One of the major issues facing ecosystem restoration and management in South Florida is the availability and distribution of clean, fresh water. The South Florida ecosystem encompasses an area of approximately 28,000 square kilometers and supports a human population that exceeds 5 million and continues to grow. The natural systems of the Kissimmee-Okeechobee-Everglades watershed compete for water resources primarily with the tourism and agricultural industries and with urbanization. Therefore, surface-water flow modeling studies, as well as ecological modeling studies, are an important means of providing scientific information needed for ecosystem restoration and modeling. Hydrologic and ecological models provide much needed predictive capabilities for evaluating management options for parks and refuges, planning land acquisition, and understanding the impacts of land management practices in surrounding areas. These models require a variety of input data, however, including elevation data that define the topography of the Florida Everglades.

Sheet flow and water surface levels in South Florida are very sensitive to any changes in topography because of the region's expansive and extremely flat terrain. Therefore, hydrologic models require very accurate elevation data for input to simulate and predict water flow direction, depth, velocity, and hydroperiod. Water resources, ecosystem restoration, and other land management decisions will rely in part on the results of these models, so it is imperative to use the most accurate elevation data available to achieve meaningful simulation results. Therefore, elevation data points are being collected every 400 meters in a grid pattern to meet the requirements of hydrologic models of various cell resolutions. The vertical accuracy specification for these elevation data is 15 centimeters (6 inches), and they are referenced to the North American Vertical Datum of 1988 (NAVD88).

Because traditional methods for collecting these data for the Everglades are impractical or too costly, the U.S. Geological Survey (USGS) did a feasibility study in late 1994 and early 1995 to determine if state-of-the-art techniques using the Global Positioning System (GPS) could be used to meet the strict vertical accuracy specifications of the elevation data. The feasibility study successfully demonstrated that differential GPS techniques, using airboats to navigate transects, could meet the data accuracy requirement. The land surface being surveyed in the Everglades is typically under water and obscured by vegetation, which precludes the use of other methods for collecting elevation data, such as photogrammetry and alternative remote sensing technologies. Therefore, topographic surveys over such a large area of the Everglades with such a stringent accuracy specification can only be efficiently accomplished by using GPS. This is especially the case in an inaccessible wilderness environment.

Because the Everglades is so expansive and remote, and includes environmentally sensitive areas, impenetrable vegetation, or other areas unapproachable by airboat, access to many places is possible only by helicopter. To solve this accessibility problem the USGS developed a helicopter-based instrument, known as the Airborne Height Finder (AHF), which is able to measure the terrain surface elevation in a noninvasive, nondestructive manner. Accuracy tests have shown that the AHF system can consistently measure elevation points to within 3 to 5 cm. An accuracy test in May 2000 measured 17 National Geodetic Survey (NGS) benchmarks twice with the AHF. The average difference between the AHF measured elevations and the NGS published data sheet values was 3.3 cm. The largest difference was 8.6 cm, and the smallest difference was 0.2 cm. These accuracy test results provide confidence that the elevation dataset being produced meets the 15 cm vertical accuracy specification.

To date, thousands of elevation data points covering significant parts of the Florida Everglades have been collected and processed using differential GPS methods, from both airboats and helicopters. These data are organized by USGS 7.5-minute quadrangles and are available from the South Florida Information Access Web Site at http://sofia.usgs.gov. Data collection will continue, with emphasis on providing coverage of the Tides and Inflows in the Mangroves of the Everglades (TIME) Model Domain (http://time.er.usgs.gov/) and on completing coverage of Water Conservation Area 3.

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))

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