USE OF THE BUTTERWORTH LOW-PASS FILTER FOR OCEANOGRAPHIC DATA.

The characteristics of the Butterworth low-pass filter are well known in electrical engineering. Its use for oceanographic records is discussed and its characteristics are compared with other low-pass filters now in use: the cosine-Lanczos filter, the Gaussian filter, and the ideal filter. The Butterworth filter is recursive, i. e. , past values of the output are used as input, so a phase shift is introduced unless the data are filtered forward and backward through the same filter. When this is done, the filtered signal differs only slightly from that of other low-pass filters. Because the Butterworth filter uses fewer multiplicative constants for the same effect, there is a reduction in computer time over other low-pass filters; the difference becomes more pronounced as more data points are used.