Tide Gauges

Tide Gauges are used to measure changes in sea level. This is important to know for many reasons, including the correction echo sounder data used in making hydrographic charts.
The first known tide measuring devices were used by the Egyptians 5,000 years ago, making tide gauges the first known marine technology. Wooden or reed sticks and later marble columns planted in the river bottom were sufficiently long enough to extend above the waterline where the fluctuating level of the Nile River could be measured against markings. The hydrological information collected is one the longest scientific time-series data sets ever collected. Today, tide staffs are still widely used.
Mechanical tide recording devices have been used for more then a century. These devices typically consist of a wire hung over a pulley wheel. At one end, there is a metal float and at the other end a counter weight. As the tide fluctuates, the wire turns the pulley that is geared to a pen that runs over a paper chart held in tension over a platen. The paper is moved from a supply spool to a take-up spool by a clock-driven mechanism. The recorder is placed in a shelter or in a security box on top of a stilling well. A stilling well consists of a pipe attached vertically to a pier or other structure. It has a sealed bottom with an inlet hole of about 1/50 of the diameter of the well. This “stills” wave action, allowing an accurate measurement to be taken.
Modern electronic tide gauges use pressure sensors and data loggers . For information on the different type of pressure sensors used, see the section on this subject in this glossary.
Long-term tide gauge records can be used to determine changes in mean sea level. However, with tide gauges alone, it is impossible to distinguish between any “true” sea level variations and vertical land movements at the tide gauge site. GPS monitoring can be used to decouple vertical land movements from changes in relative sea level, so that tide gauges can provide estimates of changes in “absolute sea level.”