Provided by: gmt_4.5.11-1build1_amd64 
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 4.5.11 5 Nov 2013 GRDBLEND(1gmt)