Tide height is typically measured relative to a reference point, often the low-water mark, but modern methods involve sophisticated instruments and data analysis techniques.
Traditional Methods of Tide Measurement
Traditionally, tide height was determined by visual observation of the water level against a graduated staff or tide gauge.
- Tide Staff: A simple pole marked with height increments used for direct visual reading.
- Tide Gauge: A more sophisticated device that automatically records the height of the tide over time. These gauges often use a float in a stilling well to dampen wave action.
Modern Methods of Tide Measurement
Modern techniques offer more accurate and continuous measurements.
- Float Gauges: These gauges consist of a float inside a stilling well, which is connected to the open water. As the water level rises and falls with the tide, the float moves accordingly. This movement is then recorded mechanically or electronically.
- Pressure Sensors: Submersible pressure sensors measure the hydrostatic pressure of the water column above them. This pressure is directly proportional to the water depth, which can be converted to tide height. These sensors are deployed underwater and transmit data via cable or wirelessly.
- Acoustic Sensors: These sensors use sound waves to measure the distance to the water surface. By emitting a sound pulse and measuring the time it takes for the echo to return, the distance, and therefore the water level, can be determined. These are often mounted above the water surface.
- Satellite Altimetry: Satellites equipped with radar altimeters can measure the sea surface height from space. By comparing these measurements with a reference geoid (a model of the Earth's gravitational equipotential surface), scientists can determine the tide height. This method provides global coverage but has lower accuracy than in-situ measurements.
Data Processing and Reference Levels
Raw tide measurements are often processed to remove noise and correct for errors. It's also crucial to establish reference levels.
- Low-Water Mark: This is often used as a reference point, designated as the zero point for tide height measurements.
- Mean Sea Level (MSL): The average height of the sea surface over a long period (typically 19 years) is another common reference level.
- Mean Lower Low Water (MLLW): The average of the lowest tide recorded at a location is used as the zero tide level for nautical charts in the United States.
Challenges in Tide Measurement
Several factors complicate tide measurement.
- Wave Action: Waves can introduce significant noise into tide measurements. Stilling wells and data filtering techniques help mitigate this issue.
- Storm Surges: Extreme weather events can cause temporary increases in sea level, which must be accounted for in tide predictions and measurements.
- Local Variations: Tides can vary significantly from one location to another due to coastal geometry, bathymetry, and other factors.
Conclusion
Tides are measured using a variety of methods, ranging from simple tide staffs to sophisticated electronic and satellite-based instruments. Modern techniques offer more accurate and continuous measurements, allowing for improved tide predictions and a better understanding of coastal processes.
