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    Question ID:   7974         Current Version: 1
Question: How are tides measured?
Category: Science > Earth Sciences
Keywords: measure, tide
Type: how
Rating:(0 ratings)    Views: 299    Discussions: 0   In Watch Lists: 1  

 
    Answer:

Old System

Since the early 1800s, NOAA (National Oceanic and Atmospheric Administration) and its predecessor organizations have been measuring, describing and predicting tides along the coasts of the United States. The longest continuous sea level records exists for the Presidio, in San Francisco, California. Records for the area date back to June 30, 1854. Today, the Center for Operational Oceanographic Products and Services (CO-OPS), which is part of NOAA's National Ocean Service (NOS), is responsible for recording and disseminating water level data.

In the past, most water level measuring systems used a recorder driven by a float in a "stilling" well. A stilling well calms the waters around the water level sensor. A typical stilling well consisted of a 12-inch wide pipe. Inside the stilling well, an 8-inch diameter float was hung by wire from the recording unit above.

Before computers were used, water level data was recorded on a continuously running pen and ink strip chart. These records were collected by observers once a month and mailed to headquarters for manual processing. In the 1960s, data were recorded onto mechanically punched paper tape that were read into a computer for processing. Water levels were recorded at 6-minute intervals. Observers maintained and adjusted the clocks, and calibrated the gauges with the tide readings. Tide stations were visited annually to maintain the tide houses and clean biological fouling from the underwater surfaces. During these annual visits, the components and support structures also were checked for stability.

Although these systems worked well, they had their limitations. Stations were subject to recording errors and marine fouling, and were constantly in need of maintenance. In addition, the measurement and data processing equipment could not provide users with information until weeks after the data was collected.

New System

Advances in technology have helped solve many of the problems associated with the old tidal recording systems. Microprocessor-based technologies allow for customized data collection and have improved measurement accuracy. While older tidal measuring stations used mechanical floats and recorders, a new generation of monitoring stations uses advanced acoustics and electronics. Today's recorders send an audio signal down a half-inch-wide sounding tube and measure the time it takes for the reflected signal to travel back from the water's surface. The sounding tube is mounted inside a 6-inch diameter protective well, which is similar to the old stilling well.

In addition to measuring tidal heights more accurately, the new system also records 11 different oceanographic and meteorological parameters. These include wind speed and direction, water current speed and direction, air and water temperature, and barometric pressure.

Like the old recorders, the new measuring stations collect data every six minutes. However, whereas the old recording stations used mechanical timers to tell them when to take a reading, timing is controlled on the new stations by a Geostationary Operational Environmental Satellite (GOES). The stations also use these satellites to transmit their data hourly to NOAA headquarters. In the event of a storm, the stations can be programmed to transmit their data every six minutes. Field teams can quickly check and maintain the systems using laptop computers. In addition, all of the raw and processed data are available over the Internet.

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