Mapping the Sydney Underground

By on 13 May, 2010

JON FAIRALL

Tough new penalties await contractors who interrupt electricity and gas supplies in Sydney. The legislation, which allows for five-year jail terms, has passed through the New South Wales Parliament and is now awaiting Royal Assent. It follows a series of power failures in Sydney’s central business district in March.

The legislation is a double-edged sword. On the one hand, inadvertent strikes on buried assets by contractors cost utility companies millions of dollars every year. No doubt they will welcome the legislative support.
 
But on the other hand, it will increase pressure on utilities to improve the performance of the Dial Before You Dig system. DBYD is designed to provide information that enables contractors to be reasonably sure that they will avoid buried utility assets when they dig. The assumption is that the utility is able to provide that information. Few people in the industry think they can.
 
The problem of mapping buried assets is not intractable, but it does require money and resources.
 
Like most urban spaces, the underground is a busy place. Tunnels, basements, pipes and conduits go here, there and everywhere. In fact, in many ways, the scene under our feet is more complex than that above ground. The underground contains the detritus of hundreds of years of occupation, long buried and forgotten. Even if an asset is not forgotten, give it a few years and its exact position can be very uncertain.
 
So long as we leave things alone, it’s not an issue. Out of sight really is out of mind. But start digging stuff up and things can go badly wrong very quickly. Unfortunately, authorities in Sydney are reaching a stage where digging stuff up is becoming essential. Apart from the impending legislation and its ramifications, the city’s transport system is dysfunctional. Much of the problem stems from the City Loop, a section of underground railway line that loops through the CBD from Central Station, providing vital connections to the rail network from six stations.
 
The loop is now at full capacity in peak hours. Unfortunately, since it connects to all lines in the city’s rail network, it puts an upper limit on the number of trains in the rest of Sydney.
 
One solution would be a new tunnel system to augment the City Loop. The first stages of the proposed City Metro link would connect the CBD to the inner western suburbs, but thereafter form the basis of a fast rail service to under-serviced north western suburbs.
 
Clearly, before such a project can be undertaken, the designers would like some indication of the implications for buried infrastructure.
 
However, they are by no means the only ones concerned. Another group with interest in Sydney underground is the Emergency Information Co-ordination Unit. It works out of the NSW Department of Lands and has responsibility for the supply of data for response in emergency situations.
 
Of particular concern to the unit is the location of infrastructure, and especially the relationship between the assets of the various suppliers. The focus of EICU, therefore, is on building databases composed of information from many different sources.
 
Two years ago, a group of people with an interest in the problem got together to devise a pilot project to co-ordinate and integrate asset information from various stakeholders. The group included participants from RailCorp, the Geospatial Information Technology Association, and private contractors.
 
A committee was established. It has representatives from Lands, and from RailCorp and the Roads and Traffic Authority. Asset owners such as Telstra, Energy Australia, Sydney Water and gas supplier Jemena were also represented, as was the new kid on the block, Sydney Metro. The research organisation CRCSI was also present.
 
Greg Oaten, the president of GITA, told the Sydney Down Under seminar (15 May) that the initial plan was to run a pilot project in the north western corners of the CBD – from north of Wynyard station down to Barrangaroo. The idea was to put something together that could be used for Sydney Metro planning.
 
However, this idea was quickly overtaken when it became apparent that most of the cost would be in setting up the project, meaning the pilot site could be extended to the rest of the city for very little extra money.
The strategy was to take data of existing underground infrastructure from owners of those assets plus aboveground building data, and put it into a single database.
 
This was no mean undertaking, Oaten says. The data was held in a mix of Smallworld and ESRI databases with idiosyncratic data models. Safe Software’s FME solution was used for the data integration to join these together.
 
The fundamental product of the work is a 3D model. It shows all the underground services in Sydney, with the buildings above extruded out of their footprints to the correct height above and depth below.
 
‘It’s a fantastic project,’ says Dominic Puiu, the NSW state manager of Dial Before You Dig, an association of asset managers.
 
At a recent workshop to publicise the development of the 3D model, Lands’ Stephen Lead said the dataset was a significant development.
 
However, it suffers from a number of problems. It is a big dataset – which requires big machines to manipulate and store it. Even with these big machines, the nature of the data means that displays are overly cluttered, making them difficult to read.
 
Another problem is that of scale. The 3D dataset contains multistorey buildings and single strands of wire. It is impossible to show both on the same screen at anything like meaningful size. The main problem, though, is accuracy and completeness. There is no guarantee that all services are recorded in the data, and for those that are, they may not be shown in the right place.
 
Of course, it is relatively simple to accurately measure the position of surface expressions of underground infrastructure – manholes, pits, and so on. It may be possible to infer the position of pipes or conduits between these points, and even the depth, but all too often it is impossible to do so accurately.
 
The database from Sydney Water, for instance, assumes that all its sewer pipes have a constant fall. The depth at one end may be known, in which case the depth at the other end can be inferred. However, errors can creep in if the surface has been raised or lowered by subsequent work or if the as-built work has moved from its designed position.
 
It is also possible that the overall accuracy of the individual datasets can be improved when several of them are being combined. Often enough, the position of one asset can be inferred from the position of another, leading to an overall improvement of accuracy.
 
There is no doubt however, that direct survey – resulting in an absolute threedimensional position – would improve matters.
 
Equipment certainly exists to do this. A combination of ground-penetrating radar and GPS with RTK will yield absolute x, y, and z to within a few centimetres.
 
For instance, in January 2009, Ground Control Aust was contracted by the Queensland Department of Transport and Main Roads to perform 3D mapping of one kilometre of the Bruce Highway near Mackay. The radar equipment called RIS MF-Himod was supplied by IDS Australia in Brisbane. The purpose of the survey was to identify all the underground services. Of particular interest was the first 600 mm below the road surface.
 
DMR undertook the survey because it may need to remove a layer from the highway at a later date. Services to this depth must be located to ensure safe excavation. Data was presented in a 3D AutoCad file.
 
An alternative to this type of work are products such as the Ezidig cable avoidance tool, which is available from Tech Rentals. It mounts on an excavator and indicates the presence of buried metallic services.
 
The device works by detecting electromagnetic emissions from cables and similar objects. The maximum range is two metres Clearly, such a device is of limited use in finding concrete or plastic conduits, but it is a good way of locating metallic assets underground.
 
No plans are yet on the public record to improve the current 3D model with active surveys of Sydney’s underground assets. But it would be the next logical step.
 
With the spectre of legal action waiting in the wings, it may also be financially responsible.
 
Jon Fairall is the editor of Position Magazine.


Issue 42; August – September 2009

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