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# Tides and Water Levels

## How are Tides Measured? - The Old System

**Click the image**

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Since the early 1800s, NOAA 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.

**Click the image**

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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.

*Special tide houses were constructed to shelter permanent water level recorders, protecting them from harsh environmental conditions. In this diagram, we can see how the analog data recorder (ADR) is situated inside the house with the float, and the stilling well located directly beneath it. Attached to one of the piers pilings is a tidal staff. Essentially a giant measuring stick, this device would allow scientists to manually observe the tidal level and then compare it to the readings taken by the analog recorder.*

### Tides and Water Levels

              - Welcome
              - What are Tides?
              - What Causes Tides?
              - Gravity, Inertia, and Bulges
              - Changing Angles and Tides
              - The Frequency of Tides
              - Tidal Variations
              - Types and Causes of Tidal Cycles
              - What Else Affects Tides?
              - Monitoring the Tides
              - Old System
              - New System
              - References
              - Roadmap to Resources 
              - Subject Review

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