- Tide gauge
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A tide gauge.
A tide gauge is a device for measuring sea level and detecting tsunamis.
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 record it to a tiny computer.
Data is 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. 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.
Tide Gauge at Fort Denison in Sydney, Australia
Tides have been measured at Fort Denison since 1857 on completion of the fort, initially using a bench mark cut into the stonework of the Martello Tower. From 1867 successive instruments were used as tide measuring technology developed.[1]
The Fort Denison photographs below show float activated tide gauge instruments[2] in a cabinet and the system's stilling well. A wire connected to the upper drum mechanism and passes out through the bottom right of the cabinet and runs over a pulley to drop down to the float system in the large pipe in the well. This system is now obsolete at Fort Denison but maintained as a museum exhibit. To the right of the large pipe in the well is an enclosed pipe which rises to the active modern system.
Tide heights and times at Fort Denison are the primary referent for published tide information for other places in the state of New South Wales.
Gallery
Tide gauge instrumentation at Fort Denison 
Tide gauge installation: old and new
Obsolete instruments
Tide gauge's stilling wellFootnotes
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