GRDBLEND(1) Generic Mapping Tools GRDBLEND(1)NAMEgrdblend - Blend several partially over-lapping grids into one large
grid
SYNOPSISgrdblend blendfile -Ggrdfile -Ixinc[unit][=|+][/yinc[unit][=|+]]
-Rwest/east/south/north[r] [ -Nnodata ] [ -Q ] [ -Zscale ] [ -V ] [ -W
] [ -fcolinfo ]
DESCRIPTIONgrdblend 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 spa‐
ces 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. How‐
ever, 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 nauti‐
cal miles, respectively, and will be converted to the equivalent
degrees longitude at the middle latitude of the region (the con‐
version 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 lati‐
tude). 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 delim‐
iters 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 geo‐
graphical 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 (abso‐
lute 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 facili‐
tates 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 spe‐
cial 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 registra‐
tion and grid spacing as the final output grid.
SEE ALSOGMT(1), grd2xyz(1), grdedit(1)grdraster(1)GMT 4.5.14 1 Nov 2015 GRDBLEND(1)
