Tide Gauges: From Pen-and-Ink Tide Houses to Satellite-Timed Sound

A tide gauge answers a simple question — how high is the water right now? — against a fixed height reference called a datum. The answer matters for safe navigation, sound engineering, and the restoration and preservation of coastal habitat. How gauges answer it has changed completely, even though they still take a reading every six minutes.

The NOAA San Francisco Tide Station, in operation for more than 150 years. Credit: NOAA.

The old way: tide houses

Before computers, permanent tide gauges lived in special "tide houses" that protected them from harsh conditions. Inside were a stilling well, a float and a mechanical pen-and-ink recorder. Outside, attached to a pier piling, stood a tide staff — essentially a giant measuring stick — so scientists could read the tide by eye and compare it with the recorder, which took a reading every six minutes on a mechanical timer. Each station needed maintenance every month, when scientists collected the data tapes and mailed them to headquarters to be processed by hand.

An older tide house: the analog recorder sits inside with the float, above the stilling well, while a tidal staff is fixed to a piling outside. Credit: NOAA.

The new way: sound and satellites

Modern gauges use microprocessors, acoustics and electronics. Today's recorders send an audio signal down a half-inch-wide "sounding tube" and measure how long the echo takes to return from the water's surface. Readings still come every six minutes, but the timing is now controlled by a Geostationary Operational Environmental Satellite (GOES), and the data goes straight back to NOAA headquarters.

A newer tide station. The stilling well has been replaced with an acoustic sounding tube, and the tide staff with a pressure sensor. Credit: NOAA.

Newer enclosures look much like the old tide houses, but they protect sensitive electronics, transmitters, backup power and data storage, and are built for the highest accuracy with the least maintenance.

More than tides

Modern stations measure tides more accurately and also record 11 other ocean and weather measurements, including wind speed and direction, air and water temperature, and barometric pressure. NOAA uses the data to keep navigation safe, record and predict sea-level trends and other conditions through nowcasts and forecasts, and publish annual tide predictions.

Sources

Based on "What is a tide gauge?" from NOAA's National Ocean Service, a work of the United States government in the public domain.