backtracker (1gmt)

backtracker [infile(s)] -Erotations.txt | -elon/lat/angle [ -A[young/old] ] [ -C ] [ -Df|b ] [ -Fdrift.txt ] [ -H[i][nrec] ] [ -Lf|bstep ] [ -Nupper_age ] [ -Qfixed_age ] [ -Sfilestem ] [ -Tzero_age ] [ -V ] [ -W[a|t] ] [ -:[i|o] ] [ -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [ -m[i|o][flag] ]

backtracker reads (longitude, latitude, age) positions from infiles [or standard input] and computes rotated (x,y,t) coordinates using the specified rotation parameters. It can either calculate final positions [Default] or create a sampled track (flowline or hotspot track) between the initial and final positions. The former mode allows additional data fields after the first 3 columns which must have (longitude,latitude,age). See option -: on how to read (latitude,longitude,age) files.

No space between the option flag and the associated arguments. Use upper case for the option flags and lower case for modifiers.

infile(s)

Data file(s) to be projected. If not given, standard input is read.

-E

Give file with rotation parameters. This file must contain one record for each rotation; each record must be of the following format:

lon lat tstart [tstop] angle [ khat a b c d e f g df ]

where tstart and tstop are in Myr and lon lat angle are in degrees. tstart and tstop are the ages of the old and young ends of a stage. If -C is set then a total reconstruction rotation is expected and tstop is implicitly set to 0 and should not be specified in the file. If a covariance matrix C for the rotation is available it must be specified in a format using the nine optional terms listed in brackets. Here, C = (g/khat)*[ a b d; b c e; d e f ] which shows C made up of three row vectors. If the degrees of freedom (df) in fitting the rotation is 0 or not given it is set to 10000. Blank lines and records whose first column contains # will be ignored.

-e

Alternatively, specify the longitude, latitude, and opening angle (all in degrees and separated by /) for a single total reconstruction rotation that should be applied to all input points.

-A

Used in conjunction with -Lb|f to limit the track output to those sections whose predicted ages lie between the specified young and old limits. If -LB|F is used instead then the limits apply to the stage ids (id 1 is the youngest stage). If no limits are specified then individual limits for each record are expected in columns 4 and 5 of the input file.

-C

Expect Total Reconstruction Rotations rather than Forward Stage Rotations [Default]. File format is similar to the stage pole format except that the tstart column is not present (assumed to be 0 Ma). Requires -E.

-D

Set the direction to go: -Df will go backward in time (from younger to older positions), while -Db will go forward in time (from older to younger positions) [Default]. Note: For -Db you are specifying the age at the given location, whereas for -Df you are not; instead you specify the age at the reconstructed point.

-F

Supply a file with lon, lat, age records that contains the history of hotspot motion for the current hotspot. If given, the reconstructions will only use the 3rd data input column (i.e., the age) to obtain the location of the hotspot at that time, via an interpolation of the hotspot motion history. This adjusted location is then used to reconstruct the point or path [No drift].

-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.

-L

Specify a sampled path between initial and final position: -Lf will draw particle flowlines, while -Lb will draw backtrack (hotspot track) paths. Append sampling interval in km. If step < 0 then only the rotation times will be returned. When -LF or -LB is used, the third output column will contain the stage id (1 is youngest) [Default is along-track predicted ages]. You can control the direction of the paths by using -D.

-N

Set the maximum age to extend the oldest stage rotation back in time [Default is no extension].

-Q

Assign a fixed age to all positions. Only lon, lat input is expected [Default expects longitude, latitude, age]. Useful when the input are points defining isochrons.

-S

When -L is set, the tracks are normally written to stdout as a multisegment file. Specify a filestem to have each track written to filestem.#, where # is the track number. The track number is also copied to the 4th output column.

-T

Set the current time [Default is 0 Ma].

-V

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

-W

Rotates the given input (lon,lat,t) and calculates the confidence ellipse for the projected point. The input point must have a time coordinate that exactly matches a particular finite rotation time, otherwise the point will be skipped. Append t or a to output time or angle, respectively, after the projected lon, lat. After these 2-3 items, we write azimuth, major, minor (in km) for the 95% confidence ellipse. See -D for the direction of rotation.

-:

Toggles between (longitude,latitude) and (latitude,longitude) input and/or output. [Default is (longitude,latitude)]. Append i to select input only or o to select output only. [Default affects both].

-bi

Selects binary input. Append s for single precision [Default is d (double)]. Uppercase S or D will force byte-swapping. 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. [Default is 3 input columns].

-bo

Selects binary output. Append s for single precision [Default is d (double)]. Uppercase S or D will force byte-swapping. Optionally, append ncol, the number of desired columns in your binary output file. [Default depends on settings].

-m

Multiple segment file(s). Segments are separated by a special record. For ASCII files the first character must be flag [Default is '>']. For binary files all fields must be NaN and -b must set the number of output columns explicitly. By default the -m setting applies to both input and output. Use -mi and -mo to give separate settings to input and output.

To backtrack the (x,y,t) points in the file seamounts.d to their origin (presumably the hotspot), using the DC85.d Euler poles, run

backtracker seamounts.d -Db -E DC85.d > newpos.d

To project flowlines forward from the (x,y,t) points stored in several 3-column, binary, double precision files, run

backtracker points.* -Df -E DC85.d -Lf 25 -bo -bi3 > lines.b

This file can then be plotted with psxy -M.

To compute the predicted Hawaiian hotspot track from 0 to 80 Ma every 1 Ma, given a history of hotspot motion file (HIdrift.txt) and a set of total reconstruction rotations for the plate (PAC_APM.d), try

echo 204 19 80 | backtracker -Df -C -E PAC_APM.d -Lb 1 > path.d

Data coordinates are assumed to be geodetic and will automatically be converted to geocentric before spherical rotations are performed. We convert back to geodetic coordinates for output. Note: If your data already are geocentric, you can avoid the conversion by using --ELLIPSOID=sphere.

GMT(1), project(1), grdrotater(1), grdspotter(1), mapproject(1), hotspotter(1), originator(1)

Wessel, P., 1999, "Hotspotting" tools released, EOS Trans. AGU, 80 (29), p. 319.