A tide gauge is a device for measuring the change in sea level relative to a vertical datum. It is also known as a mareograph, marigraph, sea-level recorder and limnimeter. When applied to freshwater continental water bodies, the instrument may also be called a limnimeter. Operation Sensors continuously record the height of the water level with respect to a height reference surface close to the geoid. 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 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 and quantify the size of tsunamis. The measurements make it possible to derive the mean sea level. 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 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 While still part of modern-day tide gauge instrumentation, these technologies have since been superseded by pressure gauges (similar to depth gauges), acoustic/ultrasonic gauges, and radar gauges.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 in Portugal, which was originally installed because of the sand bar in the River Tagus that causes difficulties for shipping entering Lisbon port. Because of similar problems many industries have installed private