The data set is labeled 1990 for easy comparison. The project duration was a decade.
Numerical circulation and sediment transport models being developed for the South Florida Ecosystem Restoration Program are being used to address water quality issues in Florida Bay. Application of these models is complicated due to the complex seafloor topography (basin/mudbank morphology) of the Bay. The only complete topography data set of the Bay is 100 years old. Consequently, an accurate, modern seafloor bathymetry map of the Bay is critical for numerical modeling research. A modern bathymetry data set will also permit a comparison to historical data in order to help access sedimentation rates within the Bay.
The objective of this research was to collect new bathymetry for all of Florida Bay, digitize the historical shoreline and bathymetric data, compare previous data to modern data, and produce maps and digital grids of historical and modern bathymetry.
DeWitt, Nancy T.
According to NOAA tidal datum information, NAVD88 is approximately 0.41m (1.35 ft) above MLW in the Florida Bay area. The 1.35 feet average is calculated using the elevation information from several tidal benchmarks throughout the Florida Keys. These tidal benchmarks are published by the National Geodetic Survey and can be located on their website <http://www.ngs.noaa.gov/cgi-bin/ngs_opsd.prl/>. Five tidal benchmarks were chosen from four different U.S. Geological Quadrangle locations including Blackwater Sound, Plantation Key, Long Key and Grassy Key. The open-file report contains a table for quick reference of information for the five tidal benchmarks.
To compare this data set with other depth information which is relative to MLW, e.g. NOAA nautical charts, 0.41 m (1.35 ft) is algebraically subtracted from each depth value or contour line.
Data was collected on a USGS 7.5-minute quadrangle-by-quadrangle basis, proceeding westward from Blackwater Sound. The trackline spacing varied depending upon the relief of the sea floor; that is, closer spacing adjacent to mudbanks and wider spacing in the basins. Tracklines were surveyed in a north-south orientation, and crossings (intersecting tracklines) were surveyed in an east-west orientation. Crossing lines are critical because they served as a check on the accuracy of the system. In theory, data values at the crossing should be exactly the same. In reality, this is not always the case due to random errors of each sensor. Eighty-five percent of the crossings in this data set were within +/- 6 cm with the balance within +/- 20 cm.
Ideally, crossings are made at the end of the survey day; however, some of the crossings were made months and even years after an area was initially surveyed. The results are very good considering the soft bottom characteristics of Florida Bay. Bathymetric positions for this study were derived using differential GPS techniques on 10-km baselines or less. One or more GPS reference stations (base stations) were continuously recording full-phase carrier data while the boat surveyed. A rover GPS receiver on the boat was simultaneously recording carrier information.
Record epoch interval was 1 second for both base station and rover receivers. Precise positions for each epoch were computed using Ashtech Precise Differential GPS Navigation and Surveying (PNAV) v2.0 software. PNAV provides a root mean squared (RMS) value for each epoch. For this study, a RMS value of 0.08 cm or less was considered reasonable. An RMS value of 1.0 indicates that the GPS data ambiguities were unresolved, and that the depths were determined by a tie-line best-fit method. For data format consistencies, RMS values for the 1890's data set are equal to 0.0. The horizontal positions of each data point were transformed from Latitude/Longitude to UTM (Zone 17) XY coordinates using NOAA/NGS software UTMSv1.0. Vertical positions were converted from ellipsoid height to orthometric height using NOAA/NGS software GEOID96, where orthometric height is considered to be equal to NAVD88. According to NOAA tidal datum information, NAVD88 is approximately 0.41m (1.35 ft) above MLW in the Florida Bay area. The 1.35 feet average is calculated using the elevation information from several tidal benchmarks throughout the Florida Keys. These tidal benchmarks are published by the National Geodetic Survey and can be located on their website <http://www.ngs.noaa.gov/cgi-bin/ngs_opsd.prl/>. Five tidal benchmarks were chosen from four different U.S. Geological Quadrangle locations including Blackwater Sound, Plantation Key, Long Key and Grassy Key. To compare this data set with other depth information, which is relative to MLW, e.g. NOAA nautical charts, 0.41 m (1.35 ft) is algebraically subtracted from each depth value or contour line.
Geodetic control in Florida Bay was lacking in number and geometric strength. In order to accomplish centimeter vertical accuracy, additional geodetic control needed to be established within the Bay. Thirteen new temporary ground-control points or benchmarks (surveyed to within 1 cm to 2 cm accuracy) were established throughout the Bay for use as reference receiver sites. The thirteen benchmarks were surveyed using Ashtech Z-12, 12 channel dual-frequency GPS receivers. Full-phase carrier data were recorded on each occupied benchmark in Ashtech proprietary BIN format with daily occupations ranging from 6 to 12 hours. BIN files were then converted to RINEX-2 format and position determined by the National Aeronautics and Space Administrations (NASA) Jet Propulsion Laboratory GIPSY system software. The GIPSY derived positions were provided in ITRF96 coordinate system for each (daily) occupation. Using National Oceanic and Atmospheric Administration/National Geodetic Survey (NOAA/NGS) software HTDPv2.3, ITRF96 positions were transformed to NAD83/GRS80 positions. The computed daily positions were then averaged to derive a final benchmark location and elevation. Daily outliers greater than 0.05cm (in the vertical component) from the average were not included in the final average.
U.S. Department of the Interior, U.S. Geological Survey
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