Provided by: gmt-common_5.4.5+dfsg-2_all bug


       grdsample - Resample a grid onto a new lattice


       grdsample  in_grdfile  -Gout_grdfile [  -Iincrement ] [  -Rregion ] [  -T ] [  -V[level] ]
       [ -fflags ] [ -nflags ] [ -rreg ] [ -x[[-]n] ]

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


       grdsample reads a grid file and interpolates it to create a new grid file with  either:  a
       different  registration  (-r  or  -T); or, a new grid-spacing or number of nodes (-I), and
       perhaps  also  a  new  sub-region  (-R).  A  bicubic  [Default],  bilinear,  B-spline   or
       nearest-neighbor  interpolation  is  used; see -n for settings. Note that using -R only is
       equivalent to grdcut or grdedit -S.  grdsample safely creates a fine mesh  from  a  coarse
       one;  the  converse  may  suffer  aliasing  unless  the  data are filtered using grdfft or

       When -R is omitted, the output grid will cover the same region as the input grid. When  -I
       is omitted, the grid spacing of the output grid will be the same as the input grid. Either
       -r or -T can be used to change the grid registration. When omitted, the output  grid  will
       have the same registration as the input grid.


              The name of the input 2-D binary grid file. (See GRID FILE FORMAT below.)

              The name of the output grid file. (See GRID FILE FORMAT below.)


              x_inc  [and  optionally  y_inc]  is  the  grid spacing. Optionally, append a suffix
              modifier. Geographical (degrees) coordinates: Append m to indicate arc minutes or s
              to  indicate  arc  seconds.  If  one  of  the  units e, f, k, M, n or u is appended
              instead, the increment is assumed to be given in meter, foot,  km,  Mile,  nautical
              mile  or  US  survey  foot,  respectively,  and will be converted to the equivalent
              degrees longitude at the middle latitude of the region (the conversion  depends  on
              PROJ_ELLIPSOID).  If  y_inc  is given but set to 0 it will be reset equal to x_inc;
              otherwise it will be converted to degrees  latitude.  All  coordinates:  If  +e  is
              appended  then the corresponding max x (east) or y (north) may be slightly adjusted
              to fit exactly the given increment  [by  default  the  increment  may  be  adjusted
              slightly  to fit the given domain]. Finally, instead of giving an increment you may
              specify the number of nodes  desired  by  appending  +n  to  the  supplied  integer
              argument;  the  increment  is  then  recalculated  from the number of nodes and the
              domain. The resulting increment value  depends  on  whether  you  have  selected  a
              gridline-registered  or  pixel-registered  grid;  see App-file-formats for details.
              Note: if -Rgrdfile is used then the grid spacing has already been initialized;  use
              -I to override the values.

       -Rxmin/xmax/ymin/ymax[+r][+uunit] (more ...)
              Specify the region of interest.

       -T     Translate between grid and pixel registration; if the input is grid-registered, the
              output will be pixel-registered and vice-versa.

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

       -f[i|o]colinfo (more ...)
              Specify data types of input and/or output columns.

       -n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more ...)
              Select interpolation mode for grids.

       -r (more ...)
              Set pixel node registration [gridline].

       -x[[-]n] (more ...)
              Limit number of cores used in multi-threaded algorithms (OpenMP required).

       -^ 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.


       Regardless  of  the  precision of the input data, GMT programs that create grid files will
       internally hold the grids in 4-byte floating point arrays. This is done to conserve memory
       and  furthermore  most  if  not  all  real  data can be stored using 4-byte floating point
       values. Data with  higher  precision  (i.e.,  double  precision  values)  will  lose  that
       precision  once  GMT  operates  on  the  grid  or  writes  out new grids. To limit loss of
       precision when processing data you should always consider normalizing the  data  prior  to


       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 ...)


       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.


       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 zero-order continuity. Use bicubic when smoothness is important. Use bilinear
       to minimize the propagation of NaNs.


       As an alternative to bicubic spline, linear spline or nearest neighbor  interpolation  one
       can instead send the entire dataset through surface for re-gridding.  This approach allows
       more control on aspects such as tension but it also leads to a solution that is not likely
       to have fully converged.  The general approach would be something like

              gmt grd2xyz old.grd | gmt surface -Rold.grd -Inewinc -Gnew.grd [other options]

       For moderate data set one could also achieve an exact solution with greenspline, such as

          gmt grd2xyz old.grd | gmt greenspline -Rold.grd -Inewinc -Gnew.grd [other options]


       To resample the 5 x 5 minute grid in onto a 1 minute grid:

              gmt grdsample -I1m

       To  translate  the gridline-registered file to pixel registration while keeping
       the same region and grid interval:

              gmt grdsample -T


       gmt, grdedit, grdfft, grdfilter, greenspline, surface


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