Tide Gauge Instrumentation

Humans have been measuring water depths for many centuries, using many different techniques. The oldest are still very common, even irreplaceable, like the tide staff. The instrumentation has naturally evolved from the simple tide staff, which still serves as a reference for modern technologies such as radar tide gauges, float type tide gauges, pressure gauges and ultrasonic tide gauges. Each measurement system is detailed in the following pages.

 

The Nilometer: the ancestor of the tide staff

The Nilometer is a way to measure changes in water level of the Nile. First used in Egypt in the 4th century BC, the Nilometer consists of a graduated scale used to quantify the flood levels of the Nile. The taxation of grains depended on the quality of the flood: the better the peak level (not too high or too low), the more silt enriched soil was deposited, so taxes were raised in anticipation of expected good harvests.

 

Nilomètre de l'île d'Eléphantine - Assouan (Crédits Olaf Tausch, mars 2011). Cliquer sur la photo pour l'agrandir

 

We do not know how the Egyptians determined the zeros for Nilometers, but the tide gauges used in France in the early 18th century were generally referenced to the base of the raft foundations of dry docks. This was true in Brest, where routine measurements began in 1711.

 

Float tide gauge and stilling well

A stilling well is needed for permanent tide stations near a coast. Most permanent gauges are located in port areas or along navigable estuaries. The use of float type tide gauges is decreasing, because of the analogue chart records, and especially because of systematic errors caused by the mechanical parts (friction of the float in the stilling well, mechanical wear, etc.). These mechanical gauges are being replaced by automatic digital data logging systems.

 

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