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GPS Plots Drainage on the Flat

Some resourceful sourceful surveyors in Florida's Orange County build a GPS system on a shoestring.

Much of Florida's rapid population growth -- 1,000 new residents a day -- is taking place in the Orlando metropolitan area of Orange County, a region noted for tourist attractions, sunshine and natural beauty. An agricultural center little more than 20 years ago, Orlando is now the hub of one of the fastest-growing areas in the United States, and a popular destination for travelers worldwide. Of the 25.5 million people who pass through Orlando's international airport, a large percentage come to experience Sea World of Florida, Walt Disney World and Universal Studios. Also within a 50-mile radius are the famed Cypress Gardens and Kennedy Spaceport.





Accompanying the rise of tourism is the continuing development of homes, businesses and supporting
infrastructure -- some of the most challenging
being drainage and storm-water runoff systems.

In the relatively flat topography of Florida, drainage is a key environmental factor, from the St. Johns River in the north to the outflow of the Everglades in the south. Since the late 1970s, engineers from the Orange County Public Works Division have been expanding the drainage network through interconnecting canals and storm-water piping; assessing the impact of continuing development on existing systems; and planning improvements. Natural basins in the county are interconnected by unlined canals bulldozed 50 feet wide by 10 feet deep. Most have continuously running water. Improvements planned for the system include additional canal interconnections and piping.

Importance of Map Contours

Determining elevation changes is an essential step in drainage pattern planning involves. To identify these, Orange County engineers use rectified aerial photography with 1-foot contour lines provided by an aerial survey firm. Before the aerials can be used in planning, however, field crews from the county survey section must check the contour elevations for accuracy. "Elevations are a critical factor," said Orange County surveyor Dann McKee. "You can go through some areas of this county where there is hardly any elevation change for hundreds of feet. A 1-foot contour change can be highly significant when you are looking at large drainage patterns, so it is necessary to have photography with accurate contours shown to a high level of detail."

Of the 980 square miles of Orange County, 728 have been surveyed for contour-elevation accuracy. The most recent survey, completed in November 1995, covered 42 square miles, encompassing rural areas, wetlands and two townships. Prior to that, surveys were conducted using the conventional level and leveling rod, a time-consuming, labor-intensive process requiring crews to spend months in the field setting benchmarks and running elevation profile lines in rural areas.

Since downsizing and privatization became operative words in state and local government, traditional surveying tools were rapidly replaced by efficient, precision GPS. In an effort to reduce the time and cost of checking elevations, the survey section turned to kinematic GPS, which records coordinates and elevations while in motion. A few days' experiments with the system produced centimeter accuracies while dramatically reducing the field survey time.

Kinematic on the Cheap

Working with limited resources, the survey section fabricated a mount for a GPS antenna and welded it to the center of a boat trailer, just forward of the axle. The mast, a two-section range pole, was then secured with twine. The interconnecting cable runs from the helix antenna to the GPS receiver mounted in a 4-wheel drive. Pulling the trailer at about 5 mph allows one person to collect elevations and associated horizontal coordinates every 15 feet.

"The heavy suspension system of the unladen boat trailer maintains a consistent height above ground, assuring a constant distance between the ground and the base of the antenna," McKee said. "We can use one section of range pole, which puts the antenna 5.8 feet above the ground, or add the second section to put it up to 9.8 feet. The second section really helps when we are driving through orange groves."

The most recent aerial photography for the project covers approximately 42 square miles, and has a scale of 1:200 (1 inch = 200 feet), with 1-foot contour intervals labeled every fifth line. The contours are delivered in digital format, but are also combined with the rectified aerial photos on Mylar, in 42 separate sections of 1 square mile. Each section is separated into quadrants.

Accuracy checks involve placing Trimble 4000 SSE GPS receivers at two known control points that serve as base stations, and pulling the roving receiver along a 1,000- to 2,000-foot path that transects the contour lines in each quadrant of the section map. The driver uses photo-identifiable objects in the aerial as a guide to navigate the preselected course.

Post Processing

Data from the rover and base stations are first processed using Trimble GPSurvey, a software package that provides baseline solutions, quality analysis and coordinate transformation from latitude-longitude coordinates to state plane coordinates. The coordinates are then exported in ASCII format to C&G Survey Software and plotted on vellum, which is placed on a light table over the Mylar of the aerial photograph. Surveyors then check the elevations to see if they fall between the right contour lines.

McKee said that at the time the contract was formulated, the Stormwater Management Department did not have an active layer on the GIS database that could be used for the project. Elevations had to be overlaid manually. "The next project," he said, "will be done using a GIS system developed specifically for stormwater management. All elevation data will become part of that system."

Benefits

According to Senior Survey Field Crew Supervisor Paul Akers, kinematic GPS is the way to go for topographic work. It is faster, less labor-intensive and provides far more elevations than can be achieved with either conventional surveying tools or static GPS measurements. "Using the conventional method, we had to put four benchmarks into each section so we could run the profile lines," Akers said. "Now, we can cover eight sections using just the two control points. Also, the old way would have taken two days just to do one section. Using kinematic, we completed four sections a day, and that's with a crew of only three -- one at each of the base stations, and one on the rover."

According to Akers, the cost benefits of the new system speak for themselves. "A past project covering 42 square miles took two crews eight weeks to complete at a cost of $28,000," he said. "Using the new system, we completed a similar project for $3,500. The net result was a savings of over $24,000 to the taxpayers of Orange County. There were other benefits as well; field crews were not exposed to dangerous traffic situations, and they were available much sooner for other projects."

If another county wanted to replicate the project, Akers estimates the cost of the three GPS receivers and software to be $40,000; a four-wheel drive, $20,000; and the boat trailer, $1,000. Two days' experimenting and training cost the survey section about $700. The GPS equipment was already available, since the county had purchased it for other projects.

"We took a job that used to take two crews one day per section to complete," said Akers. "Now, three people can do four sections a day. Increased production is the bottom line of the whole project."

Bill McGarigle is a freelance writer specializing in communication and information technology.




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