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Rainier and her launches acquire sounding data at all stages of tide during a day of surveying. Vertical corrections must be made to this data so soundings meet NOAA's nautical charting datum of Mean Lower Low Water (MLLW). Rainier uses tidal data to correct soundings to MLLW. The tidal data is obtained from local primary control stations of the National Water Level Observation Network. Since some of these stations are far from the project area, Rainier also sets up tertiary tide stations to obtain more accurate local data.
A tide station is set up prior to acquiring bathymetric data. More than one station may be set up depending on the project area. Tide stations consist of five major components: nitrogen tank, pressure gauge, bubbler orifice, satellite antenna, and tide staff. The gauge itself is a large case which protects a basic computer and pressure sensor. This pressure gauge is attached to tube from a nitrogen tank. The nitrogen enters the gauge and is outputted in small bubbles through a bubbler orifice fastened to the seafloor near the gauge setup. The gauge measures the amount of pressure it takes to force the nitrogen bubbles through the tubing and to the bubbler orifice. The greater the pressure, the more water is over the orifice and the higher the tide. The pressure gauge transmits the tide data to NOAA headquarters hourly via satellite for analysis. Lastly, a graduated tide staff is attached to bedrock at the water’s edge. The staff is used to collect manual tide observations used in quality control and calibration of the tide gauge.
A tide station crew consists of at least eight people: two to set up land-based equipment, four divers to install a bubbler orifice, one to drive the launch that carries equipment, and one to drive the skiff for shore and diver assistance. The equipment involved in an installation is extensive. Aside from the dive gear, tide staff, tide gauge, and associated tools and accessories, benchmarks and leveling equipment are used to establish elevations at the site.
NOAA's Center for Operational Oceanographic Products and Services offers a detailed description of how and why a tide station is installed.
