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.TH "GRDBLEND" "1gmt" "Jan 10, 2019" "5.4.5" "GMT"
.SH NAME
grdblend \- Blend several partially over-lapping grids into one large grid
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.SH SYNOPSIS
.sp
\fBgrdblend\fP [ \fIblendfile\fP | \fIgrid1\fP \fIgrid2\fP ... ]  \fB\-G\fP\fIoutgrid\fP
[  \fB\-I\fP\fIincrement\fP ]
[  \fB\-R\fP\fIregion\fP ]
[  \fB\-C\fP\fBf\fP|\fBl\fP|\fBo\fP|\fBu\fP ] [  \fB\-N\fP\fInodata\fP ]
[  \fB\-Q\fP ] [  \fB\-Z\fP\fIscale\fP ]
[  \fB\-V\fP[\fIlevel\fP] ]
[  \fB\-W\fP[\fBz\fP] ]
[ \fB\-f\fPflags ]
[ \fB\-n\fPflags ]
[ \fB\-r\fP ]
.sp
\fBNote:\fP No space is allowed between the option flag and the associated arguments.
.SH DESCRIPTION
.sp
\fBgrdblend\fP reads a listing of grid files and blend parameters and
creates a binary grid file by blending the other grids using
cosine\-taper weights. \fBgrdblend\fP 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\(aqs 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 \fBgrdblend\fP we only support the
netCDF and native binary grid formats for both input and output.
.SH REQUIRED ARGUMENTS
.INDENT 0.0
.TP
\fB\-G\fP\fIoutgrid\fP
\fIoutgrid\fP 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.
.UNINDENT
.INDENT 0.0
.TP
\fB\-I\fP\fIxinc\fP[\fIunit\fP][\fB+e\fP|\fBn\fP][/\fIyinc\fP[\fIunit\fP][\fB+e\fP|\fBn\fP]]
\fIx_inc\fP [and optionally \fIy_inc\fP] is the grid spacing. Optionally,
append a suffix modifier. \fBGeographical (degrees) coordinates\fP: Append
\fBm\fP to indicate arc minutes or \fBs\fP to indicate arc seconds. If one
of the units \fBe\fP, \fBf\fP, \fBk\fP, \fBM\fP, \fBn\fP or \fBu\fP 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 \fIy_inc\fP is given
but set to 0 it will be reset equal to \fIx_inc\fP; otherwise it will be
converted to degrees latitude. \fBAll coordinates\fP: If \fB+e\fP is appended
then the corresponding max \fIx\fP (\fIeast\fP) or \fIy\fP (\fInorth\fP) 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 \fInumber of nodes\fP desired by
appending \fB+n\fP 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 App\-file\-formats for
details. Note: if \fB\-R\fP\fIgrdfile\fP is used then the grid spacing has
already been initialized; use \fB\-I\fP to override the values.
.UNINDENT
.INDENT 0.0
.TP
\fB\-R\fP\fIxmin\fP/\fIxmax\fP/\fIymin\fP/\fIymax\fP[\fB+r\fP][\fB+u\fP\fIunit\fP] (more ...)
Specify the region of interest.  
.UNINDENT
.SH OPTIONAL ARGUMENTS
.INDENT 0.0
.TP
.B \fIblendfile\fP
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 \fB\-R\fP\-setting for the interior region
(optional), and the relative weight \fIwr\fP (optional). In the combined weighting scheme, this
grid will be given zero weight outside its domain, weight = \fIwr\fP
inside the interior region, and a 2\-D cosine\-tapered weight between
those end\-members in the boundary strip. However, if a negative \fIwr\fP
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 \fB\-R\fP\-setting, or
leave it off entirely.  Likewise, if a weight \fIwr\fP is not specified
we default to a weight of 1.  If the ASCII \fIblendfile\fP file is not
given \fBgrdblend\fP 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
\fIblendfile\fP\&. 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 \fB\-R\fP, \fB\-I\fP (and \fB\-r\fP) will
first be resampled via grdsample\&. Also, grids that are not in
netCDF or native binary format will first be reformatted via grdconvert\&.
.UNINDENT
.INDENT 0.0
.TP
\fB\-C\fP
Clobber mode: Instead of blending, simply pick the value of one of
the grids that covers a node. Select from the following modes: \fBf\fP
for the first grid to visit a node; \fBo\fP for the last grid to visit
a node; \fBl\fP for the grid with the lowest value, and \fBu\fP for the
grid with the uppermost value. For modes \fBf\fP and \fBo\fP the
ordering of grids in the \fIblendfile\fP will dictate which grid
contributes to the final result. Weights and cosine tapering are not
considered when clobber mode is active.
.UNINDENT
.INDENT 0.0
.TP
\fB\-N\fP\fInodata\fP
No data. Set nodes with no input grid to this value [Default is NaN].
