Water level measurement

Permanent tide gauges measure the water level (with respect to a well-established reference plane known as a datum) around coasts as part of long-term national programs that allow for the effects of changes in atmospheric pressure and the action of waves to be compensated for and to produce a tidal height. Depth measurements made by hydrographers in the area can then use the water surface as a reference surface, and the depth of a point on the seabed be determined with respect to the datum.

Pressure sensors based on strain gauges are widely used for tide gauges – a pressure transducer is fixed to the seabed and surveyed to determine its height against the local land survey datum. Meteorological instruments are often installed with tide gauges as changes in atmospheric pressure must be compensated for, and can be provided as an integrated package. Data from all instruments at the tide gauge location can be telemetered for direct use of inclusion in a wider observation network using radio or cabled connections.

As well as being used for water level measurement in their own right, radar sensors are increasingly being used to supplement pressure-based tide gauges and provide an independent source of measurement that is free from issues associated with in-water sensors such as bio-fouling and corrosion. Radar systems contain an emitter and a receiver of electromagnetic energy (typically 25Ghz wavelength) and the distance calculated to the water surface from the two-way time of flight of a pulsed signal. As per pressure-based systems, radar sensors must be surveyed to establish their relationship to the local vertical datum.

Between, or out of range of permanent tide gauges, it is often necessary for additional tidal/water level measurement to be established for a survey, construction, or research project. These systems can use pressure, radar or hybrids of both, and a range of solutions are possible including on-board logging for later retrieval of data, solar power, radio telemetry, and mounting on subsea moorings or above-water structures as appropriate.