Provided by: gmt_4.5.11-1build1_amd64 bug

NAME

       grdsample - Resample a grid file onto a new grid

SYNOPSIS

       grdsample in_grdfile -Gout_grdfile [ -F ] [ -Ixinc[unit][=|+][/yinc[unit][=|+]] ] [ -Lflag
       ] [ -Q[b|c|l|n][[/]threshold] ] [ -Rwest/east/south/north[r] ] [ -T ] [ -V ] [ -fcolinfo ]

DESCRIPTION

       grdsample reads a grid file and interpolates it to create a new grid file with either:   a
       different  registration  (-F  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 (-Q) is used, requiring boundary conditions (-L).  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 -F or -T can be used to change the grid registration. When omitted, the output grid
       will have the same registration as the input grid.

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

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

OPTIONS

       -F     Force  pixel  node  registration  on output grid.  [Default is same registration as
              input grid].

       -I     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
              c to indicate arc seconds.  If one of the units e, k, i, or n is appended  instead,
              the  increment  is  assumed  to  be  given  in meter, km, miles, or nautical miles,
              respectively, and will be converted to the  equivalent  degrees  longitude  at  the
              middle  latitude of the region (the conversion depends on 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 = 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 + 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
              Appendix B for details.  Note: if -Rgrdfile is used then grid spacing  has  already
              been initialized; use -I to override the values.

       -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
              periodic.]

       -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[: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 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).

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

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

       -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).

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
       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
       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 "packing" of grids, writing out floating point
       data as 2- or 4-byte integers. To specify the precision, scale and offset, the user should
       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.   When
       reading  grids,  the format is generally automatically recognized. If not, the same suffix
       can be added to input grid file names.  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.  The ?varname suffix can also be used
       for  output  grids  to  specify  a  variable  name  different  from the default: "z".  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.

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 [-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.

EXAMPLES

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

       grdsample hawaii_5by5_topo.grd -I 1m -Ghawaii_1by1_topo.grd

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

       grdsample surface.grd -T -G pixel.grd

SEE ALSO

       GMT(1), grdedit(1), grdfft(1), grdfilter(1)