Geodesy underpins surveying and many branches of science, yet it’s often misunderstood, underappreciated or overlooked, says Chris Rizos.

By Jonathan Nally

Chris Rizos is an Emeritus Professor at the School of Civil and Environmental Engineering at the University of NSW. His research interests are geodesy, surveying and navigation, and he’s currently serving as President of the International Union of Geodesy & Geophysics (IUGG) for the term 2023 to 2027.

Reflecting on his career and the rise of GNSS in general, he says that “basically GNSS is what made my academic reputation and it is what has given geodesy such high value and visibility”.

How do you define geodesy and its role in the sciences?

Geodesy is an applied science, because it had from its very beginnings the desire to answer the question of ‘How big is the Earth and what shape is it?’. Hence geodesy has a fundamental connection to geography, to geometry, to cartography. Then you could even say that once we started drafting accurate maps, geodesy had a role to play in land ownership and cadastral systems. That’s where the surveying connection comes in — it’s why surveying has geodesy at its core. In fact, surveying is the only profession that has geodesy as one of its foundations.

If you look at geodesy as a science on the other hand, you connect to physics and mathematics, you have people dealing with earthquakes, the physical and fluid surfaces of the Earth, the composition of the Earth’s interior, its gravity field, and many other geoscientific phenomena that have geometric or gravimetric signatures.

The undergraduate degree for geoscientists is typically from the sciences, whereas the undergraduate degree that introduces geodetic concepts is to be found in the engineering disciplines, such as surveying. Hence geodesy has been around for many, many years, and has had many, many narratives.

But the principle one has always been answering questions about the shape and size of the Earth and its variation (that is, geometry), its orientation in space (hence its connection to astronomy), its rotation (related to time), and the Earth’s variable gravity field (principally dealing with the physics of the Earth).

Ultimately, of course, we moved into the satellite era. As the accuracy of determining position and gravity increased, and became global, then geodesy went well beyond just supporting cartography to being the vital geoscience that it is today.

So when did GPS start to make its presence felt?

The first GPS surveys in Australia were geodetic or control surveys — replacing the use of terrestrial distance and theodolite angle measurements. Using GPS receivers for geodesy was the first civilian use of GPS signals, before GPS became the indispensable navigation technology that it is today.

It must be remembered that even before GPS was fully operational, in the 1980s geodesists were using it to measure coordinates with millimetre accuracy. With such ultra-high accuracy, even the movement of ‘static’ ground points could be monitored over time.

Although the US military had designed a navigation technology with 10 to 20 metres accuracy, scientists at JPL (and a few other places) developed the special GPS hardware, and then through the 1980s many of us worked on the software to be able to use differential positioning techniques, and through the processing of carrier phase measurements, to do things for which GPS was not designed.

We still use GPS (or more correctly GNSS – Global Navigation Satellite Systems) in this way for precise positioning, which is quite distinct from what you can do with GNSS in smartphones.

But what is different now is that we can do it literally with the click of our fingers in real time. Back in 1985 it took us hours — we’d have the receiver set up on a stationary (ground) mark for many hours, collecting data from all visible GPS satellites simultaneously from a number of these specialised GPS/GNSS receivers. The data was then downloaded from all receivers, and then we spent a few days processing the measurements on a computer.

In the decade that followed, companies like Trimble, Leica, Ashtech (and others) said, “Oh, maybe we can sell our equipment to surveyors because they can afford to sit still for a few hours and get coordinates which would have taken them days to determine using the old conventional techniques”. That was a productivity jump, as many survey agencies and larger companies could afford to spend up to $100,000 on a GPS/GNSS receiver.

Of course, what has happened is operations became faster, and the hardware became cheaper. Precise differential GNSS positioning is now a standard, turnkey capability, with many services available to support this; some are free open services, and some are private subscription based.

Note that the precise positioning ‘productivity’ has increased, but not the coordinate accuracy.

You mentioned millimetres in the 1980s. That might surprise some people.

When we achieved millimetre accuracy positioning in the 1980s, geodesy suddenly elicited the response “Wow!” In contrast to using satellite laser ranging stations and radio telescopes stations, which are very expensive, immobile and of which there are few — just a couple of dozen observatories around the world were able to have their position precisely measured and monitored — GPS could be utilised with highly portable ground a receiver that was relatively inexpensive, and operated for a couple of hours. GPS geodesists could obtain similar positioning accuracy to those who used multi-million-dollar telescopes.

As you can appreciate, that dramatically changed geodesy, and since then geodesists (and many other geoscientists) have benefited — just like surveying has — from the reduction in cost and the increased speed of precise positioning.

For instance, we have a heritage of decades of tracking tectonic plates, and at the end of the day we’re not going to get a more accurate estimate of the rate of change in movement of, for example, the Australian continent. What we’re going to do is increase the density of monitored ground points. That’s when you start seeing internal deformation of plates that are signatures of deeper Earth processes. In parts of the world which are tectonically active, even over a distance of a few kilometres, points might be moving at different rates.

The word ‘monitoring’ (of geometric and gravity changes) now probably best describes the capability that geodesy has developed. We’re monitoring what is a dynamic Earth — volcanoes are dynamic, the crust is dynamic, the atmosphere is dynamic, the oceans and ice sheets are dynamic. Many dynamic Earth phenomena can be investigated through continuous monitoring of changes in co-ordinates and changes in gravity. So that’s why we can measure sea level rise, because with satellites we can measure very accurately the shape of the surface of the ocean over time — not just day to day, but over many years, and we can see sea level rise in all its spatial and temporal complexity.

So geodesy underpins our understanding of the environment?

Yes, that’s the message geodesists have been trying to get out for a long time. Modern geodesy helps so many of the other geosciences. For instance, seismologists can detect tsunamis but they are likely using buoys with GNSS receivers on them, or by monitoring fluctuations in the ionosphere, again using GNSS geodesy techniques.

Hydrologists are using what are essentially geodetic satellites to measure changes in gravity to draw inferences on seasonal effects of rainfall in river basins such as the Amazon and the Ganges, or the long-term monitoring of underground water in the Great (Australian) Artesian Basin and elsewhere, or the secular loss of ice in Greenland or Antarctica. Atmospheric scientists are using networks of GNSS receivers on the ground as well as aboard orbiting satellites to monitor fluctuations in water vapour content. These data are vital for numerical weather models.

Seismologists, volcanologists, glaciologists, atmospheric and ocean scientists nowadays use geodetic techniques, technologies and data products from national agencies such as Geoscience Australia, and the global International GNSS Service (IGS) for many, many applications, from early warning systems to furthering the basic understanding of dynamic Earth processes.

But is modern geodesy too successful? Many of our peers in the geoscientific community take geodesy for granted. This is analogous to surveying — surveying is taken for granted because civil engineers assume that all this technology just works, it is easy to use and that they’ll determine co-ordinates without error, and everything will line up and fit.

The geodetic infrastructure that underpins modern geodesy is largely invisible, because in the case of GNSS you’re dealing with satellites that you can’t see, using signals that you can’t see, tracked by small inconspicuous ground receivers. And you’re dealing with geodetic reference frames that you can’t see or touch, and concepts that are extremely complex and highly mathematical, with only a very few people across them.

That’s why we’re trying to draw attention to the fact that geodesy is indispensable for the primary function of determining co-ordinates, gravity field parameters and Earth orientation, for so many societal and scientific applications.