Data in the Key of ZZZ: Development of a Network to Establish Vertical Reference Datum for Research Stations in the Southwest Coastal Everglades

Nancy T. DeWitt, B.J. Reynolds & Thomas J. Smith III

U.S. Geological Survey, Center for Coastal and Watershed Studies, St. Petersburg, FL

Gordon H. Anderson

Center for Water & Restoration Studies, c/o Everglades National Park, Homestead, FL

Some of the most remote locations in all of the National Park Service system in the lower 48 states are located in the southwestern, coastal areas of Everglades National Park. Highland Point lies on the coast between the mouths of the Lostmans and Broad Rivers and is more than 40km from the nearest trail-head. This mangrove and marsh dominated coastal wilderness is at the southern end of the Everglades, where the “River of Grass” meets the Gulf of Mexico. It encompasses over a third of the Park’s total acreage.

As the Comprehensive Everglades Restoration Plan (CERP) is implemented, questions have arisen related to monitoring restoration success in this region. Since 1996 the USGS/BRD has established and maintained a network of study sites for sampling hydrology, sediments and vegetation. Benchmarks were established and set to a relative datum at a total of 13 hydrological stations. This network of benchmarks has been used as a starting point to develop a broader survey of vertical elevation. Thirteen of the benchmarks have been installed by driving a pipe to refusal, capping it, and setting the mark with cement to maintain a stationary point of occupation that is separate from the wooden platforms constructed for fieldwork measurements. Ten other control locations are temporary locations consisting of a PK nail hammered into the top of a hydrologic station platform with the assumption that the platform remains stable through time.

The elevation factor Z is the critical variable for much of the work in the Everglades as well as Florida Bay and the Florida Keys Reef Tract. An accurate reliable key of Z is needed to convert field measurements into metrics that are cross-comparable. It also serves to satisfy the vertical variable in flow models. Essentially, an accurate elevation will become an established pivot point for data comparison among the scientific studies of various disciplines such as biology, water, remote sensing, and geology that are underway within the Everglades.

An accurate elevation for the Everglades portion currently exists for only one site of this network. To begin to bridge this gap, annual surveys have begun starting in June 2000 with long-term, repetitive occupation time from 8 hours to 24 hours upon selected benchmarks. These surveys are tied to the closest National Geodetic Survey (NGS) sites available having the best vertical control published. These sites include FLGPS Thompson, FLGPS GEB, and Tidal 8724948EH7. The surveys will begin to establish a strong reliable network for the original 13 stations. High precision Ashtech GPS receivers and antennas are the instruments used for receiving the L1 and L2 signals from the satellite constellation which are then post processed to obtain the horizontal and vertical (XYZ) information for each benchmark. Thirty-four stations in total have been occupied in the last two years three of, which are NGS control points that will be the tie to the National Geodetic Survey’s network.

Because technology is rapidly improving and this elevation survey is relatively young, the post processing method has been carried out using three different methods. The first method uses PNAV post processing software. This software calculates vectors between locations and based upon the repetition of vectors and their quality, it calculates coordinates for the location. The second method is Automated Gipsy by the Jet Propulsion Lab. This method can be applied when a location cannot be tied into an established network like an NGS location. Submission of long occupation times to Automated Gipsy will produce a location in an ITRF## format which can be transformed into NAVD88. The third method utilizes OPUS (Online Positioning User Service) located on the NGS website. This method requires submittal of a GPS file and OPUS determines the three best CORS stations (Continuously Operating Reference Station) for processing the location of the submitted file. Both OPUS and Automated Gipsy are automated and require minimal file preparation, whereas, PNAV is controlled solely by the user allowing choice selections of vector inclusion and exclusion to maintain accurate results.

Contact: DeWitt, Nancy T., U.S.Geological Survey, Center for Coastal & Watershed Studies, 600 Fourth Street, South, St. Petersburg, FL 33701. Phone: 727-803-8747, FAX 727-803-2032, Email: , POSTER, Other Coastal Ecosystems

Caption: Graphic of South Florida displaying site locations in elevation survey that have been occupied to date.