grdblend(1) GMT grdblend(1)
NAME
grdblend - Blend several partially over-lapping grids into one large
grid
SYNOPSIS
grdblend [ blendfile | grid1 grid2 a| ] -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 gridas node registration and grid spacing will automati-
cally 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, 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.
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-set-
ting, 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. Alterna-
tively, 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 refor-
matted 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 order-
ing of grids in the blendfile will dictate which grid contrib-
utes 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 a|)
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 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].
-^ 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 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. To specify the precision, scale and off-
set, 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 aza. 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
alongitudea, alatitudea, or atimea 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 grdmathas 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(1), grd2xyz(1), grdconvert(1), grdedit(1), grdraster(1),
grdsample(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 grdblend(1)
gmt5 5.4.2 - Generated Wed Jun 28 18:13:56 CDT 2017