4 minute read
SPATIAL & GIS
While global positioning systems (GPS) can currently provide 5-10m accuracy, more precise positioning is increasingly important for applications in civil engineering, construction, disaster response, flood and storm modelling, and more. Here, Nicholas Brown, Director of National Geodesy at Geoscience Australia, talks about the $225 million Positioning Australia program, and upgrades to the Australian Geospatial Reference System.
According to Mr Brown, the majority of growth in Global Navigation Satellite System (GNSS) sales is within emerging markets such as locationbased services, intelligent transport and Internet of Things (IoT) devices.
The Positioning Australia program will provide accurate and reliable positioning for everyone. The aim is to accelerate the adoption and development of location-based technology and applications.
“In anticipation for the growing use and reliance on precise positioning technology, the Permanent Committee on Geodesy is leading the upgrade of a number of elements of Australia’s Geospatial Reference System including the static datum, the introduction of a time-dependent reference frame, improved geodetic infrastructure, and standards development to improve access and efficiency of geodetic data,” Mr Brown said. The introduction of the Australian Terrestrial Reference Frame (ATRF) and the Australian Vertical Working Surface (AVWS) from 1 January 2020 will have a significant impact on the delivery of projects and operations involving precise spatial and positional data, improving accuracy, efficiency and safety.
AUSTRALIAN TERRESTRIAL REFERENCE FRAME
The Geocentric Datum of Australia 2020 (GDA2020) is a static datum, which means that the coordinates of features (e.g. roads, buildings and property boundaries), do not change with time despite the ongoing changes in the Earth’s surface.
This is beneficial for applications where it is easier if the coordinates of features do not change (e.g. a major road development project).
In contrast, GNSS base their coordinates on a framework that is fixed to the centre of the earth around which satellites are orbiting. These frameworks are called ‘timedependent reference frames’ in which the positions of features change with time due to plate tectonic motion. The ATRF is a time-dependent reference frame that means the positions of features will change with time due to plate tectonic motion, which equates to approximately 7cm per year in Australia.
“Users of ATRF are expected to be predominantly from the areas of intelligent transport services and location-based services,” Mr Brown explained.
For example, in the future, driverless cars will be receiving positioning information from GNSS satellites in ATRF and cars will be automatically requesting and receiving updated map information in ATRF to align with the car’s position.” In 2020, Australia will adopt this two-frame approach, enabling users to work with a static datum, GDA2020, or with the ATRF.
Given the diverse range of user requirements in Australia, the choice of which reference frame to use (GDA2020 or ATRF) will remain with the user for the foreseeable future.
AUSTRALIAN VERTICAL WORKING SURFACE
For all the benefits of GDA2020 and ATRF, these upgrades only provide GNSS users with the ability to compute their position and align spatial data accurately with respect to the ellipsoid – a simplified mathematical representation of the Earth’s surface.
Nonetheless, water flows in accordance with gravity, not according to the ellipsoid.
“For this reason, the ellipsoid is not an appropriate reference surface for height applications relating to water flow,” Mr Brown said.
“We use the Australian Height Datum (AHD) and a model known as AUSGeoid to convert ellipsoidal heights to AHD heights, which is roughly equivalent to the mean sea level of the ocean around Australia.
“However, AHD has its own problems which mean GNSS users are only capable of deriving AHD heights with an accuracy of 6-13cm across Australia. This is not suitable for some users.”
The AVWS on the other hand is accurate to 4-8cm and its much smoother surface doesn’t suffer from some of the problems associated with the AHD.
“Some of the biggest beneficiaries of AVWS are expected to be those who are working on projects that require modelling or mapping water flow over areas greater than 10km,” Mr Brown said. “This is because the AHD has a number of distortions in it which appear as steps or bumps in the datum at distances beyond about 10km.”
THE FOUNDATION TO DATA-DRIVEN DECISION-MAKING
The main challenge that comes with greater accuracy is that there are new considerations – with professionals from the spatial industry being the bridge between the complexities of geodesy and user requirements.
“For example, when attempting to measure something at the millimetre level compared to the centimetre level, geodesists need more stable site foundations, more expensive receivers, geodetic style antennas and better atmospheric models,” Mr Brown said. “Many users will have precise positioning technology in their hand but it will be up to the spatial industry to deliver data to the user which is aligned with the user’s position and in the user’s datum.” Mr Brown likened Australia’s Geospatial Reference System to the foundations of a house, providing a stable, accurate and reliable frame on which accurate measurements can be made and connected together.
“The importance of Geospatial Reference Systems was recognised by the United Nations in 2015 with the adoption of a General Assembly Resolution promoting the importance of an accurate, sustainable and accessible Global Geodetic Reference Frame to support science and society,” Mr Brown said.
“A Geospatial Reference System underpins the collection, management and alignment of spatial information and enables us to monitor the dynamic Earth as it breathes.
“In addition to the traditional survey, mapping and navigation fields, spatial information is increasingly critical for civil engineering, industrial automation, agriculture, construction, mining, recreation, intelligent transport systems, land use planning and administration, construction and hazard assessment, disaster response and emergency management, environmental studies, and scientific research.
“The Geospatial Reference System is the glue that allows us to align this spatial data to make better decisions.”
