Field of beams
Until just a few years ago, the Geographic Information Systems (GIS) department in Sioux Falls, S.D., had not used its Global Positioning System (GPS) to map features. The department had created GIS layers by digitizing features from development plans and by borrowing map data from other city offices. When highly accurate location data was required for a project, Sioux Falls had to use city survey equipment, which took considerable time, or contract with a private mapping firm, which was expensive.
Today, however, GPS field mapping is standard practice in Sioux Falls. And the process is customized to map feature locations and other attributes necessary for specific GIS applications, such as the utility mapping project now under way. Rather than using highly skilled employees, the GIS department is paying college interns with no prior mapping experience about $10 an hour to gather the mapping information using a handheld GPS unit.
The Overland Park, Kan., GIS Division also is using GPS in its ongoing mapping efforts. GPS-equipped volunteers have fanned out across the city recording the location of every municipally owned tree and capturing up to a dozen attributes at each stop. One volunteer, a retired resident, has the feature and data collection technique down to 90 seconds per tree. The city says it never would have been able to undertake such an ambitious GIS mapping project until it recently began using the new portable devices.
The difference between GIS mapping projects today and just a few years ago for Sioux Falls, Overland Park and hundreds of other local governments — including Dallas, whose employees after only one day of training were sent into the field to map the city’s utilities — is the GPS equipment. GPS receiver manufacturers have introduced dozens of faster and more accurate products that are easier to carry, program and operate than previous models.
Capable of sub-meter accuracy, the mapping-grade receivers were developed to mesh with the GIS users’ work environments and data collection applications. The portable units have helped spawn a new discipline, known as mobile GIS, whereby geospatial users take their data sets into the field, update and add new information and upload the files back into their enterprise systems.
Integrated mapping devices
Perhaps the most significant hardware advancement has been the integrated units that combine the GPS receiver, antenna and data collector into a handheld device. Cumbersome systems previously involving separate equipment connected by cables and weighing up to 10 pounds is now contained in a two-pound handheld product. Resembling beefed-up personal digital assistants (PDAs), the units include a touch-sensitive viewing screen, stylus and keypad.
“Before we purchased the integrated GPS products, our biggest problem was the wires running between the data collectors and the receivers,” says Chris Laingen, a Sioux Falls GIS technician. “The wires were always getting snagged on things and yanked out of their connections, and we would have to start over again.”
Using mapping-grade GPS receivers, Sioux Falls has re-mapped its entire utility infrastructure network — water, storm, sewer, traffic and light components — over the past three years to prepare for a statewide Call-Before-You-Dig program. Many of the stormwater elements, such as manhole covers and sewer drains, which had been digitized from plans submitted by developers, were mapped incorrectly by up to 60 feet. The re-mapped utility layer collected with the new technology is accurate to within two feet using differential correction (see “Correcting the raw data,” on p. 46).
For the stormwater mapping segment of the utility project, the Sioux Falls GIS Department compared the cost of performing the work internally versus using the city survey crew. Taking into account the difference in the cost of mapping- and survey-grade equipment, the hourly pay of interns and surveyors, and the speed of mobile GIS, the study calculated a savings of $131,000 using the new technology.
Easy to use
Using mobile GIS units, Overland Park inventoried more than 5,000 trees in less than nine months of fieldwork.The digital inventory will allow the city to include tree locations as a layer in the GIS along with records pertaining to the care of each individual tree. The city’s GIS analyst Robert Meier says that some of the retirees involved in the project may not have been able to participate using the older, heavier receivers. He also credits the mobile GIS software in simplifying the process.
“The data collection interface has the appearance and point-and-click operation of any other Windows application,” Meier says. “It takes us a couple of hours to train the tree inventory volunteers.”
Most developers of GIS packages and GPS equipment use the same software developed for PDAs, laptops and other portable devices — the Windows Mobile Software 2003 for Pocket PCs operating system. As a result, proprietary software that limited applications to run only on specific hardware and vice versa is no longer used.
The integrated GPS receivers, for instance, are shipped with software applications that allow users to create customized data collection menus. And the same devices also run the mobile GIS packages that have been offered by each of the major GIS software companies.
