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NAME

       x2sys_cross - Calculate crossovers between track data files

SYNOPSIS

       x2sys_cross   track(s)   -TTAG  [   -Acombi.lis  ]  [   -C[runtimes]  ]  [   -Il|a|c  ]  [
       -Jparameters ] [  -Qe|i ] [  -Sl|u|hspeed ] [  -V[level] ] [  -Wsize ] [  -Z ] [ -bobinary
       ] [ -donodata ]

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

DESCRIPTION

       x2sys_cross  is  used  to  determine all intersections between ("external cross-overs") or
       within ("internal cross-overs") tracks (Cartesian or geographic),  and  report  the  time,
       position,  distance  along  track,  heading  and  speed  along each track segment, and the
       crossover error (COE) and mean values for all observables. The names  of  the  tracks  are
       passed  on  the  command  line.  By  default,  x2sys_cross will look for both external and
       internal COEs. As an option, you  may  choose  to  project  all  data  using  one  of  the
       map-projections prior to calculating the COE.

REQUIRED ARGUMENTS

       tracks Can be one or more ASCII, native binary, or COARDS netCDF 1-D data files. To supply
              the data files via a text file with a list of tracks (one per record), specify  the
              name of the track list after a leading equal-sign (e.g., =tracks.lis). If the names
              are missing their file extension we will append the suffix specified for this  TAG.
              Track  files  will be searched for first in the current directory and second in all
              directories listed in $X2SYS_HOME/TAG/TAG_paths.txt (if it exists). [If $X2SYS_HOME
              is not set it will default to $GMT_SHAREDIR/x2sys]. (Note: MGD77 files will also be
              looked for via MGD77_HOME/mgd77_paths.txt and *.gmt files will be searched for  via
              $GMT_SHAREDIR/mgg/gmtfile_paths).

       -TTAG  Specify the x2sys TAG which tracks the attributes of this data type.

OPTIONAL ARGUMENTS

       -Acombi.lis
              Only process the pair-combinations found in the file combi.lis [Default process all
              possible combinations among the specified files]. The  file  combi.lis  created  by
              x2sys_get -L option

       -C[runtimes]
              Compute  and  append the processing run-time for each pair to the progress message.
              Append a filename to save these run-times to file.  The idea here  is  to  use  the
              knowledge  of run-times to split the main process in a number of sub-processes that
              can each be launched in a different processor of your multi-core machine.  See  the
              MATLAB function split_file4coes.m that lives in the x2sys supplement source code.

       -Il|a|c
              Sets the interpolation mode for estimating values at the crossover.  Choose among:

              l Linear interpolation [Default].

              a Akima spline interpolation.

              c Cubic spline interpolation.

       -Jparameters (more ...)
              Select map projection.

       -Qe|i  Append  e  for  external  COEs  only,  and i for internal COEs only [Default is all
              COEs].

       -Rwest/east/south/north[/zmin/zmax][+r][+uunit]
              west, east, south, and north specify the region of interest, and  you  may  specify
              them  in decimal degrees or in [±]dd:mm[:ss.xxx][W|E|S|N] format Append +r if lower
              left and upper right  map  coordinates  are  given  instead  of  w/e/s/n.  The  two
              shorthands  -Rg  and  -Rd stand for global domain (0/360 and -180/+180 in longitude
              respectively, with -90/+90 in latitude).  Alternatively  for  grid  creation,  give
              Rcodelon/lat/nx/ny,  where  code is a 2-character combination of L, C, R (for left,
              center, or right) and T, M, B for top, middle, or bottom. e.g., BL for lower  left.
              This  indicates  which  point on a rectangular region the lon/lat coordinate refers
              to, and the grid dimensions nx and ny with grid spacings via -I is used  to  create
              the corresponding region.  Alternatively, specify the name of an existing grid file
              and the -R settings (and grid spacing, if applicable) are  copied  from  the  grid.
              Appending  +uunit  expects projected (Cartesian) coordinates compatible with chosen
              -J and we inversely project to determine actual rectangular geographic region.  For
              perspective  view  (-p), optionally append /zmin/zmax.  In case of perspective view
              (-p), a z-range (zmin, zmax) can be appended to indicate the third dimension.  This
              needs to be done only when using the -Jz option, not when using only the -p option.
              In the latter case a perspective view of  the  plane  is  plotted,  with  no  third
              dimension. For Cartesian data just give xmin/xmax/ymin/ymax. This option limits the
              COEs to those that fall inside the specified domain.

