By most accounts, South Carolina is a mid-sized state, ranking 40th in land area and 26th in population in the United States. In spite of its size, South Carolina is a geodetic powerhouse that ranks among the best in the nation at providing positioning services and geodetic control for its citizens.
Much of the success comes from the South Carolina Geodetic Survey (SCGS), which is responsible for providing geodetic and land information to the state’s citizens and businesses. SCGS has worked for more than a decade to develop and enlarge the state’s geodetic framework of control monuments and Continuously Operating Reference Stations (CORS). Recently, the agency implemented a real-time network (RTN) of GNSS receivers to provide centimeter-level positioning for surveyors and other professionals working throughout the state. The new network has expanded the use of geodetic control and opened the doors to a new wave of users and applications.
The BeginningsThe driving force for improving South Carolina’s framework came from SCGS Chief Dr. Lew Lapine and SCGS Program Manager Matt Wellslager. A well-known proponent of geodetic control for surveying, Lapine came to SCGS after a storied career at the U.S. National Geodetic Survey (NGS). When he arrived in 1998, Lapine found ample opportunities for improving South Carolina’s geodetic control system. Rapid development across the state had destroyed many of the state’s conventional control and geodetic markers, and there were only a handful of CORS in operation. Ironically, the state’s strong economic growth was creating demand for surveying and construction, which relied on local control points.
Nine months after Lapine arrived, SCGS established its first CORS using a Trimble GPS Reference Station transferred from a decommissioned CORS in Maryland. The early CORS were concentrated in populated areas, and none was broadcasting corrections for real-time kinematic surveying (RTK).
Wellslager, a long-time SCGS employee, had interrupted his SCGS career to work for a private firm in Massachusetts. He spent four years working in New England, where he learned what RTK could do for surveyors. He was ready to get the same ball rolling in South Carolina.
The kicker came from the South Carolina Department of Transportation (SCDOT). SCDOT Director of Construction Danny Shealy had seen real-time networks in other states and wanted to use the technology for heavy machinery and 3D control in road construction. SCDOT offered SCGS funding to help build and manage an RTN for the state. Following an extensive evaluation and competitive bid, SCGS selected Trimble VRS technology for its statewide virtual reference network.
Construction began in 2007, with about 18 months required to install 42 Trimble NetR5 Reference Stations with Trimble Zephyr Geodetic 2 Antennas. Most of the reference stations were installed at SCDOT facilities selected to provide suitable geometry for the GNSS network. The DOT locations also had security, power and communications infrastructure already in place.
The network, known as the South Carolina Virtual Reference System (SCVRS), has since grown to 51 reference stations by incorporating data from nine stations in North Carolina. The entire network is controlled by Trimble GPSNet Software, with Wellslager managing day-to-day operations while Lapine concentrates on network integrity and geodetic aspects. To access the network, users purchase an annual subscription for each roving receiver. The subscriptions pay for maintenance and updates to the system’s software and will accumulate to pay for new or replacement receivers in the future.
Gaining MomentumAlthough SCDOT was aware of the value of the RTN, many in the state’s surveying community were not. Lapine was concerned that surveyors were not fully aware of the importance of geodetic data; Wellslager saw it simply as a missed opportunity. “When I was in Massachusetts, it was eye-opening to see how often surveyors there used GPS, even in the wetlands and forested areas,” he says. “After I moved back to SCGS, we wanted to give South Carolina surveyors a taste of what the real-time network could do for them.”
To get the word out, Lapine and Wellslager gave presentations on the benefits of RTN and RTK at local chapter meetings of the South Carolina Society of Professional Land Surveyors (SCSPLS). Then, to attract an initial base of users, SCVRS didn’t charge any access or subscription fees in its first six months of operation. Their efforts paid off, and RTK use is on the rise in South Carolina. Wellslager says that there are roughly 250 subscribers to SCVRS each year, with 25 to 30 users accessing the system on an average day. Subscribers include in-state firms as well as outside construction and engineering companies that purchase subscriptions for projects within the state.
SCVRS has made work faster and easier for many traditional surveying businesses. In Charleston, family-owned Robert Frank Surveying provides surveying services to private clients and government agencies. Lee Frank, LS, says the company owns a single Trimble 5800 GPS Receiver and began using RTK for single-base surveys tied to a CORS located in Charleston. “I did my homework before buying our system,” Frank says, “and I went into RTK with a good understanding of what GPS could and could not do for us.” He gained experience from using the CORS, and when SCVRS came online, he was ready to take advantage of it.
