grdblend
grdblend - Blend several partially over-lapping grids into one large grid
Synopsis
grdblend [ blendfile | grid1 grid2 ... ] -Goutgrid
[ -Ixinc[unit][=|+][/yinc[unit][=|+]] ]
[ -R[unit]xmin/xmax/ymin/ymax[r] ]
[ -Cf|l|o|u ] [ -Nnodata ]
[ -Q ] [ -Zscale ]
[ -V[level] ]
[ -W ]
[ -f[i|o]colinfo ]
[ -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][=|+][/yinc[unit][=|+]]
- 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 B. GMT 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.
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
grdreformat.
- -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
- Do not blend, just output the weights used for each node. This
option is valid when only one input grid is provided [Default makes the blend].
- -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.
- -r (more ...)
- Set pixel node registration [gridline].
- -^
- Print a short message about the syntax of the command, then exits (NOTE: on Windows use just -).
- +
- Print an extensive usage (help) message, including the explanation of
any module-specific option (but not the GMT common options), then exits.
- -?
- Print a complete usage (help) message, including the explanation of
options, then exits.
- --version
- Print GMT version and exit.
- --show-datadir
- Print full path to GMT share directory and exit.
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