1GRDSAMPLE(1) Generic Mapping Tools GRDSAMPLE(1)
2
6 grdsample - Resample a grid file onto a new grid
7
9 grdsample in_grdfile -Gout_grdfile [ -F ] [
10 -Ixinc[unit][=|+][/yinc[unit][=|+]] ] [ -Lflag ] [
11 -Q[b|c|l|n][[/]threshold] ] [ -Rwest/east/south/north[r] ] [ -T ] [ -V
12 ] [ -fcolinfo ]
13
15 grdsample reads a grid file and interpolates it to create a new grid
16 file with either: a different registration (-F or -T); or, a new grid-
17 spacing or number of nodes (-I), and perhaps also a new sub-region
18 (-R). A bicubic [Default], bilinear, B-spline or nearest-neighbor
19 interpolation (-Q) is used, requiring boundary conditions (-L). Note
20 that using -R only is equivalent to grdcut or grdedit -S. grdsample
21 safely creates a fine mesh from a coarse one; the converse may suffer
22 aliasing unless the data are filtered using grdfft or grdfilter.
23 When -R is omitted, the output grid will cover the same region as the
25 input grid. When -I is omitted, the grid spacing of the output grid
26 will be the same as the input grid. Either -F or -T can be used to
27 change the grid registration. When omitted, the output grid will have
28 the same registration as the input grid.
29 in_grdfile
31 The name of the input 2-D binary grid file. (See GRID FILE FOR‐
32 MAT below.)
33 -G The name of the output grid file. (See GRID FILE FORMAT below.)
35
37 -F Force pixel node registration on output grid. [Default is same
38 registration as input grid].
39 -I x_inc [and optionally y_inc] is the grid spacing. Optionally,
41 append a suffix modifier. Geographical (degrees) coordinates:
42 Append m to indicate arc minutes or c to indicate arc seconds.
43 If one of the units e, k, i, or n is appended instead, the
44 increment is assumed to be given in meter, km, miles, or nauti‐
45 cal miles, respectively, and will be converted to the equivalent
46 degrees longitude at the middle latitude of the region (the con‐
47 version depends on ELLIPSOID). If /y_inc is given but set to 0
48 it will be reset equal to x_inc; otherwise it will be converted
49 to degrees latitude. All coordinates: If = is appended then the
50 corresponding max x (east) or y (north) may be slightly adjusted
51 to fit exactly the given increment [by default the increment may
52 be adjusted slightly to fit the given domain]. Finally, instead
53 of giving an increment you may specify the number of nodes
54 desired by appending + to the supplied integer argument; the
55 increment is then recalculated from the number of nodes and the
56 domain. The resulting increment value depends on whether you
57 have selected a gridline-registered or pixel-registered grid;
58 see Appendix B for details.
59 -L Boundary condition flag may be x or y or xy indicating data is
61 periodic in range of x or y or both set by -R, or flag may be g
62 indicating geographical conditions (x and y are lon and lat).
63 [Default uses "natural" conditions (second partial derivative
64 normal to edge is zero) unless the grid is automatically recog‐
65 nised as periodic.]
66 -Q Quick mode, use bilinear rather than bicubic interpolation
68 [Default]. Alternatively, select the interpolation mode by
69 adding b for B-spline smooting, c for bicubic interpolation, l
70 for bilinear interpolation or n for nearest-neighbor value.
71 Optionally, append threshold in the range [0,1]. This parameter
72 controls how close to nodes with NaN values the interpolation
73 will go. E.g., a threshold of 0.5 will interpolate about half
74 way from a non-NaN to a NaN node, whereas 0.1 will go about 90%
75 of the way, etc. [Default is 1, which means none of the (4 or
76 16) nearby nodes may be NaN]. -Q0 will just return the value of
77 the nearest node instead of interpolating. This is the same as
78 using -Qn.
79 -R xmin, xmax, ymin, and ymax specify the Region of interest. For
81 geographic regions, these limits correspond to west, east,
82 south, and north and you may specify them in decimal degrees or
83 in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
84 and upper right map coordinates are given instead of w/e/s/n.
