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A tide gauge (also known as mareograph or marigraph〔(International Tsunami Information Center, UNESCO/IOC-NOAA )〕) is a device for measuring the change in sea level relative to a datum. 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. Data are available for over 1,750 stations worldwide. At some places records cover centuries, for example in Amsterdam where data dating back to 1700 is available.〔(【引用サイトリンク】title=Other Long Records not in the PSMSL Data Set )〕 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.〔(Tide gauge history UK National Oceanographic Centre )〕 They 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 While still part of modern-day tide gauge instrumentation, these technologies have since been superseded by pressure gauges, acoustic gauges or radar gauges.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 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「tide gauge」の詳細全文を読む スポンサード リンク
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