# gmtsplit

Split xyz[dh] data tables into individual segments

## Synopsis

gmt split [ table ] [ -A azimuth/tolerance ] [ -C course_change] [ -D minimum_distance ] [ -F xy_filter/z_filter ] [ -N[template] ] [ -Q flags ] [ -S ] [ -V[level] ] [ -b binary ] [ -d nodata[+c col] ] [ -e regexp ] [ -f flags ] [ -g gaps ] [ -h headers ] [ -i flags ] [ -q flags ] [ -s flags ] [ -:[i|o] ] [ --PAR=value ]

Note: No space is allowed between the option flag and the associated arguments.

## Description

split reads a series of (x, y[, z]) records [or optionally (x, y [, z], d, h); see -S option] from standard input [or xy[z][dh]file] and splits this into separate lists of (x, y[, z]) series, such that each series has a nearly constant azimuth through the x-y plane. There are options to choose only those series which have a certain orientation, to set a minimum length for series, and to high- or low-pass filter the z values and/or the x, y values. split is a useful filter between data extraction and wiggle plotting, and can also be used to divide a large x, y[, z] dataset into segments.

## Required Arguments

table

  One or more ASCII [or binary, see -bi] files with 2, 3, or 5 columns holding (x, y[, z[,d,h]]) data values. To use (x ,y, z, d, h) input, sorted so that d is non-decreasing, specify the -S option; default expects (x, y, z) only. If no files are specified, split will read from standard input.

## Optional Arguments

-A azimuth/tolerance

  Write out only those segments which are within ±tolerance degrees of azimuth in heading, measured clockwise from North, [0 - 360]. [Default writes all acceptable segments, regardless of orientation].

-C course_change

  Terminate a segment when a course change exceeding course_change degrees of heading is detected [ignore course changes].

-D minimum_distance

  Do not write a segment out unless it is at least minimum_distance units long [0].

-F xy_filter/z_filter

  Filter the z values and/or the x, y values, assuming these are functions of d coordinate. xy_filter and z_filter are filter widths in distance units. If a filter width is zero, the filtering is not performed. The absolute value of the width is the full width of a cosine-arch low-pass filter. If the width is positive, the data are low-pass filtered; if negative, the data are high-pass filtered by subtracting the low-pass value from the observed value. If z_filter is non-zero, the entire series of input z values is filtered before any segmentation is performed, so that the only edge effects in the filtering will happen at the beginning and end of the complete data stream. If xy_filter is non-zero, the data are first divided into segments and then the x, y values of each segment are filtered separately. This may introduce edge effects at the ends of each segment, but prevents a low-pass x, y filter from rounding off the corners of track segments. [Default = no filtering].

-N[template]

  Write each segment to a separate output file [Default writes a multiple segment file to standard output]. Optionally append a format template for the individual file names; this template must contain a C format specifier that can format an integer argument (the running segment number across all tables); this is usually %d but could be %08d which gives leading zeros, etc. [Default is gmtsplit_segment_%d.{txt|bin}, depending on -bo]. Alternatively, give a template with two C format specifiers and we will supply the table number and the segment number within the table to build the file name.

-Q flags

  Specify your desired output using any combination of xyzdh, in any order. Do not space between the letters. Use lower case. The output will be ASCII (or binary, see -bo) columns of values corresponding to xyzdh [Default is -Q xyzdh (-Q xydh if only 2 input columns)].

-S

  Both d and h are supplied. In this case, input contains x, y, z, d, h. [Default expects (x, y, z) input, and d,h are computed from delta x, delta y. Use -fg to indicate map data; then x, y are assumed to be in degrees of longitude, latitude, distances are considered to be in kilometers, and angles are actually azimuths. Otherwise, distances are Cartesian in same units as x, y and angles are counter-clockwise from horizontal].

-V[level]

  Select verbosity level [w]. (See full description) (See cookbook information).

