Our Changing World
Thursday 2 October 2014, with Alison Ballance, Ruth Beran & Veronika Meduna
On This Programme
- The Science of Knowing Where You Are
- Modelling the Canterbury Coast in the Water Lab
- Celebrating Antarctica
Geodesy - the Science of Knowing Where You Are
We take it for granted, these days, that global positioning satellites will help us know exactly where are at all times. And the science that underpins global positioning and makes it possible? Geodesy.
“Geodesy is the study of the shape and size of the earth,” says Graham Blick, the National Geodesist at LINZ, Land Information New Zealand. “We’re interested in how big the earth is, how it changes through time due to earthquakes and plate tectonics, earth’s rotation and the earth’s gravity field.”
Geodesy involves applied maths and earth sciences, and Graham says it’s the oldest known science, dating back 2500 years to Pythagorus, who was among the first to recognise that Earth was elliptical in shape and orbited the Sun.
LINZ is responsible for managing New Zealand's geodetic system. On its web site it describes how this system ‘provides the underlying measurements used to produce topographic maps and hydrographic charts. It is also an essential tool in setting and identifying property boundaries. The physical component of the geodetic system is a network of trig stations and geodetic marks that serve as physical reference points.’
“Knowing where you are and having accurate data sets is extremely important,” says Graham. “And what underlies all of that is the geodetic framework, and we often refer to that as ‘the infrastructure for infrastructure’. Before you start building infrastructure such as roads you must start with that accurate spatial reference framework.”
To see how this ‘infrastructure for infrastructure’ is used in practise Alison Ballance joined Graham Blick on a visit to one of the largest roading projects currently underway in New Zealand. MacKays to Peka Peka (M2PP) is an 18-kilometres section of the Kapiti Expressway, north of Wellington. Part of State Highway 1, it is a road of national significance.
Will Newall is survey manager for M2PP, and he says it is a world-leader in using GNSS technology. “I have a whole fleet of machines that have GNSS on board,” says Will.
Geodesy and surveying used to rely solely on physical measurements carried out on the earth’s surface. These days, the GNSS or Global Navigation Satellite System provides highly accurate location information that allows every point on Earth to be given its own unique address, namely its latitude, longitude, and height. The American GPS network is part of the global GNSS system that also includes positioning satellites from a number of other countries, such as the Russian GLONASS system.
While most consumer products in New Zealand, such as smartphones, use just the GPS system, professional surveyors such as Will Newall use the entire GNSS system, as the larger number of satellites allows for much for precise positioning.
Will Newall says GNSS has made it easier, safer and more cost-effective to build large infrastructure projects. “Most of the calculations are taken care of by the technology we use,” he says. “It effectively tells us where we are, in an instant, to within a centimetre or so.”
A number of the diggers and excavators being used in the M2PP project have GNSS fitted.
“They have the design in 3D, and they can see exactly where they are, and where the teeth of the buckets on the excavator are,” says Will. “So they can dig or fill the ground exactly to the design we’re trying to build.”
GNSS relies on referencing a highly accurate mathematical model of the Earth to calculate where positions are in relation to sea level, and this is created by precise measurements of gravity. LINZ has just finished a series of flights over New Zealand re-measuring gravity across the country, and Graham Blick says this will enable us to “transform GNSS-derived heights down to heights that approximate sea level, to an accuracy of 2-3 centimetres.”
Modelling the Canterbury Coast in the Water Lab
Until 6-8,000 or so years ago, Wairewa/Lake Forsyth, on the Canterbury Coast just south of Banks Peninsula, was open to the coast and filled with seawater. Then as Kaitorete Spit began to form, the lake became partially blocked, and connected to the sea by just an outlet river. Over the last few years this outlet has become blocked, meaning what was once brackish water has become freshwater. This has affected the plants and animals of the lake. As well, the lake is now prone to flooding over the nearby road. The lake outlet is occasionally opened using bulldozers, but blocks again within a few hours or days.
Crile Doscher, from the Faculty of Environment, Society and Design at Lincoln University, has been trying to understand the coastal processes that are taking place along this stretch of coast, to work out if it might be possible to engineer a groin or breakwater that would keep the outlet open.
Kaitorete Spit is made from rounded boulders and sand washed down the many braided rivers draining from the Southern Alps and running across the Canterbury Plains. This sediment is then moved along the coast by longshore drift.
To begin with Crile created a GIS or Geographic Information System model of the outlet and coast.
“GIS allows you to create different layers of information that reflect some characteristic about the surface of the earth,” says Crile. “So in this case what we’re really interested in is the shape of the seafloor. And that’s the most important thing because you’ve got interactions between the waves and the underwater bathymetry which affects movement of the sediment.”
Crile surveyed the stretch of coast between Birdlings Flat and the outlet by boat, collecting depth, location and sidescan sonar information on the sea floor. Within GIS he added these and layers of information such as aerial photos, which showed him the profile of the coast, and highlighted details such as large ripples in the sediment running parallel to the shore. These ripples stopped near the outlet, where there appears to be a bare rock platform at the base of the vertical cliffs.
Once he knew the profile of the shore, Crile created a 1:50 scale model of the coast in the Water Lab, using sand in lieu of boulders. The large triangular model has a sandy shore along one side, cliffs along another and a wave generator (a large metal plate that rocks back and forth to set up waves in the water). A piece of plastic spouting represents the lake outlet, while bricks are used to simulate groins and breakwaters.
“With the model, if we get the dynamics right then we can play around with different scenarios,” says Crile. “What if we put a 20 metre groin perpendicular from shore, what if we make it 40 metres, what if we put a wingwall on the end. And we just test these different scenarios to see what’s going to make the difference.”
Modelling options like this in the lab is much cheaper and faster than trying to build different options in real life. A half hour simulation equates to several days in real time, and the results have been very revealing. It seems the only option that might prevent the outlet blocking up would have to be so large and expensive that it’s not really a possibility.
This weeks marks the start of the Antarctic science season and of the 2014 New Zealand IceFest in Christchurch.
Coming Up – Thursday 9 October 2014
Mutant petunias, research to help build better roads, and the restoration of Rutherford’s Den and the Arts Centre after the earthquakes in Christchurch.