- Find and compute crossover errors
x2sys_cross track(s) -TTAG
] [ -Jparameters
] [ -L
] [ -Qe
] [ -V
] [ -2
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
- 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
- Specify the x2sys TAG which tracks the attributes of this data
No space between the option flag and the associated arguments.
- Sets the interpolation mode for estimating values at the crossover. Choose
l Linear interpolation [Default].
a Akima spline interpolation.
c Cubic spline interpolation.
- Selects the map projection. Scale is UNIT/degree, 1:xxxxx, or width in
UNIT (upper case modifier). UNIT is cm, inch, or m, depending on the
MEASURE_UNIT setting in .gmtdefaults4, but this can be overridden
on the command line by appending c, i, or m to the
scale/width value. When central meridian is optional, default is center of
longitude range on -R option. Default standard parallel is the
equator. For map height, max dimension, or min dimension, append h,
+, or - to the width, respectively.
More details can be found in the psbasemap man pages.
-Jmlon0/lat0/scale (Mercator - Give meridian and standard
-Jo[a]lon0/lat0/azimuth/scale (Oblique Mercator -
point and azimuth)
-Jo[b]lon0/lat0/lon1/lat1/scale (Oblique Mercator -
-Joclon0/lat0/lonp/latp/scale (Oblique Mercator - point and
-Jtlon0/[lat0/]scale (TM - Transverse Mercator)
-Juzone/scale (UTM - Universal Transverse Mercator)
-Jy[lon0/[lat0/]]scale (Cylindrical Equal-Area)
-Jdlon0/lat0/lat1/lat2/scale (Conic Equidistant)
-Jllon0/lat0/lat1/lat2/scale (Lambert Conic Conformal)
-Jalon0/lat0[/horizon]/scale (Lambert Azimuthal
-Jkf[lon0/]scale (Eckert IV)
-Jk[s][lon0/]scale (Eckert VI)
-Jr[lon0/]scale (Winkel Tripel)
-Jv[lon0/]scale (Van der Grinten)
(Linear, log, and power scaling)
- Only process the pair-combinations found in the file combi.lis
[Default process all possible combinations among the specified
- Output results using the old XOVER format [Default is x2sys format]. This
option should only be used with *.gmt-formatted MGD77 files. See the GMT
mgg supplement for file description; see Wessel  for details
on the XOVER format.
- Append e for external COEs only, and i for internal COEs
only [Default is all COEs].
- 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, specify the name of an existing grid file and the -R
settings (and grid spacing, if applicable) are copied from the grid. For
Cartesian data just give xmin/xmax/ymin/ymax. This option limits
the COEs to those that fall inside the specified domain.
- 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].
- Selects verbose mode, which will send progress reports to stderr [Default
- Give the maximum number of data points on either side of the crossover to
use in the spline interpolation .
- Report the values of each track at the crossover [Default reports the
crossover value and the mean value].
- 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.
The COEs found are printed out to standard output in ASCII format (unless
is set). When ASCII is chosen, the output format depends on whether
or not old-style XOVER output ( -L
) has been selected [See the
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 multisegment 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 -2
). 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.
If track_a and track_b are passed on the command line, then the COE value is
Value (track_a) - Value (track_b).
The output format of individual columns are controlled by D_FORMAT
for geographic coordinates ( OUTPUT_DEGREE_FORMAT
) and absolute
calendar time ( OUTPUT_DATE_FORMAT
sure these are set to give you enough significant digits to achieve the
To compute all internal crossovers in the gmt-formatted file c2104.gmt, and
output in the old XOVER format, using the tag GMT, try
c2104.gmt -L -T
GMT > c2104.d
To find the crossover locations with bathymetry between the two MGD77 files
A13232.mgd77 and A99938.mgd77, using the MGD77 tag, try
A13232.mgd77 A99938.mgd77 -Qe -T
Wessel, P. (2010), Tools for analyzing intersecting tracks: the x2sys package.
IT(Computers and Geosciences), BD(36), 348-354.
Wessel, P. (1989), XOVER: A cross-over error detector for track data,
Computers and Geosciences, 15