-bi record[+b|l] (more …)

  Select native binary format for primary table input. [Default is 2, 3, or 5 input columns as set by -S].

-bo record[+b|l] (more …)

  Select native binary format for table output. [Default is 1-5 output columns as set by -Q].

-d[i|o][+c col]nodata (more …)

  Replace input columns that equal nodata with NaN and do the reverse on output.

-e[~]“pattern” | -e[~]/regexp/[i] (more …)

  Only accept data records that match the given pattern.

-f[i|o]colinfo (more …)

  Specify data types of input and/or output columns.

-g x|y|z|d|X|Y|D gap[u][+a][+c col][+n|p] (more …)

  Determine data gaps and line breaks. Do not let a segment have a gap exceeding gap; instead, split it into two segments. [Default ignores gaps].

-h[i|o][n][+c][+d][+m segheader][+r remark][+t title] (more …)

  Skip or produce header record(s).

-i cols[+l][+d divisor][+s scale|d|k][+o offset][,…][,t[word]] (more …)

  Select input columns and transformations (0 is first column, t is trailing text, append word to read one word only).

-q[i|o][~]rows|limits[+c col][+a|t|s] (more …)

  Select input or output rows or data limit(s) [all].

-s[cols][+a][+r] (more …)

  Set handling of NaN records for output.

-:[i|o] (more …)

  Swap 1st and 2nd column on input and/or output.

-^ or just -

  Print a short message about the syntax of the command, then exit (Note: on Windows just use -).

-+ or just +

  Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exit.

-? or no arguments

  Print a complete usage (help) message, including the explanation of all options, then exit.

--PAR=value

  Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.

## ASCII Format Precision

The ASCII output formats of numerical data are controlled by parameters in your gmt.conf file. Longitude and latitude are formatted according to FORMAT_GEO_OUT, absolute time is under the control of FORMAT_DATE_OUT and FORMAT_CLOCK_OUT, whereas general floating point values are formatted according to FORMAT_FLOAT_OUT. Be aware that the format in effect can lead to loss of precision in ASCII 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 FORMAT_FLOAT_OUT setting.

## Distance Calculations

The type of input data is dictated by the -f option. If -fg is given then x, y are in degrees of longitude, latitude, distances are in kilometers, and angles are azimuths. Otherwise, distances are Cartesian in same units as x, y and angles are counter-clockwise from horizontal.

## Examples

Note: Below are some examples of valid syntax for this module. The examples that use remote files (file names starting with `@`) can be cut and pasted into your terminal for testing. Other commands requiring input files are just dummy examples of the types of uses that are common but cannot be run verbatim as written.

Note: Since many GMT plot examples are very short (i.e., one module call between the gmt begin and gmt end commands), we will often present them using the quick modern mode GMT Modern Mode One-line Commands syntax, which simplifies such short scripts.

Suppose you want to make a wiggle plot of magnetic anomalies on segments oriented approximately east-west from a NCEI-supplied cruise called JA020015 in the region -R300/315/12/20. You want to use a 100 km low-pass filter to smooth the tracks and a 500km high-pass filter to detrend the magnetic anomalies. Try this:

    gmt mgd77list JA020015 -R300/315/12/20 -Flon,lat,mag,dist,azim | gmt split -A90/15 -F100/-500 \
        -D100 -S -V -fg | gmt wiggle -R300/315/12/20 -Jm0.6i -Baf -B+tJA020015 -T1 \
        -W0.75p -Ggray -Z200 -pdf JA020015_wiggles

MGD-77 users: For this application we recommend that you extract dist,azim from mgd77list rather than have split compute them separately.

Suppose you have been given a binary, double-precision file containing lat, lon, gravity values from a survey, and you want to split it into profiles named survey_###.txt (when gap exceeds 100 km). Try this:

    gmt split survey.bin -Nsurvey_%03d.txt -V -gd100k -D100 -: -fg -bi3d

## See Also

gmt, filter1d, mgd77list, wiggle