A New Geospatial World
August 1, 2006
Dubbed the "Incredible County" by its residents, Warren County, Ohio between Dayton and Cincinnati is one of the fastest-growing counties in the nation. With a population of 189,276, the county covers 408.9 square miles including both suburban and rural communities. More than 10 years ago, the county developed a Geographic Information System (GIS) from its paper parcel base maps. Populated with mapping-grade accurate data, the GIS developed in ESRI (Redlands, Calif.) ArcINFO was cutting-edge and sufficient for that time. However, it provided no utility information--crucial information every county needs.
In mid-2003, the county's water and sewer department started its own GIS project that allowed workers to avoid the inconvenient process of obtaining water and sanitary sewer infrastructure information stored on 4,500 separate 24" x 36" source drawings in numerous file racks. What they didn't realize at the outset of the GIS project was that this improvement would catapult them into the future of GIS technology as well.
Developing A New Geospatial WorldOver the past decade, the use of GIS to accurately record, track and manage assets for utilities and other public work organizations' assets has exploded. Cities, counties and state agencies across the nation have come to rely on GIS for more efficient and effective asset management, maintenance, budgeting, analysis tasks and better informed decision-making. Predominantly populated by mapping-grade geospatial data, most GIS have provided submeter level accuracy. This mapping-grade data generally remains sufficient for many municipal purposes such as mapping hydrants or trees--but several other needs call for greater accuracies.
Today, many governmental agencies are requiring centimeter-level (survey-grade) accuracy, especially for utility infrastructure, either as an upgrade or to initially establish a GIS. Government regulations such as the Environmental Protection Agency's (EPA) National Pollutant Discharge Elimination System (NPDES) regulations, Sanitary Sewer Overflow/Capacity, Management, Operation and Maintenance (SSO/CMOM) regulations and the Governmental Accounting Standards Board (GASB) Statement 34 have propelled the demand for higher accuracy data. Engineering applications such as sewer and water modeling require higher accuracy data than the elevation information that mapping-grade data provides. In addition, jurisdictions are increasingly sharing data across geo-political lines; if one jurisdiction has high-accuracy GIS data based on a coordinate reference frame (datum) and another area has mapping-grade data based on a different datum, it becomes challenging to share data.
While high-accuracy geospatial data has historically been more costly to produce, advances in GPS technology have made survey-grade data increasingly cost-effective, thus improving GIS. Today, new options for gaining high-accuracy data more efficiently and effectively are available across the nation. It's a new world, geospatially.
Project SetupWith the goal of providing inventory accounting and enhancing the operations and maintenance of the water and sanitary sewer systems, Warren County System Data Manager Laura Gray knew the department needed highly accurate elevation data for modeling purposes. Given these parameters, the department decided to utilize real-time kinematic (RTK) GPS data collection, a method that provides centimeter accurate data in real time using RTK GPS survey equipment.
Woolpert Inc. of Dayton, Ohio, an engineering and surveying firm with 24 offices nationwide, had worked with Warren County for years, developing both the initial countywide GIS as well as a countywide first-order geodetic control network including 150 control monumentation pairs. Woolpert had also developed current orthophotography data including planimetric features (parcels, centerlines, etc.) and LiDAR data sets. The company began the GIS inventory project for Warren County by compiling all county source maps, construction drawings and as-builts to construct a document management system (DMS) used to index, scan, store and access all source documents. By using the base map, orthophotography and scanned documents, Woolpert surveyors created a digitized map of the utility network. Utility features were digitized and linked to an ESRI Spatial Database Engine (SDE), creating an enterprise-wide repository for spatial and attribute data. This allowed a user to click on a feature to pull up attribute information including the scanned historical information. Due to this preparation work, the consultant was able to document approximately 80 percent of the water and sanitary sewer system prior to field work. After uploading the digitized map into Trimble (Sunnyvale, Calif.) TSCe data collectors, it was time to go to the field.
