Virtual Reference Systems—Setting Machines Free to Run

A 1.331Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

It is early in the morning, the shopping center parking lot was empty as the flatbed hauling a D5 Dozer equipped with a Trimble GCS900 System pulled up next to a 1.5 acre square plot of grass. We all have seen these infill lots in strip-malls across the country, a vacant plot waiting for some commercial venture to pounce. In this case it is an auto parts store. The flatbed driver unloaded the D5, parked it in the grass and drove off. Shortly afterward a pickup pulls up next to the dozer, the driver mounts a GPS receiver on each blade mast, connects the cables and the dozer operator climbs up into the cab of the dozer. There is one stake at the edge of the lot, a 3 foot lath with Northing, Easting and Elevation written it. The dozer makes its way towards the stake. The operator places the right corner of the blade on the mark on the blacktop next to the stake. The dozer spins around and begins to cut into the manicured turf.

Nationwide there are, at last count, 10 state wide and state owned Trimble VRS systems. Leica and Topcon also have some public systems in operation. There are also numerous private VRS systems ranging from city wide to regional systems. North and South Carolina both developed and maintain statewide VRS systems. In the Carolinas the base stations that comprise the network are spaced throughout the state. They are mounted on public buildings or in open areas on public ground. The base stations are connected to servers in Raleigh or Columbia via high speed Ethernet connections. The servers provide corrected 3D signals via wireless connections to all users. Both states have placed a high value on providing accurate 3D positioning to the state DOT's, first responders and utility companies. The utilization of these state sponsored VRS systems by private industry has had a drastic effect on agriculture and earthmoving activities giving them the freedom to deploy state wide. North Carolina has had a statewide VRS System since 2002 and South Carolina since 2006. Surveyors working in the cadastral as well the construction world have been using VRS since its inception. The freedom of VRS enabled rapid site measurement as long as there was wireless data access. It has only been recently that machines have been able to utilize VRS for guidance. The key factor was the advent of 4G wireless data streaming. Many hours of testing have shown that correction data from a VRS in North or South Carolina can provide the same data latency variable as a base station. North and East positions are virtually the same, vertical positions can drift more than a local base station, sometimes by as much twice the variance. Position inside the VRS network will affect the vertical repeatability as will the quality of the high speed wireless connection. Contractors using VRS on machines are made aware of its limitations and accept them; the advantages outweigh the risks. The scene played out at the beginning of this article is not unique. Every day in the United States there are hundreds of small commercial sites going under the blade. There is nothing odd about this machine on this site but to the trained eye there is something about this site that is different from most. There are no stakes in the ground, except for the one lonely lath at the back of the property. What makes this stake-less site unique is that there is no base station either. The D5's GPS blade control system is equipped with a high speed modem that contains a wireless data plan. The GCS900 system is programmed to log onto the North Carolina Virtual Reference System on power up. The GCS900 System is also logged onto VisionLink. VisionLink is Trimble's web hosted machine monitoring system allowing for real time remote site monitoring and real time remote troubleshooting of issues encountered during construction. VisionLink has also allowed the GPS manager to upload the design data and site calibration to this D5 the night before. With 3 minutes of in cab preparation the operator had synchronized the site data from VisionLink to his in cab display, verified the data link and gotten a visual of his machine parked on the finished plans. After verifying his position with the check point at the rear of the lot; grading can begin.

The site preparation for VRS utilization is the same as if a base was being used. Good 3D control is still required for the site calibration/localization to be accomplished. A GPS hand held rover logged onto the VRS system is used to measure and verify the site control placed by the surveyors. A calibration is done and used in the same way as a base station calibration. After the site calibration is done, many times a base position is established just in case machines without a modem are deployed to the site. Placing a base station on a site with a VRS Calibration works well as long as the base station position is established using the calibration derived from VRS. Base station position integrity can be enhanced by measuring the VRS position in a real time static mode numerous times over the course of an hour and averaging the result. In almost every case the control measurement spread on the North Carolina VRS varies by 0.05 feet X, Y and 0.10 of a foot Z. Let's keep in mind, the work being done on the auto parts store site is rough grading. The site is stripped, the parking lot and retention basin are roughed in to within 0.10 by keeping the target surface 0.10 feet above sub-grade. As contractors become comfortable with VRS many sites are worked to within 0.05 feet if site tolerances allow a plus or minus ½ an inch.

Utilization of VRS corrections in the mapping and construction world has forever changed the way corrected 3D positions are calculated. The constant downsizing of electronic components and the refinement of GNSS correction signals are rapidly expanding the options for enhanced 3D data reporting. Already accurate X and Y positioning is becoming ubiquitous. It will not be long until the corrected elevation component becomes a part of our everyday lives. The states that have had the foresight to establish and maintain a public VRS system are a step ahead in making VRS an engine for commerce, but that is another story. Oh, the dozer we left grading the auto parts store? At the end of day one the site was stripped of sod. A wheel loader and dump truck had shown up after lunch and carted off the spoil. The parking lot was at subgrade, the retention basin was roughed in and the dirt placed so the next day it could be spread for the building pad. By noon on the second day the utility crew was already laying storm pipe. The progress was swift. The customer was pleased. It was just another day for this contractor that picked up and moved to another site to do it again.

With over 25 years experience, Andy Donovan is currently vice president at Sitech Mid-Atlantic. Andy as held positions at Spectra Integrated Systems, Trimble, Spectra Precision, Geotronics and Nikon.

A 1.331Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

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