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South Florida High-Accuracy Elevation Data Collection Project

By Charles Henkle and Greg Desmond

Background

The major issues facing ecosystem restoration and management in south Florida are centered on the availability and distribution of clean, fresh water. Surface water flow modeling studies are an important aspect of the scientific information needs of the ecosystem restoration initiative. Hydrologic models provide much needed predictive capabilities for evaluating options for management of parks, refuges, and lands planned for acquisition, as well as for understanding the impacts of land management practices in surrounding areas. Models must account for the expansive and extremely low relief terrain of south Florida, where surface waters of the natural system are transported by sheet flow from Lake Okeechobee through the Everglades to Florida Bay and the Gulf of Mexico. To model sheet flows, highly accurate elevation data that define the surface topography are required as the most critical input parameter to the Natural Systems Model and other hydrologic models. These data are necessary for calculating water surface slope, depth, velocity, and direction of flow. Hydrologists have stated vertical accuracy requirements of between 3 and 15 centimeters. These accuracy requirements are so stringent, because of the extremely flat terrain, that any standard or other currently available data products would not suffice for this modeling application.

USGS Role

The role of the U.S. Geological Survey (USGS) is to design, develop, and implement plans for collecting high-accuracy elevation data that define surface topography for very large areas. Use of traditional methods for collecting these data is not possible because of the high costs and difficulty of working in a challenging environment such as the Everglades.

Figure 1. Plot of representative elevation data. (Click on image for larger picture.)

Planned Approach

Therefore, it was hypothesized that emerging Global Positioning System (GPS) techniques could provide a cost-effective method for surveying elevation data while meeting the stringent vertical accuracy requirements. To test this hypothesis, an interagency proof-of-concept pilot project was designed and conducted during fiscal year 1995. The USGS, the U.S. Army Topographic Engineering Center, and the National Oceanic and Atmospheric Administration (NOAA) formed partnerships to leverage equipment and expertise (for example, dual-frequency GPS receivers, field scientists, computers, air boats, and so on). An interagency team of more than a dozen scientists and engineers was formed for planning the project, performing field work, and postprocessing data to effectively execute the project. The results of the pilot project were highly successful. The average variance to known reference control points was 1.4 centimeters. These results and the methodology developed during the pilot project proved that new GPS techniques provided a feasible, cost-effective solution to a very difficult problem. Planned Approach As a result of the successful pilot project, efforts were expanded to collect high-accuracy elevation data covering much broader areas. A requirements letter soliciting elevation needs was distributed to agencies involved with the restoration initiative. As a result, forty-eight 7.5-minute quadrangle maps, or approximately 8,100 square kilometers, were identified as priority area requirements. Because of the scope of these requirements, work to collect these data in areas that lend themselves to the developed methodology and are accessible by air, boat or truck is being contracted to the private sector. The vertical accuracy of these data is about 15 centimeters, and data points are being collected approximately every 400 meters in a grid fashion. Significant features that influence sheet flow, such as cross-sections of roads and levies, are being surveyed as well (fig. 1). In areas not accessible by these surface transportation modes, such as in heavily vegetated mangrove stands along the Buttonwood Embankment, a helicopter-based system is being designed to collect elevation data as accurately as possible. A new pilot project is now being developed to experiment with this approach for data collection and accuracy assessment.

For more information contact:

Caribbean-Florida Water Science Center
U.S. Geological Survey
3321 College Avenue
Davie, FL 33314

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