With GNSS/GPS the base station and the machine both need a view to the satellites while total station technology doesn’t. A total station needs a line of sight to the machine while a GNSS/GPS base station doesn’t. Total station technology provides greater accuracy, although GNSS/GPS augmented with laser can match it, if site conditions allow it.At Inverell Shire, four hours inland from the mid-north coast of New South Wales, the council has experience with robotic total stations and with GNSS/GPS machine control equipment.After recently hiring a Trimble SPS730 Universal Total Station and Trimble GCS900 grader system for quality assurance on a local road reconstruction contract, the council has decided to go with UTS technology for most of its work in future, because its high accuracy allows it to handle everything from bulk earthworks through to final trimming. The SPS730 can also be used for reflectorless surveying, which is particularly useful when working in hazardous or inaccessible areas.According to shire executive engineer Richard Jane, the performance of the machine control system on the contract work was a real eye opener. It was the first time they had used such a system, and the tolerance (-0mm and +10mm) on the layers of pavement was quite tight. After the first grading run on the base course, fewer than 20 levels were out of tolerance from about 900 levels taken over 700m of road. The technology allows the site foreman or operator to pick up a range pole and switch the UTS to track him or her in, quickly double checking levels behind the grader. A full conformance report can be carried out using the same process. The term “universal” is used to describe the system’s ability to switch quickly and simply between tracking the grader and checking the levels manually.Grader operator Mick Goater said he would have traditionally used a surveyor to place level pegs for each pavement layer, but by using the universal total station and GCS900 the time taken to spread the pavement material was approximately halved.“It saves you time in spreading, but it also means that each layer ends up a consistent thickness – which helps with compaction,” Goater said.“In the long run, the system cut about three weeks off a project that would otherwise have taken us about five months,” he said.After finishing the road project, Inverell Shire moved the grader onto preparing the subgrade for the town’s new synthetic hockey field.The design of the rectangular field required the finished subgrade to take the shape of a shallow hip roof: a slight ridge runs part way along the field centreline and divides at each end to run to the corners. The slope required from the central ridge down to the perimeter drain was just 0.85%.After the grader operator had got the subgrade roughly in shape, he took the machine around the field in circles and the blade automatically followed the design, even as the machine ran over the various ridge lines.Jane said the shire was tending towards using total stations for most of its construction work, and if cost was no consideration, it would probably have one of everything.“However that’s not practical,” he said. “We’re often working in heavily wooded areas where the satellite signals required for GNSS/GPS work just don’t get through. Total station technology doesn’t have those issues and it gives us higher accuracy anyway, so we’re probably better off using total stations for most of our work,” he said. Inverell Shire’s Cat 12H grader made short work of producing the complex shape required for the hockey field subgrade.
