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 gridded files and blend parameters and cre‐
16 ates 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 the gridfile name, the -R-set‐
25 ting for the interior region, and the relative weight wr, sepa‐
26 rated by spaces or tabs. In the combined weighting scheme, this
27 grid will be given weight = zero outside its domain, weight = wr
28 inside the interior region, and a 2-D cosine-tapered weight
29 between those end-members in the boundary strip. However, if a
30 negative wr is given then the sense of tapering is inverted
31 (i.e., zero weight inside its domain). If the ASCII file is not
32 given grdblend will read standard input.
33 -G grdfile is the name of the binary output grid file. (See GRID
35 FILE FORMATS below).
36 -I x_inc [and optionally y_inc] is the grid spacing. Optionally,
38 append a suffix modifier. Geographical (degrees) coordinates:
39 Append m to indicate arc minutes or c to indicate arc seconds.
40 If one of the units e, k, i, or n is appended instead, the
41 increment is assumed to be given in meter, km, miles, or nauti‐
42 cal miles, respectively, and will be converted to the equivalent
43 degrees longitude at the middle latitude of the region (the con‐
44 version depends on ELLIPSOID). If /y_inc is given but set to 0
45 it will be reset equal to x_inc; otherwise it will be converted
46 to degrees latitude. All coordinates: If = is appended then the
47 corresponding max x (east) or y (north) may be slightly adjusted
48 to fit exactly the given increment [by default the increment may
49 be adjusted slightly to fit the given domain]. Finally, instead
50 of giving an increment you may specify the number of nodes
51 desired by appending + to the supplied integer argument; the
52 increment is then recalculated from the number of nodes and the
53 domain. The resulting increment value depends on whether you
54 have selected a gridline-registered or pixel-registered grid;
55 see Appendix B for details.
56 -R xmin, xmax, ymin, and ymax specify the Region of interest. For
58 geographic regions, these limits correspond to west, east,
59 south, and north and you may specify them in decimal degrees or
60 in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
61 and upper right map coordinates are given instead of w/e/s/n.
62 The two shorthands -Rg and -Rd stand for global domain (0/360
63 and -180/+180 in longitude respectively, with -90/+90 in lati‐
64 tude). For calendar time coordinates you may either give (a)
65 relative time (relative to the selected TIME_EPOCH and in the
66 selected TIME_UNIT; append t to -JX|x), or (b) absolute time of
67 the form [date]T[clock] (append T to -JX|x). At least one of
68 date and clock must be present; the T is always required. The
69 date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
70 calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
71 string must be of the form hh:mm:ss[.xxx]. The use of delim‐
72 iters and their type and positions must be exactly as indicated
73 (however, input, output and plot formats are customizable; see
74 gmtdefaults).
75
77 -N No data. Set nodes with no input grid to this value [Default is
78 NaN].
79 -Q Create a header-less grid file suitable for use with grdraster.
81 Requires that the output grid file is a native format (i.e., not
82 netCDF).
83 -V Selects verbose mode, which will send progress reports to stderr
85 [Default runs "silently"].
86 -W Do not blend, just output the weights used for each node. This
88 option is valid when only one input grid is provided [Default
89 makes the blend].
90 -Z Scale output values by scale before writing to file. [1].
92 -f Special formatting of input and/or output columns (time or geo‐
94 graphical data). Specify i or o to make this apply only to
95 input or output [Default applies to both]. Give one or more
96 columns (or column ranges) separated by commas. Append T (abso‐
97 lute calendar time), t (relative time in chosen TIME_UNIT since
98 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
99 to each column or column range item. Shorthand -f[i|o]g means
100 -f[i|o]0x,1y (geographic coordinates).
101
103 By default GMT writes out grid as single precision floats in a COARDS-
104 complaint netCDF file format. However, GMT is able to produce grid
105 files in many other commonly used grid file formats and also facili‐
106 tates so called "packing" of grids, writing out floating point data as
107 2- or 4-byte integers. To specify the precision, scale and offset, the
108 user should add the suffix =id[/scale/offset[/nan]], where id is a two-
109 letter identifier of the grid type and precision, and scale and offset
110 are optional scale factor and offset to be applied to all grid values,
111 and nan is the value used to indicate missing data. See grdreformat(1)
112 and Section 4.17 of the GMT Technical Reference and Cookbook for more
113 information.
114 When writing a netCDF file, the grid is stored by default with the
116 variable name "z". To specify another variable name varname, append
117 ?varname to the file name. Note that you may need to escape the spe‐
118 cial meaning of ? in your shell program by putting a backslash in front
119 of it, or by placing the filename and suffix between quotes or double
120 quotes.
121
123 When the output grid type is netCDF, the coordinates will be labeled
124 "longitude", "latitude", or "time" based on the attributes of the input
125 data or grid (if any) or on the -f or -R options. For example, both
126 -f0x -f1t and -R90w/90e/0t/3t will result in a longitude/time grid.
127 When the x, y, or z coordinate is time, it will be stored in the grid
128 as relative time since epoch as specified by TIME_UNIT and TIME_EPOCH
129 in the .gmtdefaults file or on the command line. In addition, the unit
130 attribute of the time variable will indicate both this unit and epoch.
131
133 To create a grid file from the four gridded files piece_?.grd, make the
134 blendfile like this
135 piece_1.grd -R<subregion_1> 1
137 piece_2.grd -R<subregion_2> 1
138 piece_3.grd -R<subregion_3> 1
139 piece_4.grd -R<subregion_4> 1
140 Then run
142 grdblend blend.job -Gblend.grd -R<full_region> -I<dx/dy> -V
144
146 Currently, all grids processed must have the exact same node registra‐
147 tion as well as grid spacing as the final output grid.
148
150 GMT(1), grd2xyz(1), grdedit(1)
151GMT 4.3.1 15 May 2008 GRDBLEND(1)