Provided by: gmt-common_5.4.3+dfsg-1_all 
NAME
grdtrack - Sample grids at specified (x,y) locations
SYNOPSIS
grdtrack [ xyfile ] -Ggrd1 -Ggrd2 … [ -Af|p|m|r|R[+l] ] [ -Clength[u]/ds[/spacing][+a][+v] ] [
-Ddfile ] [ -Eline ] [ -N ] [ -Rregion ] [ -Smethod/modifiers ] [ -T[radius[u]][+e|p]] [ -V[level]
] [ -Z ] [ -bbinary ] [ -dnodata ] [ -eregexp ] [ -fflags ] [ -ggaps ] [ -hheaders ] [ -iflags ] [
-nflags ] [ -oflags ] [ -sflags ] [ -:[i|o] ]
Note: No space is allowed between the option flag and the associated arguments.
DESCRIPTION
grdtrack reads one or more grid files (or a Sandwell/Smith IMG files) and a table (from file or standard
input; but see -E for exception) with (x,y) [or (lon,lat)] positions in the first two columns (more
columns may be present). It interpolates the grid(s) at the positions in the table and writes out the
table with the interpolated values added as (one or more) new columns. Alternatively (-C), the input is
considered to be line-segments and we create orthogonal cross-profiles at each data point or with an
equidistant separation and sample the grid(s) along these profiles. A bicubic [Default], bilinear,
B-spline or nearest-neighbor (see -n) interpolation is used, requiring boundary conditions at the limits
of the region (see -n; Default uses “natural” conditions (second partial derivative normal to edge is
zero) unless the grid is automatically recognized as periodic.)
REQUIRED ARGUMENTS
-Ggridfile
grdfile is a 2-D binary grid file with the function f(x,y). If the specified grid is in
Sandwell/Smith Mercator format you must append a comma-separated list of arguments that includes a
scale to multiply the data (usually 1 or 0.1), the mode which stand for the following: (0) Img
files with no constraint code, returns data at all points, (1) Img file with constraints coded,
return data at all points, (2) Img file with constraints coded, return data only at constrained
points and NaN elsewhere, and (3) Img file with constraints coded, return 1 at constraints and 0
elsewhere, and optionally the max latitude in the IMG file [80.738]. You may repeat -G as many
times as you have grids you wish to sample. Alternatively, use -G+llist to pass a list of file
names. The grids are sampled and results are output in the order given. (See GRID FILE FORMAT
below.)
OPTIONAL ARGUMENTS
xyfile This is an ASCII (or binary, see -bi) file where the first 2 columns hold the (x,y) positions
where the user wants to sample the 2-D data set.
-Af|pm|r|R[+l]
For track resampling (if -C or -E are set) we can select how this is to be performed. Append f to
keep original points, but add intermediate points if needed [Default], m as f, but first follow
meridian (along y) then parallel (along x), p as f, but first follow parallel (along y) then
meridian (along x), r to resample at equidistant locations; input points are not necessarily
included in the output, and R as r, but adjust given spacing to fit the track length exactly.
Finally, append +l if distances should be measured along rhumb lines (loxodromes). Ignored unless
-C is used.
-Clength[u]/ds[/spacing][+a][+v]
Use input line segments to create an equidistant and (optionally) equally-spaced set of crossing
profiles along which we sample the grid(s) [Default simply samples the grid(s) at the input
locations]. Specify two length scales that control how the sampling is done: length sets the full
length of each cross-profile, while ds is the sampling spacing along each cross-profile.
Optionally, append /spacing for an equidistant spacing between cross-profiles [Default erects
cross-profiles at the input coordinates]. By default, all cross-profiles have the same direction
(left to right as we look in the direction of the input line segment). Append +a to alternate the
direction of cross-profiles, or v to enforce either a “west-to-east” or “south-to-north” view.
Append suitable units to length; it sets the unit used for ds [and spacing] (See UNITS below). The
default unit for geographic grids is meter while Cartesian grids implies the user unit. The
output columns will be lon, lat, dist, azimuth, z1, z2, …, zn (The zi are the sampled values for
each of the n grids)
-Ddfile
In concert with -C we can save the (possibly resampled) original lines to the file dfile [Default
only saves the cross-profiles]. The columns will be lon, lat, dist, azimuth, z1, z2, … (sampled
value for each grid)
-Eline[,line,…][+aaz][+d][+iinc[u]][+llength[u]][+nnp][+oaz][+rradius[u]
Instead of reading input track coordinates, specify profiles via coordinates and modifiers. The
format of each line is start/stop, where start or stop are either lon/lat (x/y for Cartesian data)
or a 2-character XY key that uses the pstext-style justification format format to specify a point
on the map as [LCR][BMT]. In addition, you can use Z-, Z+ to mean the global minimum and maximum
locations in the grid (only available if only one grid is given). Instead of two coordinates you
can specify an origin and one of +a, +o, or +r. You may append +iinc[u] to set the sampling
interval; if not given then we default to half the minimum grid interval. The +a sets the azimuth
of a profile of given length starting at the given origin, while +o centers the profile on the
origin; both require +l. For circular sampling specify +r to define a circle of given radius
centered on the origin; this option requires either +n or +i. The +nnp sets the desired number of
points, while +llength gives the total length of the profile. Append +d to output the along-track
distances after the coordinates. Note: No track file will be read. Also note that only one
distance unit can be chosen. Giving different units will result in an error. If no units are
specified we default to great circle distances in km (if geographic). If working with geographic
data you can prepend - (Flat Earth) or + (Geodesic) to inc, length, or radius to change the mode
of distance calculation [Great Circle]. Note: If -C is set and spacing is given the that sampling
scheme overrules any modifier in -E.
-N Do not skip points that fall outside the domain of the grid(s) [Default only output points within
grid domain].
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more …)
Specify the region of interest.
-Smethod/modifiers
In conjunction with -C, compute a single stacked profile from all profiles across each segment.
Append how stacking should be computed: a = mean (average), m = median, p = mode (maximum
likelihood), l = lower, L = lower but only consider positive values, u = upper, U = upper but only
consider negative values [a]. The modifiers control the output; choose one or more among these
choices: +a : Append stacked values to all cross-profiles. +d : Append stack deviations to all
cross-profiles. +r : Append data residuals (data - stack) to all cross-profiles. +s[file] : Save
stacked profile to file [grdtrack_stacked_profile.txt]. +cfact : Compute envelope on stacked
profile as +/- fact *deviation [2]. Notes: (1) Deviations depend on method and are st.dev (a), L1
scale (m and p), or half-range (upper-lower)/2. (2) The stacked profile file contains a leading
column plus groups of 4-6 columns, with one group for each sampled grid. The leading column holds
cross distance, while the first four columns in a group hold stacked value, deviation, min value,
and max value, respectively. If method is one of a|m|p then we also write the lower and upper
confidence bounds (see +c). When one or more of +a, +d, and +r are used then we also append the
stacking results to the end of each row, for all cross-profiles. The order is always stacked value
(+a), followed by deviations (+d) and finally residuals (+r). When more than one grid is sampled
this sequence of 1-3 columns is repeated for each grid.
-T[radius[u]][+e|p]
To be used with normal grid sampling, and limited to a single, non-IMG grid. If the nearest node
to the input point is NaN, search outwards until we find the nearest non-NaN node and report that
value instead. Optionally specify a search radius which limits the consideration to points within
this distance from the input point. To report the location of the nearest node and its distance
from the input point, append +e. To instead replace the input point with the coordinates of the
nearest node, append +p.
-V[level] (more …)
Select verbosity level [c].
-Z Only write out the sampled z-values [Default writes all columns].
-: Toggles between (longitude,latitude) and (latitude,longitude) input/output. [Default is
(longitude,latitude)].
-bi[ncols][t] (more …)
Select native binary input. [Default is 2 input columns].
-bo[ncols][type] (more …)
Select native binary output. [Default is one more than input].
-d[i|o]nodata (more …)
Replace input columns that equal nodata with NaN and do the reverse on output.
-e[~]”pattern” | -e[~]/regexp/[i] (more …)
Only accept data records that match the given pattern.
-f[i|o]colinfo (more …)
Specify data types of input and/or output columns.
