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Smooth command (Edit menu)

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Smooths the amplitudes for a selected curve.

The X values for the selected curve(s) must be monotonically increasing. If no curves possess that characteristic, this command is disabled.

NOTE: If your data is not sorted and you are certain it is appropriate to sort it, you can use the Sort command on the Edit menu to sort your data points before using this command. Sorting isn't appropriate for data sets describing measurements that, by design, have two or more Y values for any unique X, e.g. moment-thrust diagrams.

The Smooth command removes any linear trend to the data so that the first and last amplitudes are zero, uses a Fast Fourier Transform to low-pass filter the data, and finally reinserts the linear trend to the amplitudes.

The “smoothing window” is the number of points over which the data should be smoothed. Zero gives no smoothing at all, while a value approaching half the number of data points will render the data virtually featureless.

The algorithm used by the Smooth command is taken from “Numerical Recipes” by Press, Flannery, Teukolsky, and Vetterling. This reference includes the following qualification:

Data smoothing is probably most justified when it is used simply as a graphical technique, to guide the eye through a forest of data points all with large error bars. In this case, the individual points and their error bars should be plotted on the same graph, and no quantitative claims should be made on the basis of the smoothed curve. Data smoothing is least justified when it is used subjectively to massage the data this way and that, until some feature in the smoothed curve emerges and is pounced on in support of an hypothesis.

As an alternative to the Filter command with the Bandpass option, you can low-pass filter your data given the correct "smoothing window". Use a smoothing window of 1/(cutoff frequency * time increment). For example, for a 60Hz low-pass filter on data with a time increment of 1 millisecond, use 1/(60 cycles/sec * 0.001 sec/point) = 16.6667 points.


Related macro commands



See also

Fast Fourier Transform




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