Field WorkIn the field, Woolpert surveyors, equipped with the Trimble handheld field data collectors, navigated and located the digitized features and lines from the GIS. Using Trimble 5800 RTK GPS rovers, they acquired high-accuracy location and above-ground elevation data as well as additional attribute information such as new or changed structures. They also documented those areas needing additional surveying where features could not be found, were inaccessible, or where obstructions limited satellite signal reception.
Improvements through GPS InfrastructureFor years, the main option for achieving survey-grade accuracy data on projects had been to use an RTK GPS rover and portable base station; in fact, this was the approach used for the first part of the project. In the last few years, however, RTK GPS infrastructure technology has offered a new solution to projects. This technology is increasingly being implemented across the nation and the world, dramatically reducing the cost and increasing the efficiency of collecting survey-grade geospatial data. Whether a single permanent reference station or a complete Trimble VRS (Virtual Reference Station)* network, RTK GPS infrastructure technology is being used to create highly accurate GIS that provide an increasing number of benefits.
The geospatial data gained from a VRS network benefits GIS with greater quality control and higher data accuracy at longer distances. With conventional RTK, the farther users get from a reference station in the field, the more susceptible they become to reduced accuracy and performance due to ionospheric and tropospheric factors, or PPM errors. With a Trimble VRS infrastructure, network software provides a fully modeled solution that factors in potential PPM errors. Users connect into the system using a wireless connection; the software acknowledges the user's field positions and allows him to operate as though there is a reference station--a virtual reference station--right next to his rover. As a result, the PPM error is eliminated or significantly reduced, enabling users to achieve RTK precision over much greater distances with fewer reference stations.
For the user, GPS infrastructure reduces time, labor and the need for a GPS base station, making high-accuracy data more viable and affordable. It also provides ubiquitous positioning over a large area; a common datum for cross-jurisdiction data sharing; and a decreased learning curve for those with little high-accuracy surveying experience. Many municipalities find it effective to set up a single reference station when an entire VRS network may not be cost-effective. But when a Trimble VRS network is implemented, VRS technology increases network range while minimizing the number of reference stations required to cover large areas.
VRS ImpactAt the start of the Warren County Water and Sewer Department's project, the Ohio Department of Transportation (ODOT) was in the process of implementing the nation's first statewide RTK GPS infrastructure using Trimble VRS technology. With 52 RTK GPS reference stations, the VRS network provides a common coordinate system for the 44,828 square-mile state. For Warren County, the Trimble VRS network would offer a cost-effective way of collecting high-accuracy GPS data for part of the initial project as well as for subsequent updates.
Because the Trimble VRS network was not available until the latter part of the project, Woolpert was able to quantify the cost-savings provided by the infrastructure solution. Often, finding a known point, setting up a base station, validating its accuracy and returning to the survey site (and the reverse at day's end) can use up to two hours per day. And, unless the base is visible to the user, most users leave someone at the base to ward off theft. Because a portable base station's range is limited to its radio range--3-5 miles maximum--a new location is often required as the project moves throughout the county. In addition, the possibility of error increases with manual setup, meaning quality control checks become even more critical and time-consuming if errors are found.
With a VRS infrastructure, however, the user simply goes to the survey site with an RTK GPS rover, accesses the network via cell phone and is ready to collect data. As with any technical equipment, quality control and confidence checks remain part of the process. But once confidence in the system is gained, it's basically "dial in and go."
Once implemented, Warren County found that using the VRS network for the GIS project resulted in a 15-20 percent increase in productivity, equipment cost reduction of 33 percent and personnel cost reduction of 18 percent--an overall potential project savings of 25 percent. In addition, the county gained the continuing availability of the network for the county for GIS upkeep.
VRS AccuracyThe ODOT VRS system is part of the National Geodetic Survey (NGS) National Continuously Operating Reference Station (CORS) network, which offers the most accurate and up-to-date horizontal data available. The system also uses ellipsoidal height for vertical corrections. Because much of the Warren County documentation was based on NAD 83, 1986 adjustment (horizontal) and NAVD 88 datum (vertical), the consultant developed a countywide site calibration prior to any field observations to bring the VRS system to the localized project datum. Because the consultants had implemented the initial control network, Woolpert surveyors were able to check the VRS network results against their control network. Results were consistently on the order of 1-2 cm orthometric height when site calibration was applied. Without site calibration, results were in the 4-5 cm range.
At project end, Woolpert provided the county with a high-accuracy water and sanitary sewer GIS database, providing a unified utility network map that is linked to a DMS. Woolpert also supplied the high-accuracy RTK GPS equipment purchased for the project, trained the GIS staff in the collection of true features and developed a procedural manual specific to features it required. The consultant also trained the county in traditional RTK base station setup for backup use. And even where the surveyors couldn't get survey-grade elevations on some assets due to obscured GPS areas (trees, buildings, etc.), Woolpert was able to utilize tools in ESRI's ArcInfo software and the countywide DTM to interpolate elevations from these locations to develop a comprehensive X,Y,Z map.
GIS BenefitsThe new high-accuracy GIS enables the water and sewer department to more precisely locate valves in response to a water main break, to monitor any breakage and maintenance of valves and hydrants, and to provide precise location information of all utilities to expedite maintenance procedures. The accurate system provides GIS elevation data that enables modeling and other engineering applications--a huge issue for utility management. Knowing where the underground assets are located helps to prevent unnecessary utility breaks or conflicts with any future development in the area. The department is continuing to inventory its water and sanitary sewer utilities as new subdivisions are built, and it intends to make the GIS an integral part of its operations and planning.
The larger county GIS is also benefiting from the water and sewer GIS--and the RTK GPS equipment. As new high-accuracy subdivision data is added to the county database, Warren County GIS Coordinator Dawn Johnson is using it as a pilot study to compare the accuracy of county digitized data with the highly accurate water and sewer data. In some extreme cases, the new data is causing shifts to as much as 12 ft in the larger GIS because the larger GIS was based on paper parcel base maps. A county GIS steering committee is currently considering the possibility of shifting the existing parcel data set; if so, the larger county and the water and sewer department would be able to more effectively share data in the future.
Warren County's FutureInitial testing of the data identified areas in the utility model that produced "sinks" (sinkholes). While the high-accuracy data collected is sufficient for most modeling requirements, precise sewer modeling invert elevations are needed. For this initial project, full inspection of every manhole and pipe invert was not verified; instead, the top-of-manhole elevation was used along with an estimated depth of manhole from construction drawings to gain "assumed" or "derived" invert information. These construction drawings are not always representative of true utility features (unlike the more accurate as-built data would provide). So when the data is used in modeling applications, all discrepancies between original source documents and GPS data become obvious.
In the future, the county may include a full inspection analysis and condition assessment of manholes to get true invert elevation data for its system. This would also include densifying the GIS network by finding the additional features not accessible for this project. Woolpert's Smart Surveyor software linked with RTK GPS survey equipment enables the user to go directly to the structures and features on the base map and update it to survey-grade accuracy. In addition, the user can connect all utility lines; assess pipe condition, sizes and materials; gain accurate invert data; and note any new attributes or changes in the field. The software runs numerous quality control and validation tools in real-time, making sure the data is accurate and thereby dramatically reducing errors in the GIS.
The county is also considering implementing a work order management system that will enable maintenance crew members to access the GIS database in real-time in the field. For any customer service order, maintenance crews can access and resolve the order in the field instead of returning to the office, significantly decreasing labor time and increasing productivity.
The effective approach to updating the Warren County water and sewer system using RTK GPS equipment and VRS technology proved successful in integrating the high accuracy data into the county's existing GIS. By streamlining the data collection workflow, the Woolpert/Trimble VRS solution has also ensured that the county's utility data will continue to be updated.
*Trimble was the original provider of a VRS network solution and has a trademark on the term VRS; today, the term VRS is used almost universally for various network configurations.
This article originally appeared in Underground Infrastructure Management magazine (March/April 2006). It is being reprinted, with additional information, with permission.