A Monumental Effort

January 25, 2011
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For the past several years, surveyors in Wisconsin have kept pace with the national trend toward increased use of GNSS for survey positioning. Wisconsin surveyors started adopting GPS technology in the mid-1980s, and the practice has been growing ever since. 

Jim Prehn, PLS, of Spatial Data Surveys, uses WISCORS to collect topographic data in a low-lying area near Verona, Wis., to provide a storm water management consultant with surface water collection capacity details.


For the past several years, surveyors in Wisconsin have kept pace with the national trend toward increased use of GNSS for survey positioning. Wisconsin surveyors started adopting GPS technology in the mid-1980s, and the practice has been growing ever since. With the advent of real-time kinematic (RTK) GPS in the late 1990s, surveyors, GIS professionals and others have embraced the speed and accuracy provided by modern GPS and GNSS equipment. Like most positioning activities, RTK relies on accurate control. Here in Wisconsin, a team within the Geodetic Surveys Unit of the Wisconsin Department of Transportation (WisDOT) is charged with establishing and maintaining the state’s geodetic control framework.

In 1998, work began on the Wisconsin Height Modernization Program (WI-HMP), a geodetic infrastructure comprising four levels of horizontal and vertical control. The state’s high accuracy reference network (HARN), which carries the highest accuracy, is made up of stations roughly 31 miles (50 km) apart. At the next level of accuracy, the state’s primary base stations are positioned at a spacing of roughly 15 miles (25 km). Secondary base stations are spaced at 7 to 9 miles (12 to 15 km), with local network stations at still smaller intervals. The program calls for GPS or GNSS occupations of primary base, secondary base and local network stations together with differential leveling to the HARN and primary base stations. The work, which started in the southern part of the state and has progressed to the northwest, is currently about two-thirds complete. 

Prehn checks NGS control points at the beginning of each day and as he moves along the project.

The height modernization surveys exposed the need for better geodetic control in the state. In 2006, there were a few continuously operating reference stations (CORS) in the state, but their spacing and availability was inconsistent and provided no widespread coverage for RTK. As an offshoot to the height modernization, we began to develop the Wisconsin Continuously Operating Reference Stations (WISCORS) network with the intention of covering the state with real-time, survey-grade GNSS correction data. To test the concept, we conducted a pilot project to set up a temporary real-time network and used RTK to measure a number of points observed during the height modernization project. The measurements were compared against the static observations taken for HMP. The results were excellent in both horizontal position and orthometric heights, and the decision was made to proceed with the real-time network (RTN).

Today, WISCORS is a fully GNSS-capable RTN that enables its users to achieve centimeter-level accuracy using a single GPS or GNSS receiver. As the result of a concentrated effort in 2010, WISCORS has 40 reference stations in operation. Each reference station consists of a Trimble NetR5 GNSS Reference Station and a Trimble Zephyr Geodetic 2 Antenna together with the required power and communications equipment. When the network is fully complete, it will be made up of more than 70 reference stations providing RTN coverage for the entire state.

As we planned WISCORS, we identified five primary aspects to creating a real-time GNSS network (RTN):

• Network geometry. The reference stations must be arranged to provide optimal coverage and accurate positions. An advantage of an RTN is that it can provide survey-grade positions with spacing much greater than single baseline RTK.

• Communications and power. GNSS reference stations must be connected to the Internet to stream the observed data in real-time to the Network Operations Center. In addition to a reliable high-speed Internet connection, each site needs to have reliable electrical power.

• Security and stability. The GNSS antenna at each reference station must be on a permanent, stable mount. Most antennas are installed on dedicated pillars or monuments, and others are attached to buildings.

• System accuracy and reliability. Users depend on WISCORS to provide positioning data seven days a week. We needed to use hardware, software and procedures that minimize downtime and simplify processes for the users.

• Costs. As a publicly funded agency, we must keep costs at a minimum while providing the best possible benefits for the community. We focused on controlling costs for initial installation and ongoing operations of the network. 


A typical WISCORS reference station. The standard design for the pillars reduced cost and installation time.

Early in our planning, we developed a strategy to manage costs without compromising performance. We knew that our system, which uses Trimble VRS technology, would function well with reference stations placed at wider spacing than used for conventional RTK. By installing reference stations farther apart--but still within the manufacturer’s recommended spacing--we could significantly reduce upfront costs as well as gain flexibility in site selection. This allowed us to use network geometry as the primary criteria for selecting locations. Because of this criteria, we will be able reduce the number of reference stations needed to provide the required coverage. With careful planning, we will save 10 to 15 stations over the course of the project.

