixed datum is attached to the
tectonic plate and therefore also known
as a static datum. It is ‘frozen’ at a certain
instant in time (the reference epoch),
essentially preventing the coordinates
from changing over time due to (normal)
tectonic plate motion.
However, as the time difference
between the reference epoch and the
current epoch increases, the plate-fixed
datum deviates more and more from
the true position of the plate (and the
earth-fixed datum used for GNSS).
Consequently, it needs to be updated at
frequent intervals. GDA94 and GDA2020
are examples of plate-fixed datums.
GDA94 was defined in the then state-
of-the-art global reference frame, the
International Terrestrial Reference Frame
1992 (ITRF92) at epoch 1994.0.
This definition was justified by the
relatively uniform drift of the Australian
continent at about 7 cm/yr to the north-
east. Tectonic plate motion causes the
difference between ITRF coordinates and
GDA94 coordinates to increase over time,
amounting to about 1.6 m at present.
This is generally not an issue for
differential GNSS applications, however
the increasing number of real-time mass-
market applications is causing this offset
to introduce errors for the layperson.
GDA2020 is a much more homogeneous
plate-fixed datum, based on a national
least squares network adjustment that
rigorously propagates uncertainty.
GDA2020 is defined in the current state-
of-the-art global ITRF2014 reference
frame at epoch 2020.0. The coordinates
are extrapolated into the future to 1
January 2020 in order to extend the
lifespan of the datum.
GDA2020 is realised by gazetting
an expanded AFN consisting of 109
GNSS CORS, mainly including stations
contributing to the Australian Regional
GNSS Network (ARGN) and the
The UTM projection will continue to
be used to project latitude and longitude
to grid coordinates (Easting, &Northing),
albeit based on new equations. These grid
coordinates will be expressed in the Map
Grid of Australia 2020 (MGA2020).
Following a transition period,
GDA2020 is to be adopted by 1 January
2020, although most jurisdictions may
decide to move to the new datum earlier.
This move from GDA94 to GDA2020 will
cause the horizontal coordinates of a
mark to shift by approximately 1.8 m to
the north-east, while the ellipsoidal height
will decrease by about 0.1 m.
In order to connect to the Australian
Height Datum (AHD), it is therefore
crucial to apply AUSGeoid2020 to
GDA2020 ellipsoidal heights, while
AUSGeoid09 must be used to convert
GDA94 ellipsoidal heights. Due to the
aforementioned 0.1 m difference in
ellipsoidal heights between GDA94
and GDA2020, a user should never
combine AUSGeoid2020 with GDA94
or AUSGeoid09 with GDA2020. While
AUSGeoid2020 has the same extent and
density as its predecessor AUSGeoid09, it
is based on a much larger and much more
homogeneous dataset. AUSGeoid2020
also provides a rigorous uncertainty value
associated with the offset between the
ellipsoid and AHD, varying as a function
of location. In contrast, AUSGeoid09 only
provides a constant uncertainty estimate.
An earth-fixed datum accounts for the
Earth’s dynamics by allowing tectonic
plates to move within it over time, i.e. it is
4-dimensional. It is fixed to the Earth but
not its crust and therefore also known as
a ‘dynamic datum’, however this is a term
to be avoided because the datum is not
dynamic but the Earth is! Consequently,
the coordinates of a given ground mark are
constantly changing. It is critical to attach
a time stamp to each position given in an
earth-fixed datum, so a position given at a
reference epoch can be propagated to the
current or any other epoch using station
velocities. ITRF, ATRF and WGS84 are
examples of earth-fixed datums.
The International Terrestrial Reference
Frame (ITRF) is the most precise earth-
centred, earth-fixed datum currently
available – the global gold standard if
you like. It was first introduced in 1988
and has since gone through 13 versions,
including the current ITRF2014. The
ITRF is updated regularly in order to
account for the dynamics of the Earth.
The Australian Terrestrial Reference Frame
(ATRF) will be a regional realisation of
the ITRF. As a consequence, Australian
spatial information will be directly
interoperable with GNSS measurements.
It is anticipated that the ATRF will be
implemented from January 2020 with
adoption planned to be complete by 2023.
However, it is important to note that
GDA2020 and ATRF will exist in tandem
for the foreseeable future. Over time
GDA2020 will meet the fate of all plate-
fixed datums and become outdated.
The World Geodetic System 1984
(WGS84) is the nominal datum used by
GPS. It is based on the WGS84 ellipsoid,
which can generally be assumed identical
to the GRS80. The WGS84 datum was
introduced in 1987 and has since been
refined several times to be closely aligned
with the ITRF. For most purposes WGS84
and ITRF can be assumed identical,
however, WGS84 is based on a much
smaller number of reference stations and
the level of agreement worsens as the time
between realisation grows.
From GDA2020 to ATRF
It is anticipated that the transition
from GDA94 to GDA2020 will be complete
by 1 January 2020, i.e. the date ATRF
is planned to be released. However,
most states and territories are expected
to move to GDA2020 before this date.
The Intergovernmental Committee on
Surveying and Mapping’s (ICSM’s) GDA
Modernisation Implementation Working
Group (GMIWG), which is overseeing
the datum modernisation, has already
released GDA94-GDA2020 and ITRF2014-
GDA2020 transformation parameters and
the GDA2020 Interim Release Note. The
soon-to-be-released GDA2020 Technical
Manual will provide more detailed
information, while the official release of
AUSGeoid2020 is also imminent.
Once adopted GDA2020 and ATRF will
are expected to operate in tandem as a
'dual frame system' for the foreseeable
future. The ongoing datum modernisation
will provide a much improved Australian
national datum that will ensure that
Australia is well positioned into the future.
Dr Volker Janssen works at Spatial
Services, a unit of the NSW Department
of Finance, Services and Innovation
(DFSI), which operates CORSnet-NSW. ■
Links Archive Position 89 Jun-Jul 2017 Position 91 Oct-Nov 2017 Navigation Previous Page Next Page