1GRDBLEND(1) Generic Mapping Tools GRDBLEND(1)
2
6 grdblend - Blend several partially over-lapping grids into one large
7 grid
8
10 grdblend blendfile -Ggrdfile -Ixinc[unit][=|+][/yinc[unit][=|+]]
11 -Rwest/east/south/north[r] [ -Nnodata ] [ -Q ] [ -Zscale ] [ -V ] [ -W
12 ] [ -fcolinfo ]
13
15 grdblend reads a listing of grid files and blend parameters and creates
16 a binary grid file by blending the other grids using cosine-taper
17 weights. grdblend will report if some of the nodes are not filled in
18 with data. Such unconstrained nodes are set to a value specified by
19 the user [Default is NaN]. Nodes with more than one value will be set
20 to the weighted average value.
21 blendfile
23 ASCII file with one record per grid file to include in the
24 blend. Each record must contain three items, separated by spa‐
25 ces or tabs: the gridfile name, the -R-setting for the interior
26 region, and the relative weight wr. In the combined weighting
27 scheme, this grid will be given zero weight outside its domain,
28 weight = wr inside the interior region, and a 2-D cosine-tapered
29 weight between those end-members in the boundary strip. How‐
30 ever, if a negative wr is given then the sense of tapering is
31 inverted (i.e., zero weight inside its domain). If the inner
32 region should instead exactly match the grid region then specify
33 a - instead of the -R-setting. If the ASCII file is not given
34 grdblend will read standard input.
35 -G grdfile is the name of the binary output grid file. (See GRID
37 FILE FORMATS below).
38 -I x_inc [and optionally y_inc] is the grid spacing. Optionally,
40 append a suffix modifier. Geographical (degrees) coordinates:
41 Append m to indicate arc minutes or c to indicate arc seconds.
42 If one of the units e, k, i, or n is appended instead, the
43 increment is assumed to be given in meter, km, miles, or nauti‐
44 cal miles, respectively, and will be converted to the equivalent
45 degrees longitude at the middle latitude of the region (the con‐
46 version depends on ELLIPSOID). If /y_inc is given but set to 0
47 it will be reset equal to x_inc; otherwise it will be converted
48 to degrees latitude. All coordinates: If = is appended then the
49 corresponding max x (east) or y (north) may be slightly adjusted
50 to fit exactly the given increment [by default the increment may
51 be adjusted slightly to fit the given domain]. Finally, instead
52 of giving an increment you may specify the number of nodes
53 desired by appending + to the supplied integer argument; the
54 increment is then recalculated from the number of nodes and the
55 domain. The resulting increment value depends on whether you
56 have selected a gridline-registered or pixel-registered grid;
57 see Appendix B for details. Note: if -Rgrdfile is used then
58 grid spacing has already been initialized; use -I to override
59 the values.
60 -R xmin, xmax, ymin, and ymax specify the Region of interest. For
62 geographic regions, these limits correspond to west, east,
63 south, and north and you may specify them in decimal degrees or
64 in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
65 and upper right map coordinates are given instead of w/e/s/n.
66 The two shorthands -Rg and -Rd stand for global domain (0/360
67 and -180/+180 in longitude respectively, with -90/+90 in lati‐
68 tude). Alternatively, specify the name of an existing grid file
69 and the -R settings (and grid spacing, if applicable) are copied
70 from the grid. For calendar time coordinates you may either
71 give (a) relative time (relative to the selected TIME_EPOCH and
72 in the selected TIME_UNIT; append t to -JX|x), or (b) absolute
73 time of the form [date]T[clock] (append T to -JX|x). At least
74 one of date and clock must be present; the T is always required.
75 The date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
76 calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
77 string must be of the form hh:mm:ss[.xxx]. The use of delim‐
78 iters and their type and positions must be exactly as indicated
79 (however, input, output and plot formats are customizable; see
80 gmtdefaults).
81
83 -N No data. Set nodes with no input grid to this value [Default is
84 NaN].
85 -Q Create a header-less grid file suitable for use with grdraster.
87 Requires that the output grid file is a native format (i.e., not
88 netCDF).
89 -V Selects verbose mode, which will send progress reports to stderr
91 [Default runs "silently"].
92 -W Do not blend, just output the weights used for each node. This
94 option is valid when only one input grid is provided [Default
95 makes the blend].
96 -Z Scale output values by scale before writing to file. [1].
98 -f Special formatting of input and/or output columns (time or geo‐
100 graphical data). Specify i or o to make this apply only to
101 input or output [Default applies to both]. Give one or more
102 columns (or column ranges) separated by commas. Append T (abso‐
103 lute calendar time), t (relative time in chosen TIME_UNIT since
104 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
105 to each column or column range item. Shorthand -f[i|o]g means
106 -f[i|o]0x,1y (geographic coordinates).
107
109 By default GMT writes out grid as single precision floats in a COARDS-
110 complaint netCDF file format. However, GMT is able to produce grid
111 files in many other commonly used grid file formats and also facili‐
112 tates so called "packing" of grids, writing out floating point data as
113 2- or 4-byte integers. To specify the precision, scale and offset, the
114 user should add the suffix =id[/scale/offset[/nan]], where id is a two-
115 letter identifier of the grid type and precision, and scale and offset
116 are optional scale factor and offset to be applied to all grid values,
117 and nan is the value used to indicate missing data. See grdreformat(1)
118 and Section 4.17 of the GMT Technical Reference and Cookbook for more
119 information.
120 When writing a netCDF file, the grid is stored by default with the
122 variable name "z". To specify another variable name varname, append
123 ?varname to the file name. Note that you may need to escape the spe‐
124 cial meaning of ? in your shell program by putting a backslash in front
125 of it, or by placing the filename and suffix between quotes or double
126 quotes.
127
129 When the output grid type is netCDF, the coordinates will be labeled
130 "longitude", "latitude", or "time" based on the attributes of the input
131 data or grid (if any) or on the -f or -R options. For example, both
132 -f0x -f1t and -R90w/90e/0t/3t will result in a longitude/time grid.
133 When the x, y, or z coordinate is time, it will be stored in the grid
134 as relative time since epoch as specified by TIME_UNIT and TIME_EPOCH
135 in the .gmtdefaults file or on the command line. In addition, the unit
136 attribute of the time variable will indicate both this unit and epoch.
137
139 To create a grid file from the four grid files piece_?.nc, make the
140 blendfile like this
141 piece_1.nc -R<subregion_1> 1
143 piece_2.nc -R<subregion_2> 1
144 piece_3.nc -R<subregion_3> 1
145 piece_4.nc -R<subregion_4> 1
146 Then run
148 grdblend blend.job -Gblend.nc -R<full_region> -I<dx/dy> -V
150
152 Currently, all grids processed must have the exact same node registra‐
153 tion and grid spacing as the final output grid.
154
156 GMT(1), grd2xyz(1), grdedit(1) grdraster(1)
157GMT 4.5.6 10 Mar 2011 GRDBLEND(1)