To illustrate SCVRS’s value, Frank points to a rule implemented by the South Carolina Office of Ocean and Coastal Resource Management (OCRM). Frank’s company does a lot of dock location surveys, and OCRM requires all dock surveys to provide State Plane Coordinates (SPC). With SCVRS, Frank initializes his surveys directly in SPC and visits control monuments only for checking. “The SCGS has given us an abundance of monuments,” he says. “For the height modernization project, they coordinated with the state society and did a super job in putting points where they were needed.”
Frank says SCGS provides GPX files containing coordinates for the state’s control monuments. He loads that data into a handheld GPS navigation device, which then provides directions to the nearest monument. “With SCVRS, I tie all of my surveys to state plane and include it in all my proposals,” Frank says. “It adds to my level of professionalism and conveys the impression that I am providing a premium service to my clients.”
In addition to the dock work, Frank uses RTK and SCVRS for boundary surveys, flood certificates, land disturbance permits and FEMA certifications. When working in areas where forest canopy makes RTK difficult, Frank uses his GPS system to set pairs of intervisible points for his total station.
Frank also sees a future payoff from the RTN. Whenever he surveys a monument, it is automatically tied into SPC. If the monument is destroyed or altered, Frank says the SPC could help re-establish the location where the monument was originally found.
Coastal SurveyingSouth Carolina’s coastline is a constant source of work for Phillip McKee, a senior technical associate and principal at Coastal Science & Engineering Inc. in Columbia. Many of McKee’s projects call for survey lines that run across the beach and continue out into the ocean, and he combines land and marine surveying techniques with RTK to gather information and create maps and profiles. When working on land, McKee uses a Trimble R8 GNSS System with a Trimble TSC2 Controller running Trimble Survey Controller Software. On the boat, he uses the Trimble R8 GNSS to provide positioning data for the vessel’s echo sounder.
McKee, who has used GNSS and RTK for several years, says that SCVRS is an enormous time saver. “When working on the ocean, it may sometimes take a two-hour run in the boat just to get to the jobsite,” he explains. “Prior to the SCVRS, we would set up a base station that was five miles away from the work site and then run down the coast in the boat to get to our site. If you have a long coastline, then you have to get out of the boat and swim ashore to move or take the base station down. At the end of the day, nobody wants to go take down the base station--you’re just worn out.”
Because many of his projects require periodic resurveys, McKee must be sure that the horizontal and vertical datum is consistent from one visit to the next. Prior to SCVRS, McKee relied on NGS monuments for control and needed to set up base stations for each survey project. “On the coast, many of the old 1st order monuments have been washed out,” he says. “With SCVRS, I don’t have to worry about that. There’s usually enough control for checking in, and I don’t worry about finding points to start a survey. When I check into a monument, I’m usually happy with the results.” Describing an ongoing project at Edisto Beach, McKee says that they typically check in to the control with vertical accuracy of better than 0.1 feet (3 cm).
Agriculture: A Growing Opportunity for SCVRSAt Circle W Farms in Orangeburg County, Landrum Weathers raises corn, soybeans and peanuts on a family-owned farm that covers roughly 2,000 acres (810 hectares). Weathers has been using GPS to steer the farm’s equipment since 2004, when he installed one of the state’s first Trimble AgGPS Autopilot systems on a Circle W tractor.
Today, Weathers uses SCVRS as the basis for precise steering for all of the planting on the farm. With position accuracy of less than 1 inch (2 cm), Weathers operates his machinery to plant rows that are 30 inches (76 cm) apart, compared to 36 to 38 inches (91 to 97 cm) on farms not using precise guidance. According to Weathers, precise steering is especially valuable in managing the Circle W peanut crop. “Peanut bushes are planted close together and have very dense foliage,” he says. “When they are mature and ready for harvest, it is impossible to retrace the rows without RTK and SCVRS.” Weathers said that the system increases his harvest in peanut farming by 200 to 400 pounds (90 to 180 kilograms) per acre. In addition to increased yields, Weathers noted that the system provides savings in fertilizer and fuel consumption.
Weathers credits SCVRS with providing good service to keep his operation going. He recalls a recent day when the network stopped working late in the afternoon. Weathers called Wellslager, who was already at work on the problem and had it fixed in 15 minutes. Weathers also appreciates the e-mail status updates provided by SCGS, which let him plan his work around the network’s scheduled outages for maintenance and upgrades.
