A tide gauge is a device for measuring the change in

sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...

relative to a

vertical datum In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built stru ...

. It is also known as a mareograph, marigraph, and sea-level recorder. When applied to

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

continental

water bodies A body of water or waterbody is any significant accumulation of water on the surface of Earth or another planet. The term most often refers to oceans, seas, and lakes, but it includes smaller pools of water such as ponds, wetlands, or more rare ...

, the instrument may also be called a limnimeter.

Operation

Sensors continuously record the height of the

water level Water level, also known as gauge height or stage, is the elevation of the free surface of a sea, stream, lake or reservoir relative to a specified vertical datum. Over long distances, neglecting external forcings (such as wind), water level ten ...

with respect to a height reference surface close to the

geoid The geoid ( ) is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is exte ...

. Water enters the device by the bottom pipe (far end of the tube, see picture), and electronic sensors measure its height and send the data to a tiny computer. Historical data are available for about 1,450 stations worldwide, of which about 950 have provided updates to the global data center since January 2010. At some places records cover centuries, for example in

Amsterdam Amsterdam ( , ; ; ) is the capital of the Netherlands, capital and Municipalities of the Netherlands, largest city of the Kingdom of the Netherlands. It has a population of 933,680 in June 2024 within the city proper, 1,457,018 in the City Re ...

where data dating back to 1700 is available. When it comes to estimating the greater ocean picture, new modern tide gauges can often be improved upon by using satellite data. Tide gauges are used to measure

tides Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...

and quantify the size of

tsunami A tsunami ( ; from , ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and underwater explosions (including detonations, ...

s. The measurements make it possible to derive the mean

. Using this method, sea level slopes up to several 0.1 m/1000 km and more have been detected. A tsunami can be detected when the sea level begins to rise, although warnings from

seismic activity An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...

can be more useful.

History

Sea-level measurements were made using simple measuring poles or "tide staffs" until around 1830, when self-recording gauges with mechanical floats and stilling wells were introduced. Tidal poles and float gauges were the primary means of sea-level measurement for over 150 years and continue to operate at some locations today. While still part of modern-day tide gauge instrumentation, these technologies have since been superseded by pressure gauges (similar to

depth gauge A depth gauge is an instrument for measuring depth (coordinate), depth below a vertical reference surface. They include depth gauges for underwater diving and similar applications. A diving depth gauge is a pressure gauge that displays the equ ...

s), acoustic/ultrasonic gauges, and radar gauges. The following types of tide gauges have been used historically: * Kelvin type tide gauge * Fuess type tide gauge * GSI type tide gauge * High accuracy automatic tide gauge * High resolution automatic tide gauge

Common applications

Tide gauges have a practical application in the shipping and fishing industries where low or high tide levels can hinder or prohibit access to shallow bays or locations with bridges. An example is the

Cascais tide gauge The Cascais tide gauge () was the first tide gauge (also known as a mareograph or marigraph, as well as a sea-level recorder) installed in Portugal and dates back to 1882. It is situated in Cascais Municipality, Lisbon District. It was one of the f ...

in Portugal, which was originally installed because of the sand bar in the River

Tagus The Tagus ( ; ; ) is the longest river in the Iberian Peninsula. The river rises in the Montes Universales between Cuenca and Teruel, in mid-eastern Spain, flows , generally westward, and empties into the Atlantic Ocean in Lisbon. Name T ...

that causes difficulties for shipping entering

Lisbon Lisbon ( ; ) is the capital and largest city of Portugal, with an estimated population of 567,131, as of 2023, within its administrative limits and 3,028,000 within the Lisbon Metropolitan Area, metropolis, as of 2025. Lisbon is mainlan ...

port. Because of similar problems many industries have installed private tide gauges in ports around the world, and also rely on government agencies (such as

NOAA The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with forecasting weather, monitoring oceanic and atmospheric conditions, charting the seas, conducting deep-sea exploratio ...

). Data collected from tide gauges is also of interest to scientists measuring global weather patterns, the mean sea water level, and trends - notably those potentially associated with global warming.

Modern gauges

In recent years new technologies have developed allowing for real-time, remote tide information to be published online via a solar powered wireless connection to a tide sensor. Acoustic/ultrasonic sensors have already been deployed to great effect and the data is regularly broadcast via

Twitter Twitter, officially known as X since 2023, is an American microblogging and social networking service. It is one of the world's largest social media platforms and one of the most-visited websites. Users can share short text messages, image ...

and also displayed online.

References

External links

Historical Examples
Brown University
Mean Sea Level ExplanationNOAA Tide Data
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