Results from recent GPS Tides Projects

The U.S. Army Corps of Engineers (USACE) has completed tidal datum elevation models using Differential Global Positioning System (DGPS) carrier phase methods in three ocean approach channels in the southeastern United States. These channels are all contained entirely or in part within the state of Georgia. Tide observations and tidal datum determinations were coordinated with the Center for Operational Oceanographic Products and Services (COOPS), National Oceanic and Atmospheric Administration NOAA. The datum elevation models confirms that the local mean sea level and the mean tide level differ by a small amount on the open coast and shows that the sea level is approximately an equipotential surface over a large area of the Georgia coastline. The survey results also show an anomalous depression in mean lower low water offshore, which may be due to DGPS limitations when using a 20 km baseline distance. The USACE has found the use of carrier phase DGPS and tidal datum models a useful method for maintaining vertical control for dredging operations and intends to use carrier phase DGPS for projects along the entire east coast of the United States. The paper discusses the experiences in three projects: Ferandina, Florida, and north to Brunswick, Georgia and Savannah, Georgia. The large range light towers on the approach to Fernandina with installed acoustic tide gauges provided vertical datum references for the GPS Tides Project, which enabled the offshore GPS measurement results to be bounded. The GPS Tides measurements in Georgia were unbounded; however, the mean tide levels are correlated with the Florida results