Smooth OperationsFrom the beginning, Lapine has emphasized the importance of tight control and monitoring. Of the 42 SCVRS points in South Carolina, nine are fixed as fiducial stations and operate as part of the NGS CORS system. To keep track of the RTN accuracy, Lapine uses Trimble Integrity Manager Software to conduct regular assessments of the network. He has broken the network into several smaller sub-networks and set up filters in the software to detect unexpected changes in antenna positions. Lapine says that the software enables him to detect the motion of a GNSS antenna mounted atop a 90-foot (27-m) spun concrete pole on Hilton Head Island. As the sun moves across the sky each day, Lapine can see 0.07 to 0.1 foot (2 to 3 cm) movement of the antenna as the pole flexes due to solar heating. By watching the amplitude of the pole’s motion, he even can tell whether clouds in the area are reducing the heating on the pole.
Lapine believes that it is important to have separate conventional (or “passive”) monuments in addition to the RTN and plans to provide enough control to make it convenient for optical and GNSS surveyors to check their work. Lapine also intends to provide a dense network of vertical benchmarks (see below, “GNSS and Height Modernization in South Carolina”). “It’s an ideal combination,” he says. “We have a real-time network of more than 50 stations operating at the centimeter level, and it agrees exceptionally well with the passive monuments. And we have demonstrated that it’s possible to get good positions and heights using an RTN. As a result, our clients can use the CORS, RTN and local monuments in a hybrid fashion.”
Prepared for GrowthLee Frank estimates that 20 to 25 percent of the surveyors in his area are using RTK with SCVRS, and he expects the number to increase. Wellslager agrees, noting that the community is becoming more comfortable with the technology and that surveyors see it as a way to work faster while tying directly to state plane coordinates. At Circle W Farms, Weathers is also bullish. “Many farmers don’t like the added cost and inconvenience of running their own base stations,” he said. “SCVRS will make it more enticing for people to adopt RTK in agriculture.”
While Wellslager and Lapine see strong growth coming from agricultural users, they also expect SCVRS will serve an increasing number of GIS applications with real-time positioning at centimeter and decimeter levels. They point to wetlands surveys, utilities, stormwater management and academia as major users of GIS in the state. Wellslager says the network is set up to handle up to 200 real-time users at one time and can expand beyond that when needed.
While South Carolina’s RTN infrastructure is primed to keep pace as technology improves, its users must stay active as well. “You’ve got to push to keep up with technology, because it will change faster than you can,” Wellslager says. “If you don’t grasp technology and try to learn it as you go--and I can feel it blowing by me on a daily basis--then you are at a disadvantage.”
GNSS and Height Modernization in South CarolinaWhen Lew Lapine arrived at SCGS in 1998, height modernization was already an issue. Using procedures defined by NGS, SCGS crews are working to establish a height reference network tied to the existing CORS and South Carolina High Accuracy Reference Network (HARN). Lapine has set a goal to establish roughly 250 monuments in each of the state’s 46 counties. The NGS procedures call for two one-hour occupations on different days with GPS followed by post processing and adjustment of the resulting vectors. The results are put into bluebook format and submitted to NGS. “Out of the 250 stations, at least 100 are new,” Lapine says. “For the other 150, we will upgrade existing horizontal stations to create a network of very modern passive monuments.” Because South Carolina is a coastal state, Lapine wants to recover as many of the old tidal benchmarks as possible. Many lie on the barrier islands or in marshes, and have never been connected by levels. With the height modernization, all of the old benchmarks will be on the same datum with the rest of the state.
Today, SCGS is taking heights to the next level with RTK. In addition to the NGS procedures, SCGS crews ran several surveys using RTK and SCVRS to measure heights on the known points. To conduct the work, the crews used Trimble R7 GNSS Receivers with Trimble Zephyr GNSS antennas to collect two five-minute RTK sessions at each point. The sessions were taken roughly 27 hours apart, which provided different satellite geometry and sky conditions for the observations.
The results are impressive. The ellipsoid heights between static and RTK agreed at the 2-centimeter level 97 percent of the time. “With two five-minute sessions,” Lapine says, “we were able to do the same thing--with no post processing--that it takes months to do with the NGS protocol.” It’s a big step toward the SCGS goal of providing high-quality heights with GNSS and SCVRS.