BACKTRACKER(1gmt)					       Generic Mapping Tools						 BACKTRACKER(1gmt)

NAME

       backtracker - Reconstruct points, flowlines and hotspot tracks

SYNOPSIS

       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] ]

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 (longi-
       tude,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  for-
	      mat:

		   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 free-
	      dom  (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 reconstruc-
	      tion rotation that should be applied to all input points.

OPTIONS

       -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  speci-
	      fied 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 for-
	      mat 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  recon-
	      structions will only use the 3rd data input column (i.e., the age) to obtain the location of the hotspot at that time, via an inter-
	      polation 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.  Option-
	      ally, 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.  Option-
	      ally, 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.

EXAMPLES

       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 -EDC85.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 -EDC85.d -Lf25 -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 -EPAC_APM.d -Lb1 > path.d

COORDINATES

       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 --ELLIP-
       SOID=sphere.

SEE ALSO

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

REFERENCES

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

GMT 4.5.7							    15 Jul 2011 						 BACKTRACKER(1gmt)