ISSUE DEFINITION

Sheet flow and water surface levels in South Florida are very sensitive to changes in elevation because of the region's expansive and extremely low-relief terrain. Hydrologists have determined that the minimum vertical accuracy requirement for elevation data used as input to hydrologic models is +15 cm. This requirement is complicated by the fact that the surface being measured in inundated wetlands is the water/substrate interface, which is often soft muck. Further, since the surface being measured is obscured by water and vegetation, the application of remote sensing techniques to derive these data (for example, photogrammetric stereo correlation and profiling, light detection and ranging, and interferometric synthetic aperture radar) is not possible. Traditional spirit-level surveying is not feasible either because of the very large area to be surveyed and because of cost and time constraints. Therefore, the primary issue is how to accomplish topographic surveys and provide elevation data over large areas of the inhospitable Florida Everglades and adjacent lands--that is, surveys and data that meet the stringent vertical accuracy specification required for hydrologic modeling.

BACKGROUND

The main objective of this project is to derive very accurate elevation data using state-of-the-art differential global positioning system (GPS) technology covering large regions specified by program hydrologists. The elevation data sets being generated by this project are the most accurate ever produced for the Everglades and surrounding environs. These data will be used in developing digital elevation models (DEM's), which in turn will be used to parameterize hydrologic models to simulate and predict water flow direction, depth, velocity, and hydroperiod. Water resources and land management decisions will rely on these models, so it is imperative to use the most accurate elevation data available to achieve meaningful results.

Techniques are being developed and used to collect and process GPS derived elevation data that have a vertical accuracy of 15 cm or better referenced to the North American Vertical Datum of 1988. The primary data collection strategy is to use the helicopter-based airborne height finder (AHF), developed by the U.S. Geological Survey (USGS), to collect data and produce elevation points spaced approximately 400 m apart throughout the Everglades National Park and other inaccessible areas. Data collection in areas accessible by airboat, all-terrain vehicle, or truck, is also being accomplished through a USGS contract with the private sector. A second major goal of the project, now completed, was the surveying and positioning of very accurate elevation reference marks to support data collection by the Florida Bay Marine Monitoring Network (MMN). The MMN, consisting of 29 instrumented platforms and gages distributed throughout Florida Bay, collects data on water levels as well as other hydrodynamic and meteorological variables. For the first time, these elevation marks allow all the MMN water levels to be referenced to an accurate datum so that water-level comparisons between gages can be more accurately determined.

STATUS/PROGRESS TO DATE

The project began as an interagency effort (USGS, U.S. Army Topographic Engineering Center (USATEC), and National Geodetic Survey) that was undertaken in fiscal year 1995 to demonstrate whether the new on-the-fly differential GPS (DGPS) technology could be used to perform topographic surveys that meet the requirements for high-accuracy elevation data. This demonstration project used airboats and a truck to collect DGPS data in wetland and residential areas, and the results proved very successful. However, large areas in the region, especially Everglades National Park, are not accessible by ground transportation. Therefore, another approach was needed for areas that are only accessible by helicopter. To meet this need, the AHF was engineered and fabricated. A second demonstration project using the AHF was conducted in fiscal year 1997. Again the results proved successful, and the AHF has since been undergoing evolutionary modifications resulting in the fabrication of an improved second system.

Since the beginning of the project, significant parts of eighteen 7.5-minute quadrangles have been surveyed by both government and contract survey parties, and field work is continually progressing. By the close of fiscal year 1999, it is estimated that parts of more than two dozen quadrangles will have been surveyed, including complete coverage of the Southern Inland and Coastal System model study area within the Everglades National Park. Lastly, the survey of the MMN waterlevel gage reference marks was completed in fiscal year 1997 and required survey resources from three agencies, the USGS, USATEC, and the National Park Service, and more than 1,700 hours of field work to complete.

FUTURE EFFORTS

Future efforts will include the continuation and expansion of ongoing field data collection and processing work. Accuracy assessments of the growing data sets will be greatly increased. It will also be necessary to expand the existing vertical geodetic control network for use as vertical reference points during AHF and airboat data collection operations and other surveying applications. An evaluation of light detection and ranging technology for deriving elevation data for agricultural lands is planned as well. Lastly, a patent application for the AHF is being investigated.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)