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] [
-N nodata ] [
-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).
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)