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grdsample(1)                          GMT                         grdsample(1)




NAME

       grdsample - Resample a grid onto a new lattice


SYNOPSIS

       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.


DESCRIPTION

       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 grdfilter.

       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.


REQUIRED ARGUMENTS

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

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


OPTIONAL ARGUMENTS

       -Ixinc[unit][+e|n][/yinc[unit][+e|n]]
              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,  nau-
              tical  mile  or  US  survey foot, respectively, and will be con-
              verted to the equivalent degrees longitude at the  middle  lati-
              tude  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 coordi-
              nates: 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 recalcu-
              lated from the number of nodes and  the  domain.  The  resulting
              increment  value  depends  on  whether you have selected a grid-
              line-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 a|)
              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 a|)
              Select verbosity level [c].

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

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

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

       -x[[-]n] (more a|)
              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  explana-
              tion  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.


GRID VALUES PRECISION

       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 preci-
       sion once GMT operates on the grid or writes out new  grids.  To  limit
       loss  of precision when processing data you should always consider nor-
       malizing the data prior to processing.


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  apackinga of grids, writing out floating point
       data as 1- or 2-byte integers. (more a|)


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


EXAMPLES

       To resample the 5 x 5 minute  grid  in  hawaii_5by5_topo.nc  onto  a  1
       minute grid:

              gmt grdsample hawaii_5by5_topo.nc -I1m -Ghawaii_1by1_topo.nc

       To translate the gridline-registered file surface.nc to pixel registra-
       tion while keeping the same region and grid interval:

              gmt grdsample surface.nc -T -Gpixel.nc


SEE ALSO

       gmt(1), grdedit(1), grdfft(1), grdfilter(1)


COPYRIGHT

       2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe



5.4.2                            Jun 24, 2017                     grdsample(1)

gmt5 5.4.2 - Generated Thu Jun 29 08:08:49 CDT 2017
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