Meier is using a mobile package developed to complement the city’s existing enterprise GIS system. “I simply opened the field software on my desktop computer and used Visual Basic to write a series of 11 forms that guide the volunteers through the tree inventory process,” Meier says.
In the field, the user clicks a button that directs the device to collect the tree’s location coordinates. While that occurs, the volunteer selects the tree species from a feature menu, which asks for details such as condition, diameter, height, nearby obstructions and signs of infestation. Clicking screen buttons enters the information. Miscellaneous notes can be written on the screen with the stylus.
“I can use the mobile GIS software to download part of a data layer or orthophoto base map from the enterprise GIS into the field device,” Meier says. “Our volunteers can view the GIS information live on screen to keep from getting lost or to see which trees they’ve already mapped.”
Once the volunteers return from the field, the GPS device is snapped into a docking port wired to Meier’s desktop computer where the feature location and attribute data are automatically uploaded to the enterprise GIS as new or updated files. The operator can view and edit the data before completing the upload, Meier says. “This seamless upload [that we use today] saves us five to 10 hours a week,” he says.
From laptop to mobile GIS
Because its code enforcement personnel uses large volumes of GIS data and computer applications in the field, the Planning and Building Department in San Luis Obispo County, Calif., has converted its laptop computers into GPS-enabled field tools for locating infractions and collecting data. And because workers travel primarily by car, portability was not a major consideration. “Code enforcement officers need the digital parcel map and database with them, especially in the rural areas, to determine which property has the violation and who the owner is,” says Andrea Miller, the department’s mapping/graphics systems specialist.
When the code officer arrives at the site of a reported violation, a GPS receiver about the size of a computer mouse is attached to the laptop containing the parcel map and attributes for that officer’s region.
Working inside the customized GIS mapping program, the officer uses the GPS to obtain location coordinates for the property. The system then automatically accesses and displays the parcel map onscreen along with the owner’s name and address.
The next screen displays a series of pull-down menus of violation types and descriptions. The parcel is flagged in the database, confirming the infraction and the description is entered into an attribute table. Once the officer docks the laptop and updates the city’s GIS parcels, a notification letter automatically is prepared for mailing to the owner.
“Our next project is to establish a wireless Bluetooth connection between the laptop and a cell phone so that data can be transmitted directly from the user in the field to the office,” Miller says.
While new GPS equipment and GIS software have enhanced the accuracy and efficiency of GIS field mapping, Uncle Sam has played a role in improving GPS technology. The U.S. government, which operates the constellation of orbiting GPS satellites, has continued to build and upgrade the system.
In 2000, the government removed signal limitations for civilian GPS users, boosting location accuracy from 100 feet to less than 30 feet without differential correction. The United States also has recently increased the number of GPS satellites in orbit to 28, giving GIS users a greater chance of connecting with multiple satellites — a necessity for high-quality location measurements.
The advancements in hardware, software and technology have made mobile GIS a reality. Now, data layers and applications can be taken into the field to collect information and perform the kinds of mapping at the level of accuracy that the local government needs to perform its job faster and more effectively. Perhaps, most importantly, geospatial data users do not have to be GPS experts to do it.
Kevin Corbley is the principal of Corbley Communications based in Winchester, Va.
Correcting the raw data
Differential processing is a technique used to improve the accuracy of raw GPS location coordinates. In mapping-grade applications, that technique typically improves the accuracy of GPS position points to less than one meter. Differential correction can occur in real-time during field mapping or in post-processing. The technology surrounding both methods of differential correction has been enhanced in recent years.
In post-processing, the acquired coordinate points are corrected with another set of points collected at a specific location by a second GPS, called a base station. In many U.S. cities, correction points from local base station sources can be downloaded from the Internet for free. Editing and correcting data from mobile GIS devices which once took hours, now takes minutes and occurs automatically with software running as an extension inside popular GIS packages.
Mobile GIS users have several choices of real-time correction, which involves simultaneously receiving and processing the correction signal by the GPS unit. Two of the most popular systems are the U.S. Coast Guard’s beacon network, covering most of North America, and the Wide Area Augmentation System (WAAS) operated by the U.S. government. Both systems are free.
Most mapping-grade GPS receivers are capable of both real-time and post-processing differential correction methods, but not all products receive WAAS signals. Prospective buyers should consider that when comparing mapping-level GPS units.
— Kevin Corbley