-g[a]x|y|d|X|Y|D|[col]z[+|-]gap[u] (more …)
Determine data gaps and line breaks.
-h[i|o][n][+c][+d][+rremark][+rtitle] (more …)
Skip or produce header record(s).
-icols[+l][+sscale][+ooffset][,…] (more …)
Select input columns and transformations (0 is first column).
-n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more …)
Select interpolation mode for grids.
-ocols[,…] (more …)
Select output columns (0 is first column).
-s[cols][a|r] (more …)
Set handling of NaN records.
-^ 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.
UNITS
For map distance unit, append unit d for arc degree, m for arc minute, and s for arc second, or e for
meter [Default], f for foot, k for km, M for statute mile, n for nautical mile, and u for US survey foot.
By default we compute such distances using a spherical approximation with great circles. Prepend - to a
distance (or the unit is no distance is given) to perform “Flat Earth” calculations (quicker but less
accurate) or prepend + to perform exact geodesic calculations (slower but more accurate).
ASCII FORMAT PRECISION
The ASCII output formats of numerical data are controlled by parameters in your gmt.conf file. Longitude
and latitude are formatted according to FORMAT_GEO_OUT, absolute time is under the control of
FORMAT_DATE_OUT and FORMAT_CLOCK_OUT, whereas general floating point values are formatted according to
FORMAT_FLOAT_OUT. Be aware that the format in effect can lead to loss of precision in ASCII output, which
can lead to various problems downstream. If you find the output is not written with enough precision,
consider switching to binary output (-bo if available) or specify more decimals using the
FORMAT_FLOAT_OUT setting.
GRID FILE FORMATS
By default GMT writes out grid as single precision floats in a COARDS-complaint netCDF file format.
However, GMT is able to produce grid files in many other commonly used grid file formats and also
facilitates so called “packing” of grids, writing out floating point data as 1- or 2-byte integers. (more
…)
CONSEQUENCES OF GRID RESAMPLING
Resample or sampling of grids will use various algorithms (see -n) that may lead to possible distortions
or unexpected results in the resampled values. One expected effect of resampling with splines is the
tendency for the new resampled values to slightly exceed the global min/max limits of the original grid.
If this is unacceptable, you can impose clipping of the resampled values values so they do not exceed the
input min/max values by adding +c to your -n option.
HINTS
If an interpolation point is not on a node of the input grid, then a NaN at any node in the neighborhood
surrounding the point will yield an interpolated NaN. Bicubic interpolation [default] yields continuous
first derivatives but requires a neighborhood of 4 nodes by 4 nodes. Bilinear interpolation [-n] uses
only a 2 by 2 neighborhood, but yields only zeroth-order continuity. Use bicubic when smoothness is
important. Use bilinear to minimize the propagation of NaNs, or lower threshold.
EXAMPLES
To sample the file hawaii_topo.nc along the SEASAT track track_4.xyg (An ASCII table containing
longitude, latitude, and SEASAT-derived gravity, preceded by one header record):
grdtrack track_4.xyg -Ghawaii_topo.nc -h > track_4.xygt
To sample the Sandwell/Smith IMG format file topo.8.2.img (2 minute predicted bathymetry on a Mercator
grid) and the Muller et al age grid age.3.2.nc along the lon,lat coordinates given in the file
cruise_track.xy, try
grdtrack cruise_track.xy -Gtopo.8.2.img,1,1 -Gage.3.2.nc > depths-age.d
To sample the Sandwell/Smith IMG format file grav.18.1.img (1 minute free-air anomalies on a Mercator
grid) along 100-km-long cross-profiles that are orthogonal to the line segment given in the file
track.xy, erecting cross-profiles every 25 km and sampling the grid every 3 km, try
grdtrack track.xy -Ggrav.18.1.img,0.1,1 -C100k/3/25 -Ar > xprofiles.txt
To sample the grid data.nc along a line from the lower left to the upper right corner, using a grid
spacing of 1 km, and output distances as well, try
grdtrack -ELB/RT+i1k+d -Gdata.nc > profiles.txt
SEE ALSO
gmt, gmtconvert, pstext, sample1d, surface
COPYRIGHT
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe