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harmonic constants

Sometimes shortened to just "harmonics," this refers to the set of numbers that are needed to compute tide height and tidal current at one of the NOAA reference stations, now called harmonic stations. This is the data set that navigation programs need in order to compute tides and currents and display them on the screen. See samples at tidesandcurrents.noaa.gov. NOAA makes public these constants for US waters, but most other nations do not share their corresponding data.

Harmonic constants are sometimes called harmonic constituents, but some would make a distinction. The final values of the tide and currents are the result of two major factors. One is the relative positions of the sun, moon, and earth, which might be called the astronomical component, and the other is the geographical location of the point of interest within a waterway, including its shape, and bathymetry. This second factor can be called the hydrodynamic component. Some discussions refer to the combination of these two as the constituents, with the hydrodynamic components alone being called the constants.

This distinction does not matter to us, as we have to have both parts, and they will be in either a single file or a pair of files, usually called "harmonic constants." The single file format will have an extension TCD (tidal constituent database), or it will be in two files, one called HARMONIC and the other HARMONIC.idx. The single file version covering all US tides and currents is about 4 MB in size.

The harmonics for most major ports of the world were at one time public domain, arriving there from various published research papers. But at some point several nations decided to treat this data as their own intellectual property that they use to create and sell tide and current forecasts each year. Some, led by the UK, aggressively prohibit the sale or even distribution of this one time public data. The US is leader of a small, elite club of nations who still provide this data at no charge to mariners.

Since this data was at one time public, there are still unofficial sets international harmonic constants available from various unofficial sources. Any use of such data must be done with extreme caution as there can and even likely will be errors in it. Our course on Electronic Chart Navigation shows ways to test such data if available.




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