       -Sl|u|hspeed
              Defines window of track speeds. If  speeds  are  outside  this  window  we  do  not
              calculate a COE. Specify

              -Sl sets lower speed [Default is 0].

              -Su sets upper speed [Default is Infinity].

              -Sh  does  not limit the speed but sets a lower speed below which headings will not
              be computed (i.e., set to NaN) [Default calculates headings regardless of speed].

       -V[level] (more ...)
              Select verbosity level [c].

       -Wsize Give the maximum number of data points on either side of the crossover  to  use  in
              the spline interpolation [3].

       -Z     Report  the  values  of  each track at the crossover [Default reports the crossover
              value and the mean value].

       -bo[ncols][type] (more ...)
              Select native binary output.

       -donodata (more ...)
              Replace output columns that equal NaN with nodata.

       -^ 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  explanation  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.

REMARKS

       The  COEs  found  are  printed out to standard output in ASCII format (unless -bo is set).
       When ASCII is chosen, the output format depends on whether or not old-style  XOVER  output
       (-L)  has  been  selected  [See  the x_over man page for more details]. If ASCII, then the
       first record contains the name of the tag used, the second  records  specifies  the  exact
       command  line  used  for this run, and the third record contains the names of each column.
       For each track pair, there will be a segment header record containing the two  file  names
       and  their  start/stop/dist information (start/stop is absolute time or NaN if unavailable
       while dist is the total track length), whereas subsequent records have the data  for  each
       COE  encountered.  The  fields  written out are x, y, time along track #1 and #2, distance
       along track #1 and #2, heading along track #1 and #2, velocity along track #1 and #2,  and
       then  pairs  of  columns  for  each  selected  observable. These are either pairs of (COE,
       average value) for each data type (or track-values #1 and #2; see -Z). It  is  recommended
       that  the  Akima  spline  is  used  instead  of the natural cubic spline, since it is less
       sensitive to outliers that tend to introduce wild oscillations in the interpolation.

SIGN CONVENTION

       If track_a and track_b are passed on the  command  line,  then  the  COE  value  is  Value
       (track_a) - Value (track_b).

PRECISION AND FORMAT

       The  output  format  of  individual  columns are controlled by FORMAT_FLOAT_OUT except for
       geographic coordinates  (FORMAT_GEO_OUT)  and  absolute  calendar  time  (FORMAT_DATE_OUT,
       FORMAT_CLOCK_OUT).   Make  sure  these  are  setto  give  you enough significant digits to
       achieve the desired precision.

EXAMPLES

       To compute all internal crossovers in the gmt-formatted file c2104.gmt, and using the  tag
       GMT, try

              gmt x2sys_cross c2104.gmt -TGMT > c2104.d

       To  find  the crossover locations with bathymetry between the two MGD77 files A13232.mgd77
       and A99938.mgd77, using the MGD77 tag, try

              gmt x2sys_cross A13232.mgd77 A99938.mgd77 -Qe -TMGD77 > crossovers.d

REFERENCES

       Wessel, P. (2010), Tools for analyzing intersecting tracks: the x2sys  package.  Computers
       and Geosciences, 36, 348-354.

       Wessel,  P.  (1989),  XOVER:  A  cross-over  error  detector for track data, Computers and
       Geosciences, 15(3), 333-346.

SEE ALSO

       gmt,  x2sys_binlist,  x2sys_init,  x2sys_datalist,   x2sys_get,   x2sys_list,   x2sys_put,
       x2sys_report, x2sys_solve, x_over

COPYRIGHT

       2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe