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


grdblend [ blendfile | grid1 grid2 ... ] outgrid [ increment ] [ region ] [ f|l|o|u ] [
nodata ] [ ] [ scale ] [ [level] ] [ [z] ] [ -f<flags> ] [ -r ]

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


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


outgrid is the name of the binary output grid file. (See GRID FILE FORMATS below).
Only netCDF and native binary grid formats are can be written directly. Other
output format choices will be handled by reformatting the output once blending is

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 = 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 App-file-formats for details.
Note: if -Rgrdfile is used then the grid spacing has already been initialized; use
-I to override the values.

-R[unit]xmin/xmax/ymin/ymax[r] (more ...)
Specify the region of interest.


ASCII file with one record per grid file to include in the blend. Each record may
contain up to three items, separated by spaces or tabs: the gridfile name
(required), the -R-setting for the interior region (optional), and the relative
weight wr (optional). In the combined weighting scheme, this grid will be given
zero weight outside its domain, weight = wr inside the interior region, and a 2-D
cosine-tapered weight between those end-members in the boundary strip. However, if
a negative wr is given then the sense of tapering is inverted (i.e., zero weight
inside its domain). If the inner region should instead exactly match the grid
region then specify a - instead of the -R-setting, or leave it off entirely.
Likewise, if a weight wr is not specified we default to a weight of 1. If the
ASCII blendfile file is not given grdblend will read standard input. Alternatively,
if you have more than one grid file to blend and you wish (a) all input grids to
have the same weight (1) and (b) all grids should use their actual region as the
interior region, then you may simply list all the grids on the command line instead
of providing a blendfile. You must specify at least 2 input grids for this
mechanism to work. Any grid that is not co-registered with the desired output
layout implied by -R, -I (and -r) will first be resampled via grdsample. Also,
grids that are not in netCDF or native binary format will first be reformatted via

-C Clobber mode: Instead of blending, simply pick the value of one of the grids that
covers a node. Select from the following modes: f for the first grid to visit a
node; o for the last grid to visit a node; l for the grid with the lowest value,
and u for the grid with the uppermost value. For modes f and o the ordering of
grids in the blendfile will dictate which grid contributes to the final result.
Weights and cosine tapering are not considered when clobber mode is active.

No data. Set nodes with no input grid to this value [Default is NaN].

-Q Create a header-less grid file suitable for use with grdraster. Requires that the
output grid file is a native format (i.e., not netCDF).

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

-W[z] Do not blend, just output the weights used for each node [Default makes the blend].
Append z to write the weight*z sum instead.

Scale output values by scale before writing to file. [1].

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

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

-^ or just -
Print a short message about the syntax of the command, then exits (NOTE: on Windows
use just -).

-+ 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 options, then

Print GMT version and exit.

Print full path to GMT share directory and exit.


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. 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. See
grdconvert and Section grid-file-format of the GMT Technical Reference and Cookbook for
more information.

When writing a netCDF file, the grid is stored by default with the variable name "z". To
specify another variable name varname, append ?varname to the file name. 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.


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


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


To create a grid file from the four grid files piece_?.nc, giving them each the different
weights, make the blendfile like this

piece_1.nc -R<subregion_1> 1
piece_2.nc -R<subregion_2> 1.5
piece_3.nc -R<subregion_3> 0.9
piece_4.nc -R<subregion_4> 1

Then run

gmt grdblend blend.job -Gblend.nc -R<full_region> -I<dx/dy> -V

To blend all the grids called MB_*.nc given them all equal weight, try

gmt grdblend MB_*.nc -Gblend.nc -R<full_region> -I<dx/dy> -V


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

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