BOX 2.2
Tide Gage Measurements

Tide gages measure the height of the water relative to a monitored geodetic benchmark on land (Figure). Tide gages originally used a float to track the water level inside a vertical tube. The bottom of the tube was closed except for a hole that permitted a small amount of water to enter the tube with time, thus serving as a temporal filter. Slow changes in the sea surface caused by tides or storm surges have sufficient time to fill the tube, while passing waves do not. Today, electronic sensors or bubbler gages have replaced tide gage floats.

Two organizations collect and preserve tide gage records from around the world: the Global Sea Level Observing System, which has established a network of 290 tide gages worldwide; and the Permanent Service for Mean Sea Level, which stores and disseminates the tidal records from more than 2,000 stations around the world.

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FIGURE Examples of tide gage stations. (a) A float and stilling-well gage at Holyhead, UK. SOURCE: UK National Oceanography Centre. (b) A float gage at Vernadsky, Antarctica. SOURCE: British Antarctic Survey. (c) A radar tide gage at Alexandria, Egypt. SOURCE: Courtesy of T. Aarup, Intergovernmental Oceanographic Commission. (d) An acoustic gage at Vaca Key, Florida. Acoustic gages now form the majority of the U.S. sea-level network. SOURCE: National Oceanic and Atmospheric Administration.



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