Provided by: gmt-common_5.4.3+dfsg-1_all 
NAME
grdblend - Blend several partially over-lapping grids into one large grid
SYNOPSIS
grdblend [ blendfile | grid1 grid2 … ] -Goutgrid [ -Iincrement ] [ -Rregion ] [ -Cf|l|o|u ] [
-Nnodata ] [ -Q ] [ -Zscale ] [ -V[level] ] [ -W[z] ] [ -fflags ] [ -nflags ] [ -r ]
Note: No space is allowed between the option flag and the associated arguments.
DESCRIPTION
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.
REQUIRED ARGUMENTS
-Goutgrid
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 complete.
-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, 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.
OPTIONAL ARGUMENTS
blendfile
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
grdconvert.
-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.
-Nnodata
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.
-Zscale
Scale output values by scale before writing to file. [1].
-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].
-^ 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.
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 1- or 2-byte integers. To
specify the precision, scale and offset, the user should add the suffix
=ID[+sscale][+ooffset][+ninvalid], 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 invalid
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.
GEOGRAPHICAL AND TIME COORDINATES
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.
TAPERING
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 operator.
EXAMPLES
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
WARNING ON LARGE FILE SETS
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.
SEE ALSO
gmt, grd2xyz, grdconvert, grdedit, grdraster, grdsample
COPYRIGHT
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe