1GRDBLEND(1)                           GMT                          GRDBLEND(1)
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NAME

6       grdblend  -  Blend  several partially over-lapping grids into one large
7       grid
8

SYNOPSIS

10       grdblend [ blendfile | grid1 grid2 ... ]  -Goutgrid [  -Iincrement ]  [
11       -Rregion  ]  [   -Cf|l|o|u  ]  [   -Nnodata  ]  [   -Q ] [  -Zscale ] [
12       -V[level] ] [  -W[z] ] [ -fflags ] [ -nflags ] [ -r ]
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14       Note: No space is allowed between the option flag  and  the  associated
15       arguments.
16

DESCRIPTION

18       grdblend reads a listing of grid files and blend parameters and creates
19       a binary grid file by  blending  the  other  grids  using  cosine-taper
20       weights.  grdblend  will  report if some of the nodes are not filled in
21       with data. Such unconstrained nodes are set to a value specified by the
22       user  [Default  is  NaN]. Nodes with more than one value will be set to
23       the weighted average value. Any input grid  that  does  not  share  the
24       final  output  grid's node registration and grid spacing will automati‐
25       cally be resampled via calls to grdsample. Note: Due to the  row-by-row
26       i/o  nature  of  operations  in grdblend we only support the netCDF and
27       native binary grid formats for both input and output.
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REQUIRED ARGUMENTS

30       -Goutgrid
31              outgrid is the name of the binary output grid  file.  (See  GRID
32              FILE  FORMATS below). Only netCDF and native binary grid formats
33              are can be written directly. Other output format choices will be
34              handled by reformatting the output once blending is complete.
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36       -Ixinc[unit][+e|n][/yinc[unit][+e|n]]
37              x_inc  [and  optionally  y_inc] is the grid spacing. Optionally,
38              append a suffix modifier.  Geographical  (degrees)  coordinates:
39              Append  m  to indicate arc minutes or s to indicate arc seconds.
40              If one of the units e, f, k, M, n or u is appended instead,  the
41              increment  is assumed to be given in meter, foot, km, Mile, nau‐
42              tical mile or US survey foot, respectively,  and  will  be  con‐
43              verted  to  the equivalent degrees longitude at the middle lati‐
44              tude of the region (the conversion depends  on  PROJ_ELLIPSOID).
45              If  y_inc is given but set to 0 it will be reset equal to x_inc;
46              otherwise it will be converted to degrees latitude. All  coordi‐
47              nates:  If +e is appended then the corresponding max x (east) or
48              y (north) may be slightly adjusted  to  fit  exactly  the  given
49              increment  [by default the increment may be adjusted slightly to
50              fit the given domain]. Finally, instead of giving  an  increment
51              you  may  specify the number of nodes desired by appending +n to
52              the supplied integer argument; the increment  is  then  recalcu‐
53              lated  from  the  number  of nodes and the domain. The resulting
54              increment value depends on whether you  have  selected  a  grid‐
55              line-registered  or  pixel-registered grid; see App-file-formats
56              for details. Note: if -Rgrdfile is used then  the  grid  spacing
57              has already been initialized; use -I to override the values.
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59       -Rxmin/xmax/ymin/ymax[+r][+uunit] (more ...)
60              Specify the region of interest.
61

OPTIONAL ARGUMENTS

63       blendfile
64              ASCII  file  with  one  record  per  grid file to include in the
65              blend.  Each record may contain up to three items, separated  by
66              spaces or tabs: the gridfile name (required), the -R-setting for
67              the interior region  (optional),  and  the  relative  weight  wr
68              (optional).  In the combined weighting scheme, this grid will be
69              given zero weight outside its domain, weight  =  wr  inside  the
70              interior  region,  and a 2-D cosine-tapered weight between those
71              end-members in the boundary strip. However, if a negative wr  is
72              given  then the sense of tapering is inverted (i.e., zero weight
73              inside its domain). If the inner region should  instead  exactly
74              match  the  grid  region then specify a - instead of the -R-set‐
75              ting, or leave it off entirely.  Likewise, if a weight wr is not
76              specified  we  default to a weight of 1.  If the ASCII blendfile
77              file is not given grdblend will read  standard  input.  Alterna‐
78              tively,  if  you  have  more than one grid file to blend and you
79              wish (a) all input grids to have the same weight (1) and (b) all
80              grids  should  use  their  actual region as the interior region,
81              then you may simply list all  the  grids  on  the  command  line
82              instead  of  providing  a blendfile. You must specify at least 2
83              input grids for this mechanism to work. Any  grid  that  is  not
84              co-registered  with  the desired output layout implied by -R, -I
85              (and -r) will first be resampled via grdsample. Also, grids that
86              are  not  in netCDF or native binary format will first be refor‐
87              matted via grdconvert.
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89       -C     Clobber mode: Instead of blending, simply pick the value of  one
90              of  the  grids  that  covers  a  node. Select from the following
91              modes: f for the first grid to visit a node; o for the last grid
92              to visit a node; l for the grid with the lowest value, and u for
93              the grid with the uppermost value. For modes f and o the  order‐
94              ing  of  grids in the blendfile will dictate which grid contrib‐
95              utes to the final result. Weights and cosine  tapering  are  not
96              considered when clobber mode is active.
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98       -Nnodata
99              No  data. Set nodes with no input grid to this value [Default is
100              NaN].
101
102       -Q     Create a header-less grid file suitable for use with  grdraster.
103              Requires that the output grid file is a native format (i.e., not
104              netCDF).
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106       -V[level] (more ...)
107              Select verbosity level [c].
108
109       -W[z]  Do not blend,  just  output  the  weights  used  for  each  node
110              [Default  makes  the blend].  Append z to write the weight*z sum
111              instead.
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113       -Zscale
114              Scale output values by scale before writing to file. [1].
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116       -f[i|o]colinfo (more ...)
117              Specify data types of input and/or output columns.
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119       -n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more ...)
120              Select interpolation mode for grids.
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122       -r (more ...)
123              Set pixel node registration [gridline].
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125       -^ or just -
126              Print a short message about the  syntax  of  the  command,  then
127              exits (NOTE: on Windows just use -).
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129       -+ or just +
130              Print  an extensive usage (help) message, including the explana‐
131              tion of any module-specific  option  (but  not  the  GMT  common
132              options), then exits.
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134       -? or no arguments
135              Print a complete usage (help) message, including the explanation
136              of all options, then exits.
137

