The Networking Nucleus

October 2, 2003
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Progressive states prove that RTK GPS networks help users to achieve centimeter-level precision over great distances and with fewer base stations.



It’s only the beginning. But the vision is big.

The city of Renton, Wash., is currently undertaking a major update of the city’s Geographic Information System (GIS) data. Spearheaded by Robert MacOnie, PLS, mapping coordinator for the city, the four-year project involves gathering Real-Time Kinematic Global Positioning System (RTK GPS) data to reassess and verify all the city’s information on its utilities and assets. The city was motivated by its need to comply with financial reporting requirements for state and local government infrastructure assets. Additionally, Renton’s assets had been recorded in a variety of databases over the years; the GPS/GIS project will provide a single Oracle database repository for all assets.

“There’s been confusion between utilities as far as which asset is theirs,” says MacOnie. “We’re also finding assets we had no idea we had. With GPS, all [of] our assets will now have location and verified feature information.”

Hired by the city of Renton specifically for the GPS/GIS project, Engineering Specialist II Samantha Swinehart calls the project ‘one huge as-built.’

“Even if it’s not our [the city’s] asset, I still locate it: storm sewers, water lines, street signs, telephone and cable assets, everything in our right of way,” Swinehart says. To date, she’s surveyed four of the city’s 31 sections.

Renton purchased three Trimble (Sunnyvale, Calif.) GPS Total Station 5700 systems with Zephyr antennas for the project and uses one as a Continuously Operating Reference Station (CORS) for RTK correction generation. Using Trimble’s GPSBase software, the city’s new RTK CORS allows real-time surveying using a wireless connection—in Renton’s case, Cellular Digital Packet Data (CDPD). GPSBase software allows for up to 10 simultaneous users with circuit-switched technology or unlimited users with wireless Internet connectivity; the system also allows for traditional broadcasting using UHF or spread spectrum radios as well as data storage for post-processing applications.

But the reference station for Renton is just the start of a vision MacOnie and others in the state have for a multi-station regional—and ultimately statewide—GPS reference station network.

GPS reference station installments can range from a single reference station to multi-station VRS networks, Trimble’s high-accuracy RTK GPS network. Currently being developed across the nation by private firms as well as large agencies and municipalities, GPS infrastructures can benefit users in a wide variety of applications, including surveying and mapping, navigation, precision agriculture, seismic monitoring and many others.

A variety of municipalities, organizations and agencies in Washington have been trying to create a cooperative GPS network for some time—but with little success. When MacOnie first heard of VRS technology in 2001, he discussed the technology as a viable network option with Dave Riecken, PLS, GPS surveying lead for the King County and Seattle Public Utility’s (SPU) Gavin Schrock, PLS. The group thought the cost benefits of the new real-time VRS technology might provide the incentive for others to join in creating a regional network. They then developed the plan for a Puget Reference Station Utility (PRSU), which would build and monitor a GPS network providing complete RTK coverage for the entire Puget Sound area. Today a consortium of about 10 different area agencies is working to incorporate PRSU; simultaneously, SPU has independently funded a pilot four-station VRS network that will ultimately be expanded by PRSU. The completed network is projected to include 16 base stations and be available to anyone in the area; it will cover an area of about 100 miles running east/west and 150 miles running north/south.

“Our goal is to provide a reliable network with solid integrity monitoring for static and real-time GPS operations in the Puget Sound area,” says Schrock, SPU survey/GIS analyst. “We already have greater demand from public and private agencies than we can yet provide for—and that’s helping guide the network’s development.”

Status map of the PRSU projected network.

From Single Station to Statewide Network

VRS technology is the next step in Trimble’s evolution of GPS reference station networks. Like other reference station networks, VRS enables surveyors to gain centimeter-level positioning without investing in—or setting up—a local reference station. Most reference station networks require surveyors to be within approximately 10 km (6.2 miles) of one of the network base stations; anything further and the user may be vulnerable to reduced accuracy and performance. With VRS, a user connects into the system using a wireless connection and the software lets the user operate as though there is a base station close by their location. As a result, errors related to distance from the base station are significantly reduced. VRS thus enables users to achieve RTK precision over much greater distances with fewer base stations.

“We’re excited about the prospect of a regional VRS network in terms of monitoring and being able to serve up GPS correction factors,” MacOnie says. “It makes GPS surveying available to every surveyor who can afford a single GPS unit instead of needing two, or even three, to operate. We feel that’s very important, both for GPS technology, and the survey community at large.”

It would also put all area agencies on a regional reference frame, using the same units and coordinate space. MacOnie believes benefits will be extensive.

“In most major metro areas there are projects that go on for miles: highway corridors, pipelines, communication infrastructures and mass transit systems,” he says. “If we’re all on the same system, everyone involved would be able to relate to the network and therefore to each other.”

GPSBase software enables the city of Renton’s reference station to be easily connected to other networks and makes GPS reference stations easily scalable. Trimble’s GPSNet software enables single CORS to be combined into a multi-station GPS reference station network, with each CORS connected in real-time to a central operational facility. Trimble’s RTKNet software adds the benefits of VRS functionality to networks covered by GPSNet. Renton’s reference station has been connected to SPU’s VRS since the beginning; while MacOnie is not using the larger network yet, he hopes to do so by the end of 2003.

“By using the Seattle VRS network I can have greater confidence in the accuracy of my values, because the system monitors all base stations continually to determine the quality of information,” he says.

While the PRSU network is still in its early stages, others in Washington are already looking to a statewide network. The Spatial Reference Center of Washington (SRCW) is a statewide consortium of state agencies and organizations ultimately focused on building a 75-station CORS network. The network, which will include the PRSU network as well as others, will provide 70-km (43.5-mile) baseline coverage for any location in the state. The design will also make it possible to offer an RTK component in the future. SRCW’s network development will also benefit from federal funds secured to implement a Height Modernization program with guidance from the National Geodetic Survey (NGS), according to Schrock.

So it is that Washington’s networking vision is growing from a city’s single reference station to a statewide network able to serve all those needing GPS correction data for their applications in the future.

Joyce Hildebrandt and Ethan Owyang of Seattle Public Utilities observe an NGS monument outside of Seattle using the PRSU VRS network and a Trimble GPS Total Station 5700.

VRS Supports Pioneering North Carolina

On the other side of the country, North Carolina is making its own networking vision a reality.

As director of North Carolina’s Geodetic Survey Section (NCGS), Gary Thompson, PLS, along with major support from the North Carolina Department of Transportation (NCDOT), has led the development of a GPS network for the state that currently includes 18 base stations over a 4,290 square mile (11,110 sq km) area—with full state coverage as the goal.

Among the first states to implement a VRS, North Carolina is a natural tech leader. Along with its historic status as ‘First in Flight,’ where Orville and Wilbur Wright made their first flight from North Carolina’s Outer Banks near Kitty Hawk, North Carolina has made a name for itself by keeping a fast pace in survey as well.

An early adopter of EDM technology since 1982, North Carolina was also an early user of data collectors (1986) as well as GPS survey equipment (1988), according to Charlie Brown, PE, PLS, the state location survey engineer for NCDOT, who works closely with Thompson on the VRS network. Even earlier, North Carolina was the state that initially developed the State Plane Coordinate System through the vision of NCDOT engineers and surveyors.

“We like to benefit from using new technology,” Brown says. “We don’t just go out and try the latest and greatest because it’s the latest and greatest. We’re expected to research and quantify the benefits. Upper management has been very supportive of new technology and where it would take us.”

And the new VRS network is taking the state whose mountain regions are known as “Land of the Sky” back to the sky—this time to navigate on the ground.

From CORS to VRS

North Carolina is no stranger to the benefits of reference station technology. North Carolina has a 24-station CORS network for post-processing located throughout the state. Initially, the impetus for a reference station network came from the state’s mapping and GIS needs, mainly for 1 to 5 meter applications. So in 1992, NCGS installed three Trimble CORS base stations and expanded the network from there, converting the base stations to survey grade receivers as the need for centimeter-grade post-processing became apparent. The network is used by both the public and private sectors for resource mapping and surveying; the network is also part of the NGS CORS network.

But as GPS use proliferated throughout the state, including for NGS’ Height Modernization program, “we saw the need of our DOT and private sector surveyors for real-time methods,” Thompson says. “We had post-processing with CORS, and VRS was the next step.”

North Carolina’s history of floods also moved the GPS network forward. After Hurricane Floyd in 1999 revealed the state’s flood hazard map data to be limited, North Carolina became the first Cooperating Technical State (CTS) with the Federal Emergency Management Agency (FEMA) to develop National Flood Insurance Program (NFIP) digital Flood Insurance Rate Maps (FIRM) for all areas. NCGS is the technical lead for the survey/mapping part of the project; the maps will help predict where floods might occur in each area. The NC Center for Geographic Information & Analysis (CGIA) and NC floodplain mapping are also involved with the goal of remapping 100 counties by 2007. All the surveying and mapping elements of the project use GPS; the state’s CORS and new VRS networks are “critical elements” in supporting the project, according to Thompson.

“GPS plays an integral part in the floodplain mapping program by providing accurate data as well as important cost savings for us,” Thompson says. “Our VRS network funding benefitted from that need.”

The North Carolina VRS system was initially set up in early 2002 with five CORS base stations in the Triangle Research Park area, covering Raleigh-Durham and Chapel Hill. After a year of testing, the network was expanded to 13 additional CORS sites from Raleigh westward to Charlotte. The plan is to cover areas that undergo major development first: the VRS will expand along North Carolina’s interstate system and spread out from there. In each phase, NCGS has partnered with local, state and federal agencies to install base stations in their facilities.

“The power of VRS is that it extends the range of RTK and gives our users RTK quality positioning as if a base station is installed right next to them,” Thompson says. “The pilot showed us we’d get the accuracy and precision we wanted: 1 to 3 cm max for horizontal and 2 to 5 cm max for vertical at the 95 percent (2 sigma) confidence level. Now, for any application where you need real-time positioning accuracy, VRS will play a role.”

Tim Merrill, technical trainer for NCDOT Location & Surveys Unit, works with a Trimble 5800 RTK GPS surveying system and VRS locating photo ties in Corolla, N.C.

Saving Time and Money

The NCDOT is gaining from the GPS network as well. VRS provides the DOT with a solid control network for survey stakeout crews as well as highway construction crews. It also puts everyone on the same network system starting out, making project planning and communication more productive.

“We at the DOT see a major benefit of having a network like this,” Brown says. “With VRS you can turn a base station into a rover and it becomes a more productive piece of equipment—and you can do GPS with one person. So surveyors are major recipients of the VRS system.”

NCDOT surveyor Emory Kincaid, PLS, wanted VRS for its time-savings.

“My big push for VRS has been to eliminate the time and need for setting up a base station,” says Kincaid, NCDOT project data support engineer.

Prior to VRS, NCDOT surveyors used the State’s High Accuracy Reference Network (HARN) for all control, bringing it into projects statically. “It was time-consuming having to traverse a long distance just to bring control in,” he says. “With GPS we could bring in control quickly; VRS is extending that process more. Instead of a few hours, we’re down to a few minutes to get position and control. And we don’t have to set up a base station either—that easily saves us another hour or more a day.”

The NCDOT purchased 52 Trimble 5800 RTK GPS surveying systems and 23 5700 RTK GPS surveying systems; the 5700 systems are being used as conventional RTK base stations until the VRS network attains full coverage. The 5800s are being used by both the NCDOT’s location surveys and construction units as rovers.

“We know different parts of the state will gain complete VRS coverage quicker than others,” Kincaid says. “So we’re using the 5700s first as base stations and then rovers. It’s working well for us.”

North Carolina’s VRS system will be available to all users who pay a single one-time fee. The system allows for an unlimited number of users; NCGS oversees the development and maintenance of the network and will assess the one-time fee.

“VRS has allowed us to take the network step-by-step where we want it to go,” Thompson says. “It’s been very cost-effective for us.”

His advice?

“As we say in the south, don’t bite off too much at once,” he says. “Take it step-by-step and have a plan. Look where your needs are, where your infrastructure is, and decide what applications you want to use it for. Then work closely with local government to determine needs—and go build your system.”

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