1GRDSAMPLE(1) GMT GRDSAMPLE(1)
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6 grdsample - Resample a grid onto a new lattice
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9 grdsample in_grdfile -Gout_grdfile [ -Iincrement ] [ -Rregion ] [
10 -T ] [ -V[level] ] [ -fflags ] [ -nflags ] [ -rreg ] [ -x[[-]n] ]
11 Note: No space is allowed between the option flag and the associated
13 arguments.
14
16 grdsample reads a grid file and interpolates it to create a new grid
17 file with either: a different registration (-r or -T); or, a new
18 grid-spacing or number of nodes (-I), and perhaps also a new sub-region
19 (-R). A bicubic [Default], bilinear, B-spline or nearest-neighbor
20 interpolation is used; see -n for settings. Note that using -R only is
21 equivalent to grdcut or grdedit -S. grdsample safely creates a fine
22 mesh from a coarse one; the converse may suffer aliasing unless the
23 data are filtered using grdfft or grdfilter.
24 When -R is omitted, the output grid will cover the same region as the
26 input grid. When -I is omitted, the grid spacing of the output grid
27 will be the same as the input grid. Either -r or -T can be used to
28 change the grid registration. When omitted, the output grid will have
29 the same registration as the input grid.
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32 in_grdfile
33 The name of the input 2-D binary grid file. (See GRID FILE FOR‐
34 MAT below.)
35 -Gout_grdfile
37 The name of the output grid file. (See GRID FILE FORMAT below.)
38
40 -Ixinc[unit][+e|n][/yinc[unit][+e|n]]
41 x_inc [and optionally y_inc] is the grid spacing. Optionally,
42 append a suffix modifier. Geographical (degrees) coordinates:
43 Append m to indicate arc minutes or s to indicate arc seconds.
44 If one of the units e, f, k, M, n or u is appended instead, the
45 increment is assumed to be given in meter, foot, km, Mile, nau‐
46 tical mile or US survey foot, respectively, and will be con‐
47 verted to the equivalent degrees longitude at the middle lati‐
48 tude of the region (the conversion depends on PROJ_ELLIPSOID).
49 If y_inc is given but set to 0 it will be reset equal to x_inc;
50 otherwise it will be converted to degrees latitude. All coordi‐
51 nates: If +e is appended then the corresponding max x (east) or
52 y (north) may be slightly adjusted to fit exactly the given
53 increment [by default the increment may be adjusted slightly to
54 fit the given domain]. Finally, instead of giving an increment
55 you may specify the number of nodes desired by appending +n to
56 the supplied integer argument; the increment is then recalcu‐
57 lated from the number of nodes and the domain. The resulting
58 increment value depends on whether you have selected a grid‐
59 line-registered or pixel-registered grid; see App-file-formats
60 for details. Note: if -Rgrdfile is used then the grid spacing
61 has already been initialized; use -I to override the values.
62 -Rxmin/xmax/ymin/ymax[+r][+uunit] (more ...)
64 Specify the region of interest.
65 -T Translate between grid and pixel registration; if the input is
67 grid-registered, the output will be pixel-registered and
68 vice-versa.
69 -V[level] (more ...)
71 Select verbosity level [c].
72 -f[i|o]colinfo (more ...)
74 Specify data types of input and/or output columns.
75 -n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more ...)
77 Select interpolation mode for grids.
78 -r (more ...)
80 Set pixel node registration [gridline].
81 -x[[-]n] (more ...)
83 Limit number of cores used in multi-threaded algorithms (OpenMP
84 required).
85 -^ or just -
87 Print a short message about the syntax of the command, then
88 exits (NOTE: on Windows just use -).
89 -+ or just +
91 Print an extensive usage (help) message, including the explana‐
92 tion of any module-specific option (but not the GMT common
93 options), then exits.
94 -? or no arguments
96 Print a complete usage (help) message, including the explanation
97 of all options, then exits.
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100 Regardless of the precision of the input data, GMT programs that create
101 grid files will internally hold the grids in 4-byte floating point
102 arrays. This is done to conserve memory and furthermore most if not all
103 real data can be stored using 4-byte floating point values. Data with
104 higher precision (i.e., double precision values) will lose that preci‐
105 sion once GMT operates on the grid or writes out new grids. To limit
106 loss of precision when processing data you should always consider nor‐
107 malizing the data prior to processing.
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110 By default GMT writes out grid as single precision floats in a
111 COARDS-complaint netCDF file format. However, GMT is able to produce
112 grid files in many other commonly used grid file formats and also
113 facilitates so called "packing" of grids, writing out floating point
114 data as 1- or 2-byte integers. (more ...)
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117 Resample or sampling of grids will use various algorithms (see -n) that
118 may lead to possible distortions or unexpected results in the resampled
119 values. One expected effect of resampling with splines is the tendency
120 for the new resampled values to slightly exceed the global min/max lim‐
121 its of the original grid. If this is unacceptable, you can impose
122 clipping of the resampled values values so they do not exceed the input
123 min/max values by adding +c to your -n option.
124
126 If an interpolation point is not on a node of the input grid, then a
127 NaN at any node in the neighborhood surrounding the point will yield an
128 interpolated NaN. Bicubic interpolation [default] yields continuous
129 first derivatives but requires a neighborhood of 4 nodes by 4 nodes.
130 Bilinear interpolation [-n] uses only a 2 by 2 neighborhood, but yields
131 only zero-order continuity. Use bicubic when smoothness is important.
132 Use bilinear to minimize the propagation of NaNs.
133
135 As an alternative to bicubic spline, linear spline or nearest neighbor
136 interpolation one can instead send the entire dataset through surface
137 for re-gridding. This approach allows more control on aspects such as
138 tension but it also leads to a solution that is not likely to have
139 fully converged. The general approach would be something like
140 gmt grd2xyz old.grd | gmt surface -Rold.grd -Inewinc -Gnew.grd [other options]
142 For moderate data set one could also achieve an exact solution with
144 greenspline, such as
145 gmt grd2xyz old.grd | gmt greenspline -Rold.grd -Inewinc -Gnew.grd [other options]
147
149 To resample the 5 x 5 minute grid in hawaii_5by5_topo.nc onto a 1
150 minute grid:
151 gmt grdsample hawaii_5by5_topo.nc -I1m -Ghawaii_1by1_topo.nc
153 To translate the gridline-registered file surface.nc to pixel registra‐
155 tion while keeping the same region and grid interval:
156 gmt grdsample surface.nc -T -Gpixel.nc
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160 gmt, grdedit, grdfft, grdfilter, greenspline, surface
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163 2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
1645.4.5 Feb 24, 2019 GRDSAMPLE(1)