Langdon noted that Millimeter GPS+ was allowing on-demand elevation checks -- in varying locations within a pass -- using GX-60 control boxes and the rover. "With this system, you stick the [rover] anywhere onto the surface and it gives us the number," he said. "It takes a little pressure off [the side men], not having to constantly go up and down with the toggle switch and sensor."
"You could almost say that GPS isn't any more accurate than manual at station, but between stations, it is," says Jim Cleary Intercounty's Topcon dealer, Cleary Machinery Co., Inc., South Bound Brook, N.J. "It performs like a virtual stringline, calculating smooth transitions from station to station. And it eliminates the occasional blown grade."
The system's accuracy prevents over-milling, Langdon points out. "If we're high, we just go back and trim it. If we overcut, we've got to go back and pave and then mill again. That costs time and money."
"The entire area was milled without a single mark on the ground, which is roughly 8,000-plus shots in a 25-by-12-foot grid over 11,000-plus feet," Cleary concluded. "You could assume that two cuts would have been required with any other method, given the hardness of the surface. The mark-out costs associated with 16,000 shots is actually insignificant compared with the potential downtime resulting from waiting for the marks to be made, due to the large penalties enforced for not finishing the job on time. When you consider increased machine productivity and the concrete material savings the accurate milling provided, you could say that these contractors reaped the same benefits that GPS machine control has been providing grading and excavation contractors for the past decade."
Don Talend of Write Results Inc., West Dundee, IL, is a print and e-content developer specializing in construction, technology and innovation.
An Elevation Control Alternative
The Topcon Positioning Systems Millimeter GPS+ system is increasingly being used for milling and paving. Just as Millimeter GPS+ has been used for fine grading, contractors are beginning to use it for "fine milling" -- and achieving accuracies within a quarter-inch in contrast to the tenth-of-a foot precision inherent in conventional machine control.
Millimeter GPS+ combines GNSS and laser. In addition to a GNSS base and rover, the system uses a PZL-1 Lazer Zone transmitter and a PZS-MC machine-control sensor or PZS-1 rover sensor that gets integrated with the contractor's GNSS receiver. The PZL-1 transmitter sends out a wall of laser light 33 ft. tall and up to 2,000 ft. in diameter. The contractor can link up to four transmitters for a total reach of 8,000 horizontal ft. and 132 vertical ft. The PZL-1 transmitter can operate multiple machines equipped to accept its signals.
The GNSS component of the system plots the location of the machine while the laser component guides the grader to position and elevate the blade precisely. The system "knows" the three-dimensional position of the laser transmitter and the three-dimensional position of the machine and is then able to calculate the vertical angle from the laser up to the sensor on the machine and provide a vertical correction.