The input of the transformation is two scalar time series. The first one is non-equidistant and contains the times of the astronomical low tide. The values are not the levels, but the tide number. The second input is a regular, continuous time series that contains water levels. This can be a forecast, an observed or hindcast series.
The transformation will use the time stamps of the astronomical tide number input, and it will search for low tide values in the continuous time series around each of those times. The search window can be skewed around this astronomical extreme as the period before and after the extreme can be specified individually using minutesBeforeTidalExtreme and minutesAfterTidalExtreme.
Within this search window, the slope of the continuous timeseries is determined over the regression period (regressionPeriodInMinutes). This slope is determined looking forward for every timestep in the continuous series. A minimum or maximum is found if the slope is horizontal (thus the value of the angle is 0). If the slope value changes from positive to negative, it is a maximum, if it changes from negative to positive, it is a minimum.
A low tide value is valid if:
- There is a value within the search window both before and after the extreme where the difference between the peak and the value is greater than the validDifferenceExceedence. The value before the peak is called firstThresholdValue in this method, the value after the peak is called the secondThreshold value.
- Missing values are not allowed between the peak and the two threshold values.
If a missing value is found after the minimum and before the second threshold, the peak is automatically invalid.
The transformation produces two non-equidistant output series with timestamps of the low tide extremes: one serie holds the water level, the orther series holds the associated tide number.
<transformation id="SelectionTideNumbersAndLows">
<selection>
<selectNumberedTidalLows>
<numberedTidalLows>
<variableId>numberedTidalLows</variableId>
</numberedTidalLows>
<continuousLevelSeries>
<variableId>continuousLevelSeries</variableId>
</continuousLevelSeries>
<minutesBeforeTidalExtreme>50</minutesBeforeTidalExtreme>
<minutesAfterTidalExtreme>50</minutesAfterTidalExtreme>
<regressionPeriodInMinutes>30</regressionPeriodInMinutes>
<validDifferenceExceedence>0.02</validDifferenceExceedence>
<lowWaterLevelOutput>
<variableId>outputTideValue</variableId>
</lowWaterLevelOutput>
<lowWaterTideNumberOutput>
<variableId>outputTideNumber</variableId>
</lowWaterTideNumberOutput>
</selectNumberedTidalLows>
</selection>
</transformation>
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