GRID FILE FORMATS

139       By default GMT  writes  out  grid  as  single  precision  floats  in  a
140       COARDS-complaint  netCDF  file  format. However, GMT is able to produce
141       grid files in many other commonly  used  grid  file  formats  and  also
142       facilitates  so  called  "packing" of grids, writing out floating point
143       data as 1- or 2-byte integers. To specify the precision, scale and off‐
144       set,  the user should add the suffix =ID[+sscale][+ooffset][+ninvalid],
145       where ID is a two-letter identifier of the grid type and precision, and
146       scale  and offset are optional scale factor and offset to be applied to
147       all grid values, and invalid is the  value  used  to  indicate  missing
148       data.  See grdconvert and Section grid-file-format of the GMT Technical
149       Reference and Cookbook for more information.
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151       When writing a netCDF file, the grid is  stored  by  default  with  the
152       variable  name  "z".  To  specify another variable name varname, append
153       ?varname to the file name. Note that you may need to escape the special
154       meaning  of  ? in your shell program by putting a backslash in front of
155       it, or by placing the filename and  suffix  between  quotes  or  double
156       quotes.
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GEOGRAPHICAL AND TIME COORDINATES

159       When  the  output  grid type is netCDF, the coordinates will be labeled
160       "longitude", "latitude", or "time" based on the attributes of the input
161       data  or  grid  (if  any) or on the -f or -R options. For example, both
162       -f0x -f1t and -R90w/90e/0t/3t will result  in  a  longitude/time  grid.
163       When  the  x, y, or z coordinate is time, it will be stored in the grid
164       as relative time since epoch as specified by TIME_UNIT  and  TIME_EPOCH
165       in  the  gmt.conf  file  or  on the command line. In addition, the unit
166       attribute of the time variable will indicate both this unit and epoch.
167

TAPERING

169       While the weights computed are tapered from 1 to 0,  we  are  computing
170       weighted  averages,  so  if  there is only a single grid given then the
171       weighted output will be identical to the input.  If you are looking for
172       a way to taper your data grid, see grdmath's TAPER operator.
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EXAMPLES

175       To  create a grid file from the four grid files piece_?.nc, giving them
176       each the different weights, make the blendfile like this
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178              piece_1.nc -R<subregion_1> 1
179              piece_2.nc -R<subregion_2> 1.5
180              piece_3.nc -R<subregion_3> 0.9
181              piece_4.nc -R<subregion_4> 1
182
183       Then run
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185              gmt grdblend blend.job -Gblend.nc -R<full_region> -I<dx/dy> -V
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187       To blend all the grids called MB_*.nc given them all equal weight, try
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189              gmt grdblend MB_*.nc -Gblend.nc -R<full_region> -I<dx/dy> -V
190

WARNING ON LARGE FILE SETS

192       While grdblend can process any number of files,  it  works  by  keeping
193       those  files  open  that  are being blended, and close files as soon as
194       they are finished.  Depending on your session, many  files  may  remain
195       open  at  the  same  time.   Some  operating  systems set fairly modest
196       default limits on how many concurrent files can be open, e.g., 256.  If
197       you  run  into  this  problem  then you can change this limit; see your
198       operating system documentation for how to change system limits.
199

SEE ALSO

201       gmt, grd2xyz, grdconvert, grdedit, grdraster, grdsample
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204       2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
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2095.4.5                            Feb 24, 2019                      GRDBLEND(1)
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