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GRDBLEND(1) Generic Mapping Tools GRDBLEND(1)
NAME
grdblend - Blend several partially over-lapping grids into one large
grid
SYNOPSIS
grdblend blendfile -Ggrdfile -Ixinc[unit][=|*][/yinc[unit][=|+]]
-Rwest/east/south/north[r] [ -Nnodata ] [ -Q ] [ -Zscale ] [ -V ] [ -W
] [ -fcolinfo ]
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. 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.
blendfile
ASCII file with one record per grid file to include in the
blend. Each record must contain three items, separated by
spaces or tabs: the gridfile name, the -R-setting for the
interior region, and the relative weight wr. 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. If the ASCII file
is not given grdblend will read standard input.
-G grdfile is the name of the binary output grid file. (See GRID
FILE FORMATS below). Only netCDF and native binary grid formats
are supported.
-I 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 c to indicate arc seconds.
If one of the units e, k, i, or n is appended instead, the
increment is assumed to be given in meter, km, miles, or
nautical miles, respectively, and will be converted to the
equivalent degrees longitude at the middle latitude of the
region (the conversion depends on 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 Appendix B for details. Note: if -Rgrdfile
is used then grid spacing has already been initialized; use -I
to override the values.
-R xmin, xmax, ymin, and ymax specify the Region of interest. For
geographic regions, these limits correspond to west, east,
south, and north and you may specify them in decimal degrees or
in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
and upper right map coordinates are given instead of w/e/s/n.
The two shorthands -Rg and -Rd stand for global domain (0/360
and -180/+180 in longitude respectively, with -90/+90 in
latitude). Alternatively, specify the name of an existing grid
file and the -R settings (and grid spacing, if applicable) are
copied from the grid. For calendar time coordinates you may
either give (a) relative time (relative to the selected
TIME_EPOCH and in the selected TIME_UNIT; append t to -JX|x), or
(b) absolute time of the form [date]T[clock] (append T to
-JX|x). At least one of date and clock must be present; the T
is always required. The date string must be of the form
[-]yyyy[-mm[-dd]] (Gregorian calendar) or yyyy[-Www[-d]] (ISO
week calendar), while the clock string must be of the form
hh:mm:ss[.xxx]. The use of delimiters and their type and
positions must be exactly as indicated (however, input, output
and plot formats are customizable; see gmtdefaults).
OPTIONS
-N 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 Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
-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].
-Z Scale output values by scale before writing to file. [1].
-f Special formatting of input and/or output columns (time or
geographical data). Specify i or o to make this apply only to
input or output [Default applies to both]. Give one or more
columns (or column ranges) separated by commas. Append T
(absolute calendar time), t (relative time in chosen TIME_UNIT
since TIME_EPOCH), x (longitude), y (latitude), or f (floating
point) to each column or column range item. Shorthand -f[i|o]g
means -f[i|o]0x,1y (geographic coordinates).
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 2- or 4-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
grdreformat(1) and Section 4.17 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 -R 90w/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 .gmtdefaults file or on the command line. In addition, the unit
attribute of the time variable will indicate both this unit and epoch.
EXAMPLES
To create a grid file from the four grid files piece_?.nc, make the
blendfile like this
piece_1.nc -R<subregion_1> 1
piece_2.nc -R<subregion_2> 1
piece_3.nc -R<subregion_3> 1
piece_4.nc -R<subregion_4> 1
Then run
grdblend blend.job -G blend.nc -R<full_region> -I<dx/dy> -V
RESTRICTIONS
Currently, all grids processed must have the exact same node
registration and grid spacing as the final output grid.
SEE ALSO
GMT(1), grd2xyz(1), grdedit(1) grdraster(1)
GMT 4.5.14 1 Nov 2015 GRDBLEND(1)