.UNINDENT
.INDENT 0.0
.TP
\fB\-Q\fP
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).
.UNINDENT
.INDENT 0.0
.TP
\fB\-V\fP[\fIlevel\fP] (more ...)
Select verbosity level [c].  
.UNINDENT
.INDENT 0.0
.TP
\fB\-W\fP[\fBz\fP]
Do not blend, just output the weights used for each node [Default makes the blend].
Append \fBz\fP to write the weight*z sum instead.
.UNINDENT
.INDENT 0.0
.TP
\fB\-Z\fP\fIscale\fP
Scale output values by \fIscale\fP before writing to file. [1].
.UNINDENT
.INDENT 0.0
.TP
\fB\-f\fP[\fBi\fP|\fBo\fP]\fIcolinfo\fP (more ...)
Specify data types of input and/or output columns.  
.UNINDENT
.INDENT 0.0
.TP
\fB\-n\fP[\fBb\fP|\fBc\fP|\fBl\fP|\fBn\fP][\fB+a\fP][\fB+b\fP\fIBC\fP][\fB+c\fP][\fB+t\fP\fIthreshold\fP] (more ...)
Select interpolation mode for grids.
.UNINDENT
.INDENT 0.0
.TP
\fB\-r\fP (more ...)
Set pixel node registration [gridline].  
.UNINDENT
.INDENT 0.0
.TP
\fB\-^\fP or just \fB\-\fP
Print a short message about the syntax of the command, then exits (NOTE: on Windows just use \fB\-\fP).
.TP
\fB\-+\fP or just \fB+\fP
Print an extensive usage (help) message, including the explanation of
any module\-specific option (but not the GMT common options), then exits.
.TP
\fB\-?\fP or no arguments
Print a complete usage (help) message, including the explanation of all options, then exits.
.UNINDENT
.SH GRID FILE FORMATS
.sp
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 1\- or 2\-byte integers. To specify the precision, scale and
offset, the user should add the suffix
=\fIID\fP[\fB+s\fP\fIscale\fP][\fB+o\fP\fIoffset\fP][\fB+n\fP\fIinvalid\fP], where
\fIID\fP is a two\-letter identifier of the grid type and precision, and
\fIscale\fP and \fIoffset\fP are optional scale factor and offset to be applied
to all grid values, and \fIinvalid\fP is the value used to indicate missing
data. See grdconvert and Section
grid\-file\-format
of the GMT Technical Reference and Cookbook for more information.
.sp
When writing a netCDF file, the grid is stored by default with the
variable name "z". To specify another variable name \fIvarname\fP, append
\fB?\fP\fIvarname\fP to the file name. Note that you may need to escape the
special meaning of \fB?\fP in your shell program by putting a backslash in
front of it, or by placing the filename and suffix between quotes or
double quotes.
.SH GEOGRAPHICAL AND TIME COORDINATES
.sp
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 \fB\-f\fP or \fB\-R\fP options. For example,
both \fB\-f0x\fP \fB\-f1t\fP and \fB\-R\fP90w/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 \fBunit\fP attribute of the time variable
will indicate both this unit and epoch.
.SH TAPERING
.sp
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\(aqs TAPER operator.
.SH EXAMPLES
.sp
To create a grid file from the four grid files piece_?.nc, giving them each the different
weights, make the blendfile like this
.INDENT 0.0
.INDENT 3.5
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
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
.ft P
.fi
.UNINDENT
.UNINDENT
.UNINDENT
.UNINDENT
.sp
Then run
.INDENT 0.0
.INDENT 3.5
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
gmt grdblend blend.job \-Gblend.nc \-R<full_region> \-I<dx/dy> \-V
.ft P
.fi
.UNINDENT
.UNINDENT
.UNINDENT
.UNINDENT
.sp
To blend all the grids called MB_*.nc given them all equal weight, try
.INDENT 0.0
.INDENT 3.5
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
gmt grdblend MB_*.nc \-Gblend.nc \-R<full_region> \-I<dx/dy> \-V
.ft P
.fi
.UNINDENT
.UNINDENT
.UNINDENT
.UNINDENT
.SH WARNING ON LARGE FILE SETS
.sp
While \fBgrdblend\fP can process any number of files, it works by keeping those
files open that are being blended, and close files as soon as they are finished.
Depending on your session, many files may remain open at the same time.
Some operating systems set fairly modest
default limits on how many concurrent files can be open, e.g., 256.  If you
run into this problem then you can change this limit; see your operating system
documentation for how to change system limits.
.SH SEE ALSO
.sp
gmt,
grd2xyz,
grdconvert,
grdedit,
grdraster,
grdsample
.SH COPYRIGHT
2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
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