There are many phenomena including big waves and small waves.
Fourier Transformation is used to analysis periodic phenomena.
Periodicity is showed as frequency. This analysis is called "Frequency analysis" or "Spectrum analysis"
Even if the data is not periodic, approximation of periodic data is effective.
Data has range of numbers. Using the range as the maximum periodicity, we can use the method of periodic data.
If there are big waves and small waves along time series, spectrum analysis is used as Time Series Analysis .
This method is also used for spatial analysis of big waves and small waves.
The data of waves contains various frequencies. If we need to study specific frequency, other frequencies are noise .
"Low pass filter" used to analyze high low frequency like warp or crookedness. It eliminates high frequency.
"High pass filter" used to analyze high low frequency like roughness. It eliminates low frequency.
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