FILTER1D

filter1d − Time domain filtering of 1-D time series

filter1d [ infile ] −F<type><width>[mode] [ −Dincrement ] [ −E ] [ −H[nrec] ] [ −Iignore_val ] [ −Llack_width ] [ −Nn_cols/t_col ] [ −Qq_factor ] [ −Ssymmetry_factor ] [ −Tstart/stop/int ] [ −V ] [ −bi[s][n] ] [ −bo[s][n] ] [ −f[i|o]colinfo ]

filter1d is a general time domain filter for multiple column time series data. The user specifies the number of columns of input and which column is the time. (See −N option below). The fastest operation occurs when the input time series are equally spaced and have no gaps or outliers and the special options are not needed. filter1d has options -L, -Q, and -S for unevenly sampled data with gaps.

infile

Multi-column ASCII (or binary, see −b) file holding data values to be filtered.

−F

Sets the filter type. Choose among convolution and non-convolution filters. Append the filter code followed by the full filter width in same units as time column. Available convolution filters are:

(b) Boxcar: All weights are equal.
(c) Cosine Arch: Weights follow a cosine arch curve.
(g) Gaussian: Weights are given by the Gaussian function.
(f) Custom: Instead of width give name of a one-column file of your own weight coefficients.
Non-convolution filters are:
(m) Median: Returns median value.
(p) Maximum likelihood probability (a mode estimator): Return modal value. If more than one mode is found we return their average value. Append - or + to the filter width if you rather want to return the smallest or largest of the modal values.
(l) Lower: Return the minimum of all values.
(L) Lower: Return minimum of all positive values only.
(u) Upper: Return maximum of all values.
(U) Upper: Return maximum or all negative values only.
Upper case type B, C, G, M, P, F will use robust filter versions: i.e., replace outliers (2.5 L1 scale off median) with median during filtering.
In the case of L|U it is possible that no data passes the initial sign test; in that case the filter will return 0.0.

−D

increment is used when series is NOT equidistantly sampled. Then increment will be the abscissae resolution, i.e., all abscissae will be rounded off to a multiple of increment. Alternatively, resample data with sample1d.

−E

Include Ends of time series in output. Default loses half the filter-width of data at each end.

−H

Input file(s) has Header record(s). Number of header records can be changed by editing your .gmtdefaults4 file. If used, GMT default is 1 header record. Use −Hi if only input data should have header records [Default will write out header records if the input data have them].

−I

To ignore values; If an input value equals ignore_val it will be set to NaN.

−L

Checks for Lack of data condition. If input data has a gap exceeding width then no output will be given at that point [Default does not check Lack].

−N

Sets number of columns in input and which column contains the independent variable (time). The left-most column is # 0, the right-most is # (n_cols - 1). [Default is n_cols = 2, t_col = 0; i.e., file has t, f(t) pairs].

−Q

assess Quality of output value by checking mean weight in convolution. Enter q_factor between 0 and 1. If mean weight < q_factor, output is suppressed at this point [Default does not check Quality].

−S

Checks symmetry of data about window center. Enter a factor between 0 and 1. If ( (abs(n_left - n_right)) / (n_left + n_right) ) > factor, then no output will be given at this point [Default does not check Symmetry].

−T

Make evenly spaced timesteps from start to stop by int [Default uses input times].

−V

Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].

−bi

Selects binary input. Append s for single precision [Default is double]. Uppercase S (or D) will force byte-swapping. Append n for the number of columns in the binary file(s).

−bo

Selects binary output. Append s for single precision [Default is double]. Uppercase S (or D) will force byte-swapping. Append n for the number of columns in the binary file(s).

−f

Special formatting of input and output columns (time or geographical data). Specify i(nput) or o(utput) [Default is both input and output]. Give one or more columns (or column ranges) separated by commas. Append T (Absolute calendar time), t (time relative to chosen TIME_EPOCH), x (longitude), y (latitude), or f (floating point) to each column or column range item. Shorthand −f[i|o]g means −f[i|o]0x,1y (geographic coordinates).

The ASCII output formats of numerical data are controlled by parameters in your .gmtdefaults4 file. Longitude and latitude are formatted according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted according to D_FORMAT. Be aware that the format in effect can lead to loss of precision in the output, which can lead to various problems downstream. If you find the output is not written with enough precision, consider switching to binary output (−bo if available) or specify more decimals using the D_FORMAT setting.

To filter the data set in the file cruise.gmtd containing evenly spaced gravity, magnetics, topography, and distance (in m) with a 10 km Gaussian filter, removing outliers, and output a filtered value every 2 km between 0 and 100 km:

filter1d cruise.gmtd −T0/1.0e5/2000 −FG10000 −N4/3 −V > filtered_cruise.gmtd

Data along track often have uneven sampling and gaps which we do not want to interpolate using sample1d. To find the median depth in a 50 km window every 25 km along the track of cruise v3312, stored in v3312.dt, checking for gaps of 10km and asymmetry of 0.3:

filter1d v3312.dt −FM50 −T0/100000/25 −L10 −S0.3 > v3312_filt.dt

GMT(l), sample1d(l)