How harmonic constituents work

Tide predictions are created by adding up the effects of potentially hundreds of factors like the location of the sun, the moon, and the benthic environment. These constituents are derived from analyzing past water levels.

You can think of each tidal constituent as a sine wave. For example, below is a single constituent, M2, or "Principal lunar semidiurnal constituent" for Monterey, California during the first week of 2018:

If this constituent, which measures the regular effect of the moon on water levels, were the only one that affected the tides, then tides would go up and down in a regular pattern.

However, once you add more constituents to measure more effects, it gets complicated.To simplify, let us take just four constituents and plot them for the same location:

Just for fun, let's take a look at all the defined constituents:

If we add all these constituents together, they cancel each other or amplify each other, leading to an accurate tide prediction:

To read more about tidal constituents, the following resources are helpful:

• Theory of the harmonic model of tides - An exhaustive description of how Pytide and Neaps handle tidal constituents.
• NOAA overview of harmonics
• About harmonic constants and sub station corrections - The classic Xtide description of harmonic constituents.

Where tidal constituents come from

Tidal constituents are derived by running linear regressions (read: lots of maths) on observed water levels. Some agencies, like NOAA, publish tidal constituents that are avialble on their website and via an API. Other agencies do not provide constituents, but do provide data on observed levels.

We are also working on a world-wide database of tidal constiutents.

If you have at least a year of observations (one an hour will usually do), you can generate your own constituents on this website. These can even be added to a growing database of global tide stations.

Constituents supported by Neaps

Neaps supports all the major tidal constituents, and we are still adding the more esoteric ones that affect large shallow bays.