25 May 2008

The tide predicter of the U.S. Coast and Geodetic Survey

Robert Merkin of Northampton, Massachusetts, USA, recently sent me a photo of the tide predictor that was in use at the U.S. Coast and Geodetic Survey from 1912 to 1965. It was built by Rollin A. Harris and E. G. Fisher, who modeled it on Lord Kelvin's Tide Predicter, and can handle 37 tidal constituents. It is also known as "Old Brass Brains". Here is Bob Merkin's photo.

Bob also pointed me to a site that offers an animation of Kelvin's Tide Predicter. It shows the workings of the Predicter much clearer than my own description and offers a choice of seven port locations to try it out. Unfortunately it comes under the heading "Fourier Analysis of Ocean Tides".

Fourier Analysis uses multiples of a base period; but the tidal constituents are not multiples of a base period, they are set by the movement of the earth and the moon. Tides are therefore predicted by adding the effect of the various tidal constituents with their known periods. This method is called Harmonic Analysis in oceanography.

In mathematics the term Harmonic Analysis describes an extension of classical Fourier Analysis. When the term Harmonic Analysis is used in the context of tide prediction it does not carry that meaning; it refers to a method that is based on the known periods of the tidal constituents. This has lead to confusion on occasions. Kelvin's Tide Predicter is a very helpful tool to clarify the situation.

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