Provided by: gmt_4.5.11-1build1_amd64 bug


       grdtrack - Sampling of a 2-D grid file along 1-D trackline (a sequence of x,y points)


       grdtrack  xyfile  -Ggrdfile  [  -H[i][nrec]  ]  [ -Lflag ] [ -Q[b|c|l|n][[/]threshold] ] [
       -Rwest/east/south/north[r]  ]   [   -S   ]   [   -V   ]   [   -Z   ]   [   -:[i|o]   ]   [
       -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [ -f[i|o]colinfo ] [ -m[i|o][flag] ]


       grdtrack  reads  a  grid  file  (or  a  Sandwell/Smith IMG file) and a table (from file or
       standard input) with (x,y) positions in  the  first  two  columns  (more  columns  may  be
       present).  It interpolates the grid at the positions in the table and writes out the table
       with the interpolated values added as a new column.  A  bicubic  [Default],  bilinear,  B-
       spline  or  nearest-neighbor (see -Q) interpolation is used, requiring boundary conditions
       at the limits of the region (see -L).

       xyfile This is an ASCII (or binary, see -b) file where the first 2 columns hold the  (x,y)
              positions where the user wants to sample the 2-D data set.

       -G     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].   (See  GRID
              FILE FORMAT below.)


       No space between the option flag and the associated arguments.

       -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     Boundary condition flag may be x or y or xy indicating data is periodic in range of
              x  or  y  or both set by -R, or flag may be g indicating geographical conditions (x
              and y are lon  and  lat).   [Default  uses  "natural"  conditions  (second  partial
              derivative  normal  to edge is zero) unless the grid is automatically recognised as

       -Q     Quick  mode,  use   bilinear   rather   than   bicubic   interpolation   [Default].
              Alternatively,  select the interpolation mode by adding b for B-spline smoothing, c
              for bicubic interpolation, l for bilinear interpolation or n  for  nearest-neighbor
              value.   Optionally,  append threshold in the range [0,1].  This parameter controls
              how close to nodes with NaN values the interpolation will go.  E.g., a threshold of
              0.5  will interpolate about half way from a non-NaN to a NaN node, whereas 0.1 will
              go about 90% of the way, etc. [Default is 1, which means none  of  the  (4  or  16)
              nearby  nodes  may  be  NaN].   -Q0  will just return the value of the nearest node
              instead of interpolating.  This is the same as using -Qn.

       -R     xmin, xmax, ymin, and ymax specify the Region of interest.  For geographic regions,
              these limits correspond to west, east, south, and north and you may specify them in
              decimal degrees or in [+-]dd:mm[][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  calendar  time  coordinates  you  may  either  give  (a) relative time
              (relative to the selected TIME_EPOCH and in the selected  TIME_UNIT;  append  t  to
              -JX|x),  or  (b)  absolute time of the form [date]T[clock] (append T to -JX|x).  At
              least one of date and clock must be present; the T is always  required.   The  date
              string must be of the form [-]yyyy[-mm[-dd]] (Gregorian calendar) or yyyy[-Www[-d]]
              (ISO week calendar), while the clock string must be  of  the  form  hh:mm:ss[.xxx].
              The  use  of  delimiters  and their type and positions must be exactly as indicated
              (however, input, output and plot formats are customizable; see gmtdefaults).

       -S     Suppress the output of interpolated points that result in NaN values.

       -V     Selects verbose mode, which will send progress  reports  to  stderr  [Default  runs

       -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    Selects binary input.  Append s for  single  precision  [Default  is  d  (double)].
              Uppercase  S or D will force byte-swapping.  Optionally, 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 2 input columns].

       -bo    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.  [Default is one more than input].

       -f     Special formatting of input and/or output  columns  (time  or  geographical  data).
              Specify  i  or  o  to  make  this apply only to input or output [Default applies to
              both].  Give one or more columns (or column ranges) separated by commas.  Append  T
              (absolute calendar time), t (relative time in chosen TIME_UNIT since TIME_EPOCH), x
              (longitude), y (latitude), or f (floating point) to each  column  or  column  range
              item.  Shorthand -f[i|o]g means -f[i|o]0x,1y (geographic coordinates).

       -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.


       The  ASCII  output  formats  of  numerical  data  are  controlled  by  parameters  in your
       .gmtdefaults4   file.    Longitude   and   latitude    are    formatted    according    to
       OUTPUT_DEGREE_FORMAT,  whereas other values are formatted according to D_FORMAT.  Be aware
       that the format in effect can lead to loss of precision in the 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
       D_FORMAT setting.


       GMT  is  able  to  recognize  many  of the commonly used grid file formats, as well as the
       precision, scale and offset of the values contained in the grid file.  When  GMT  needs  a
       little help with that, you can add the suffix =id[/scale/offset[/nan]], where id is a two-
       letter identifier of the grid type and precision, and scale and offset are optional  scale
       factor  and offset to be applied to all grid values, and nan is the value used to indicate
       missing data.  See grdreformat(1) and Section 4.17 of  the  GMT  Technical  Reference  and
       Cookbook for more information.

       When  reading  a  netCDF file that contains multiple grids, GMT will read, by default, the
       first 2-dimensional grid that can find in that file. To  coax  GMT  into  reading  another
       multi-dimensional  variable  in  the  grid  file,  append ?varname to the file name, where
       varname is the name of the variable. Note that you may need to escape the special  meaning
       of  ?  in  your  shell  program  by  putting a backslash in front of it, or by placing the
       filename and suffix between quotes or double quotes.  See grdreformat(1) and Section  4.18
       of  the  GMT Technical Reference and Cookbook for more information, particularly on how to
       read splices of 3-, 4-, or 5-dimensional grids.


       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 [-Q] 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.


       To  sample  the  file  hawaii_topo.grd  along the SEASAT track track_4.xyg (An ASCII table
       containing longitude,  latitude,  and  SEASAT-derived  gravity,  preceded  by  one  header

       grdtrack track_4.xyg -G hawaii_topo.grd -H > track_4.xygt

       To  sample  the Sandwell/Smith IMG format file topo.8.2.img (2 minute predicted bathymetry
       on a Mercator grid) along the lon,lat coordinates given in the file cruise_track.xy, try

       grdtrack cruise_track.xy -G topo.8.2.img,1,1 > obs_and_predicted.d


       GMT(1), surface(1), sample1d(1)