

infile  Multicolumn ASCII (or binary, see b) file holding data values to be filtered. 
F 
Sets the filter type. Choose among convolution and nonconvolution 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 onecolumn file with your own weight coefficients. Nonconvolution 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 LU 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 filterwidth of data at each end. H Input file(s) has header record(s). If used, the default number of header records is N_HEADER_RECS. Use Hi if only input data should have header records [Default will write out header records if the input data have them]. Blank lines and lines starting with # are always skipped. 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 leftmost column is # 0, the rightmost 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 d (double)]. Uppercase S or D will force byteswapping. Optionally, append ncol, the number of columns in your binary input file if it exceeds the columns needed by the program. Or append c if the input file is netCDF. Optionally, append var1/var2/... to specify the variables to be read. bo Selects binary output. Append s for single precision [Default is d (double)]. Uppercase S or D will force byteswapping. Optionally, append ncol, the number of desired columns in your binary output file. [Default is same as input]. f Special formatting of input and/or output columns (time or geographical data). Specify i or o to make this apply only to input or output [Default applies to both]. Give one or more columns (or column ranges) separated by commas. Append T (absolute calendar time), t (relative time in chosen TIME_UNIT since TIME_EPOCH), x (longitude), y (latitude), or f (floating point) to each column or column range item. Shorthand f[io]g means f[io]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 T 0/1.0e5/2000 FG 10000 N 4/3 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 FM 50 T 0/100000/25 L 10 S 0.3 > v3312_filt.dt
GMT(1), sample1d(1)
GMT 4.5.14  FILTER1D (1)  1 Nov 2015 
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