The decision to use geometry as the prime criteria led to some interesting opportunities. We talked with RTN operators in other states and learned that many networks were constrained in their site selection. For example, some states have installed reference stations only at DOT locations. While this may simplify communications or other aspects, these locations can be suboptimal in terms of geometry or in reducing the number of reference stations. In our approach, we identified the preferred location for reference stations and then worked with local entities to get the stations built.

To establish a reference station, we first used large-scale maps to plan the network geometry and identify potential sites for reference stations. Once the general location of a proposed reference station was known--usually to within a radius of 3 to 5 miles (5 to 8 km)--we would make a reconnaissance visit to the area and look for potential sites. We focused on public facilities rather than private property and looked for civic buildings, fire departments, water treatment plants and similar facilities as potential partner organizations for the network. Our initial contact with a partner was usually informal; we wanted to establish a relationship and make sure that the partner understood the purpose and value of the reference station. From there, we negotiated the requirements and activities for both sides. We developed a standard partner agreement to spell out the needs for physical installations, access, utility service and information technology (IT) support, modifiying it as needed to suit each partner.

Building a real-time network involves at least as much knowledge of IT as geodesy or surveying, and one of our early lessons came from working with the various partners’ IT groups. We realized that to efficiently operate and manage a statewide RTN, we needed bi-directional communication with the reference stations. This required our partners to allow our computer systems access to the reference station located on their networks. At the same time, our partners needed to maintain secure control over their IT infrastructure. Although we could communicate easily with surveying or engineering teams, we lacked the expertise to deal with concerns and terminology of partners’ IT staffs. We quickly learned to have the WISDOT IT staff work with their counterparts at partner organizations to implement the access and security arrangements needed to connect our reference stations to the Internet. After several installations, we had learned how to work effectively with partner IT departments and were able to take on more of the load ourselves.

In some cases, we needed to place reference stations in locations with no existing Internet service. Because of the system’s ability to provide high accuracy with widely spaced reference stations, we decided to carry the cost of installation and monthly fees for Internet service to these sites. Our financial analysis showed that the cost of installing and providing Internet service at a number of remote locations would be less than the cost of buying, installing and maintaining the additional reference stations needed to give higher density. By the time the network is completed, we anticipate that WISCORS will have installed new Internet service for roughly 30 reference station sites.

Charlie Denu of KL Engineering (foreground) and Bob Johnson of Patrick Engineering conduct second order, class I geodetic leveling on an NGS bench mark located on the Hotel Rodgers building in Beaver Dam, Wis., to support the Wisconsin Height Modernization Program.

As the network came online, we increased our efforts to bring users to WISCORS. We conducted seminars and attended surveyor society meetings to spread the word. In the surveying community, we encountered companies familiar with GPS as well as firms that had not yet adopted the technology. Surprisingly, it was difficult to convince even experienced GPS users of WISCORS’ benefits. In addition to concerns about performance and positioning accuracy, many users were concerned about the phone service. Cellular coverage was spotty in many locations, and the cellular providers weren’t familiar with WISCORS. In some cases, the providers would erroneously tell surveyors that RTN wouldn’t work.

One successful tactic was to identify a number of key users who would adopt the RTN technology and become champions for the system. Survey equipment vendors were useful in this effort, as they could visit a surveying company and use that firm’s equipment to operate with WISCORS. WISCORS has no restrictions on the brands of GPS and GNSS rovers, and we encouraged vendors representing all major manufacturers to take advantage of the system. This was an effective approach, as it demonstrated to the surveyors how a minimal investment (often just upgrading firmware or telephone equipment) would translate into saving an hour or more each day.

WISCORS has emerged as an important tool for firms seeking increased productivity and cost control. The survey equipment vendors also contacted firms that had not yet purchased GNSS and explained how WISCORS improves the financial justification of using GNSS. Because WISCORS eliminates the need for a base station, a firm can move into the technology with just one receiver. The network automatically puts the user into the high-accuracy geodetic framework, so costs for training and developing coordinate transformations are reduced. As a result, the cost of entry is effectively halved.

One of the early WISCORS subscribers is Jim Prehn, PLS, of Spatial Data Surveys in Verona. Prehn, who frequently works as a one-person crew, uses GNSS as his primary tool. His major clients are utility companies and consulting engineers. Prehn does a lot of route surveys, and his projects can run for 20 to 40 miles (32 to 64 km). Before WISCORS was available in his area, Prehn used two Trimble R8 GNSS receivers and UHF radios for RTK operations, setting up and moving his RTK base station as he advanced along a route. Shortly after WISCORS became available, Prehn purchased a second data collector and divided his base/rover configuration into two rovers; each rover operates independently using WISCORS. He hires an assistant to operate the second rover on large projects. “Using this approach, I can collect twice the data in half the time,” he says.

