backtracker(1) GMT backtracker(1)
NAME
backtracker - Generate forward and backward flowlines and hotspot
tracks
SYNOPSIS
backtracker [ table ] -Erot_file|lon/lat/angle [ -A[young/old] ] [
-Df|b ] [ -Fdrift.txt ] [ -Lf|b[step] ] [ -Nupper_age ] [
-Qfixed_age ] [ -Sfilestem ] [ -Tzero_age ] [ -V[level] ] [ -W[a|t]
] [ -bbinary ] [ -dnodata ] [ -eregexp ] [ -hheaders ] [ -iflags ] [
-oflags ] [ -:[i|o] ]
Note: No space is allowed between the option flag and the associated
arguments.
DESCRIPTION
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,lati-
tude,age). See option -: on how to read (latitude,longitude,age) files.
REQUIRED ARGUMENTS
-Erotfile
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 tstop is not present in the record then a total
reconstruction rotation is expected and tstop is implicitly set
to 0 and should not be specified for any of the records 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. You may prepend a leading + to the filename to
indicate you wish to invert the rotations. Alternative 1: Give
the filename composed of two plate IDs separated by a hyphen
(e.g., PAC-MBL) and we will instead extract that rotation from
the GPlates rotation database. We return an error if the rota-
tion cannot be found. Alternative 2: Specify lon/lat/angle,
i.e., the longitude, latitude, and opening angle (all in degrees
and separated by /) for a single total reconstruction rotation.
OPTIONAL ARGUMENTS
table One or more ASCII (or binary, see -bi[ncols][type]) data table
file(s) holding a number of data columns. If no tables are given
then we read from standard input.
-A[young/old]
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 lim-
its are specified then individual limits for each record are
expected in columns 4 and 5 of the input file.
-Df|b 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.
-Fdrift.txt
Supply a file with lon, lat, age records that describe the his-
tory of hotspot motion for the current hotspot. The reconstruc-
tions will only use the 3rd data input column (i.e., the age) to
obtain the location of the hotspot at that time, via an interpo-
lation of the hotspot motion history. This adjusted location is
then used to reconstruct the point or path [No drift].
-Lf|b[step]
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 or not provided 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.
-Nupper_age
Set the maximum age to extend the oldest stage rotation back in
time [Default is no extension].
-Qfixed_age
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.
-Sfilestem
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.
-Tzero_age
Set the current time [Default is 0 Ma].
-V[level] (more a|)
Select verbosity level [c].
-W[a|t]
Rotates the given input (lon,lat,t) and calculates the confi-
dence ellipse for the projected point. The input point must have
a time coordinate that exactly matches a particular total recon-
struction 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.
-bi[ncols][t] (more a|)
Select native binary input. [Default is 3 input columns].
-bo[ncols][type] (more a|)
Select native binary output. [Default is same as input].
-d[i|o]nodata (more a|)
Replace input columns that equal nodata with NaN and do the
reverse on output.
-e[~]^<i>apattern^<i>a | -e[~]/regexp/[i] (more a|)
Only accept data records that match the given pattern.
-h[i|o][n][+c][+d][+rremark][+rtitle] (more a|)
Skip or produce header record(s).
-icols[+l][+sscale][+ooffset][,^<i>a|] (more a|)
Select input columns and transformations (0 is first column).
-ocols[,a|] (more a|)
Select output columns (0 is first column).
-:[i|o] (more a|)
Swap 1st and 2nd column on input and/or output.
-^ or just -
Print a short message about the syntax of the command, then
exits (NOTE: on Windows just use -).
-+ or just +
Print an extensive usage (help) message, including the explana-
tion of any module-specific option (but not the GMT common
options), then exits.
-? or no arguments
Print a complete usage (help) message, including the explanation
of all options, then exits.
GEODETIC VERSUS GEOCENTRIC COORDIINATES
All spherical rotations are applied to geocentric coordinates. This
means that incoming data points and grids are considered to represent
geodetic coordinates and must first be converted to geocentric coordi-
nates. Rotations are then applied, and the final reconstructed points
are converted back to geodetic coordinates. This default behavior can
be bypassed if the ellipsoid setting PROJ_ELLIPSOID is changed to
Sphere.
EXAMPLES
To backtrack the (x,y,t) points in the file seamounts.txt to their ori-
gin (presumably the hotspot), using the DC85.txt Euler poles, run
gmt backtracker seamounts.txt -Db -EDC85.txt > newpos.txt
To project flowlines forward from the (x,y,t) points stored in several
3-column, binary, double precision files, run
gmt backtracker points.\* -Df -EDC85.txt -Lf25 -bo -bi3 > lines.b
This file can then be plotted with psxy. 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.txt), try
echo 204 19 80 | gmt backtracker -Df -EPAC_APM.txt -Lb1 > path.txt
NOTES
GMT distributes the EarthByte rotation model Global_Earth-
Byte_230-0Ma_GK07_AREPS.rot. To use an alternate rotation file, create
an environmental parameters named GPLATES_ROTATIONS that points to an
alternate rotation file.
SEE ALSO
gmt(1), gmtpmodeler(1), grdpmodeler(1), grdrotater(1), grdspotter(1),
hotspotter(1), mapproject(1), originator(1), project(1), psxy(1)
REFERENCES
Wessel, P., 1999, aHotspottinga tools released, EOS Trans. AGU, 80
(29), p. 319.
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 backtracker(1)
gmt5 5.4.2 - Generated Wed Jun 28 14:41:52 CDT 2017