Tide predictor

A Javascript tide harmonic calculator.

🚨Warning🚨

Do not use calculations from this project for navigation, or depend on them in any situation where inaccuracies could result in harm to a person or property.

Tide predictions are only as good as the harmonics data available, and these can be inconsistent and vary widely based on the accuracy of the source data and local conditions.

The tide predictions do not factor events such as storm surge, wind waves, uplift, tsunamis, or sadly, climate change. 😢

Installation

#npm
npm install @neaps/tide-predictor

# yarn
yarn add @neaps/tide-predictor

Importing

You can import the module using Ecmascript, or CommonJS. Note that the CommonJS export is transpiled, so deep debugging the module that way will be difficult.

import TidePredictor from '@neaps/tide-predictor'
const TidePredictor = require('@neaps/tide-predictor')

There are also packaged and minified versions for the browser in dist/web.

Usage

Neaps requires that you provide your own tidal harmonics information to generate a prediction.

Because many constituent datum come with multiple phases (in the case of NOAA's data, they are phase_local and phase_GMT), there is a phaseKey option for choosing which to use.

Note that, for now, Neaps will not do any timezone corrections. This means you need to pass date objects that align with whatever timezone the constituents are in.

import TidePredictor from '@neaps/tide-predictor'

const constituents = [
  {
    phase_GMT: 98.7,
    phase_local: 313.7,
    amplitude: 2.687,
    name: 'M2',
    speed: 28.984104
  }
  //....there are usually many, read the docs
]

const highLowTides = TidePredictor(constituents, {
  phaseKey: 'phase_GMT'
}).getExtremesPrediction({
  start: new Date('2019-01-01'),
  end: new Date('2019-01-10')
})

Tide prediction object

Calling tidePredictor will generate a new tide prediction object. It accepts the following arguments:

• constituents - An array of constituent objects
• options - An object with one of:
  • phaseKey - The name of the parameter within constituents that is considered the "phase"
  • offset - A value to add to all values predicted. This is useful if you want to, for example, offset tides by mean high water, etc.

Tide prediction methods

The returned tide prediction object has various methods. All of these return regular JavaScript objects.

High and low tide - getExtremesPrediction

Returns the predicted high and low tides between a start and end date.

const startDate = new Date()
const endDate = new Date(startDate + 3 * 24 * 60 * 60 * 1000)
const tides = TidePredictor(constituents).getExtremesPrediction({
  start: startDate,
  end: endDate,
  labels: {
    //optional human-readable labels
    high: 'High tide',
    low: 'Low tide'
  }
})

If you want predictions for a subservient station, first set the reference station in the prediction, and pass the subservient station offests to the getExtremesPrediction method:

const tides = TidePredictor(constituents).getExtremesPrediction({
  start: startDate,
  end: endDate,
  offset: {
    height_offset: {
      high: 1,
      low: 2
    },
    time_offset: {
      high: 1,
      low: 2
    }
  }
})

Options

The getExtremesPrediction accepts a single object with options:

• start - Required - The date & time to start looking for high and low tides
• end - Required - The date & time to stop looking for high and low tides
• timeFidelity - Number of seconds accurate the time should be, defaults to 10 minutes.
• labels - An object to define the human-readable labels for the tides
  • high - The human-readable label for high tides
  • low - The human-readable label for low tides
• offset - The offset values if these predictions are for a subservient station

Return values

High and low tides are returned as arrays of objects:

• time - A Javascript Date object of the time
• level - The water level
• high - true if this is a high tide, false if not
• low - true if this is a low tide, false if not
• label - The human-readable label (by default, 'High' or 'Low')

Water level at time - getWaterLevelAtTime

Gives you the predicted water level at a specific time.

const waterLevel = TidePredictor(constituents).getWaterLevelAtTime({
  time: new Date()
})

Options

The getWaterLevelAtTime accepts a single object of options:

• time - A Javascript date object of the time for the prediction

Return values

A single object is returned with:

• time - A Javascript date object
• level - The predicted water level

Data definitions

Constituent definition

Tidal constituents should be an array of objects with at least:

• name - string - The NOAA constituent name, all upper-case.
• amplitude - float - The constituent amplitude
• [phase] - float - The phase of the constituent. Because several services provide different phase values, you can choose which one to use when building your tide prediction.

[
  {
    name: '[constituent name]',
    amplitude: 1.3,
    phase: 1.33
  },
  {
    name: '[constituent name 2]',
    amplitude: 1.3,
    phase: 1.33
  }
]

Subservient station definitions

Some stations do not have defined harmonic data, but do have published offets and a reference station. These include the offsets in time or amplitude of the high and low tides. Subservient station definitions are objects that include:

• height_offset - object - An object of height offets, in the same units as the reference station.
  • high - float - The offset to be added to high tide (can be negative)
  • low - float - The offset to be added to low tide (can be negative)
• time_offset - object - An object of time offets, in number of minutes
  • high - float - The number of minutes to add to high tide times (can be negative)
  • low - float - The number of minutes to add to low tide times (can be negative)

{
  height_offset: {
    high: 1,
    low: 2
  },
  time_offset: {
    high: 1,
    low: 2
  }
}

Shout out

All the really hard math is based on the excellent Xtide and pytides.