Prehn estimates that--even in one-person mode--he saves at least one hour per day by not setting up a base station. “It was an easy decision to adopt WISCORS,” he says. “When you look at the amount of time spent setting up a base station and the limited area that you can cover, it amazes me that more people don’t use the VRS.” Using WISCORS, Prehn can operate freely along an entire route, checking into NGS control points at the beginning of each day and as he moves along the project. Prehn’s positions using the network typically agree with the published control to within 0.03 to 0.1 foot (1 to 3 cm).

Denu (left foreground) and Bob Johnson (far right) along with Carrie Johnson of KL Engineering conduct second order, class I geodetic leveling on an NGS bench mark located on the Chamber of Commerce building in Beaver Dam.

At Northeast Wisconsin Technical College (NWTC) in Green Bay, WISCORS has become part of the surveying curriculum. Rick VanGoethem, LS, an instructor in the Civil Engineering Technology department, said that WISCORS service came to Green Bay in 2008. By spring of 2009, VanGoethem had incorporated the use of the network into the college’s coursework. “WISCORS is part of the progression in surveying,” he says, “and it’s important to expose students to the technologies they will be using once they graduate.”

VanGoethem’s courses make sure the surveyors understand that WISCORS is not a magic box. Students start with static post-processed GPS and then move to fast static techniques and RTK. Throughout the courses, VanGoethem emphasizes the need for rigorous checking and verification, even when using WISCORS. “I’m concerned that people are taking shortcuts without understanding the risks,” he says. “Even with a good, reliable system like WISCORS, they need to understand the technology and use a rigorous approach.”

WisDOT continues to work on height modernization as construction of new WISCORS reference stations moves forward. The goal is to establish orthometric heights with accuracy of 0.06 foot (2 cm) or better on primary, secondary and local passive markers throughout the state. Work is completed and published for nearly 29,000 square miles (75,000 km2), and GNSS field work is complete on an additional 13,800 square miles (35,700 km2). Much of the leveling work is done by contractors; Diane Arendt, the third member of our team, manages the leveling and GPS contracts and provides technical support to the passive network.

The height modernization has created tight ties between WISCORS and the existing HARN stations as well as the primary, secondary and local monuments. As a result, the HMP benefits anyone that needs accurate elevation or positions in their work. For example, the modern height data can result in more accurate flood plain information. A study in Winnebago County showed more than 1,000 structures were incorrectly included in old floodplain maps. By using the improved accuracy, the owners of these structures were freed from the financial burden of purchasing flood insurance.

WISCORS currently has more than 625 subscribers. On a typical day, 40 to 50 users will be accessing the network at a given time; the system can also expand to support more users as demand continues to grow. The system’s users include surveyors, engineering firms, construction companies, universities and municipal organizations. One of the most important user segments is the farming community, where the use of RTK for precision agriculture is growing rapidly. By providing RTK coverage in farmland regions, WISCORS benefits are reaching a new, broad range of users. As the use of WISCORS continues to expand, Wisconsin citizens will enjoy the benefits of higher productivity and lower costs in an array of industries based on reliable, accurate positioning.

The Wisconsin State Cartographer's Office

While several other states have similar sources for GIS or mapping information, Wisconsin provides geodetic data along with more general interest information. WISCORS operates a website where users can download RINEX data for post processing and receive information about tracking conditions and network status. To provide access to a broader range of positioning information, the WISCORS team works closely with the Wisconsin State Cartographer’s Office (SCO), which operates a website and information center from its headquarters at the University of Wisconsin in Madison.

The purpose of SCO is to gather and disseminate information for use in surveying, mapping and other geographic-related applications. The office acts as a liaison between public and private organizations that provide and use geospatial data. SCO’s information sources include the Department of Natural Resources, municipal and county GIS departments, universities and consortiums. It provides extensive information on geodetic control in the state including WISDOT monuments, NGS stations and other control. WISCORS users can go to the SCO website to obtain information on Public Land Survey System (PLSS) corners, county coordinate reference systems and the height modernization program.

According to Wisconsin State Cartographer Howard Veregin, SCO fields frequent inquiries for geodetic control, aerial photography and GIS information. Veregin said that the office is unique in its ability to deliver information to positioning professionals. “The user community in Wisconsin has become accustomed to turning to SCO for geographic information,” he says. “No other source in the state has the federal, state and local control consolidated into a single free application.”

For more information about the Wisconsin SCO, visit www.sco.wisc.edu.

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