85 The two shorthands -Rg and -Rd stand for global domain (0/360
86 and -180/+180 in longitude respectively, with -90/+90 in lati‐
87 tude). For calendar time coordinates you may either give (a)
88 relative time (relative to the selected TIME_EPOCH and in the
89 selected TIME_UNIT; append t to -JX|x), or (b) absolute time of
90 the form [date]T[clock] (append T to -JX|x). At least one of
91 date and clock must be present; the T is always required. The
92 date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
93 calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
94 string must be of the form hh:mm:ss[.xxx]. The use of delim‐
95 iters and their type and positions must be exactly as indicated
96 (however, input, output and plot formats are customizable; see
97 gmtdefaults).
98 -T Translate between grid and pixel registration; if the input is
100 grid-registered, the output will be pixel-registered and vice-
101 versa.
102 -V Selects verbose mode, which will send progress reports to stderr
104 [Default runs "silently"].
105 -f Special formatting of input and/or output columns (time or geo‐
107 graphical data). Specify i or o to make this apply only to
108 input or output [Default applies to both]. Give one or more
109 columns (or column ranges) separated by commas. Append T (abso‐
110 lute calendar time), t (relative time in chosen TIME_UNIT since
111 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
112 to each column or column range item. Shorthand -f[i|o]g means
113 -f[i|o]0x,1y (geographic coordinates).
114
116 Regardless of the precision of the input data, GMT programs that create
117 gridded files will internally hold the grids in 4-byte floating point
118 arrays. This is done to conserve memory and futhermore most if not all
119 real data can be stored using 4-byte floating point values. Data with
120 higher precision (i.e., double precision values) will lose that preci‐
121 sion once GMT operates on the grid or writes out new grids. To limit
122 loss of precision when processing data you should always consider nor‐
123 malizing the data prior to processing.
124
126 By default GMT writes out grid as single precision floats in a COARDS-
127 complaint netCDF file format. However, GMT is able to produce grid
128 files in many other commonly used grid file formats and also facili‐
129 tates so called "packing" of grids, writing out floating point data as
130 2- or 4-byte integers. To specify the precision, scale and offset, the
131 user should add the suffix =id[/scale/offset[/nan]], where id is a two-
132 letter identifier of the grid type and precision, and scale and offset
133 are optional scale factor and offset to be applied to all grid values,
134 and nan is the value used to indicate missing data. When reading
135 grids, the format is generally automatically recognized. If not, the
136 same suffix can be added to input grid file names. See grdreformat(1)
137 and Section 4.17 of the GMT Technical Reference and Cookbook for more
138 information.
139 When reading a netCDF file that contains multiple grids, GMT will read,
141 by default, the first 2-dimensional grid that can find in that file. To
142 coax GMT into reading another multi-dimensional variable in the grid
143 file, append ?varname to the file name, where varname is the name of
144 the variable. Note that you may need to escape the special meaning of ?
145 in your shell program by putting a backslash in front of it, or by
146 placing the filename and suffix between quotes or double quotes. The
147 ?varname suffix can also be used for output grids to specify a variable
148 name different from the default: "z". See grdreformat(1) and Section
149 4.18 of the GMT Technical Reference and Cookbook for more information,
150 particularly on how to read splices of 3-, 4-, or 5-dimensional grids.
151
153 If an interpolation point is not on a node of the input grid, then a
154 NaN at any node in the neighborhood surrounding the point will yield an
155 interpolated NaN. Bicubic interpolation [default] yields continuous
156 first derivatives but requires a neighborhood of 4 nodes by 4 nodes.
157 Bilinear interpolation [-Q] uses only a 2 by 2 neighborhood, but yields
158 only zeroth-order continuity. Use bicubic when smoothness is impor‐
159 tant. Use bilinear to minimize the propagation of NaNs.
160
162 To resample the 5 x 5 minute grid in hawaii_5by5_topo.grd onto a 1
163 minute grid:
164 grdsample hawaii_5by5_topo.grd -I1m -Ghawaii_1by1_topo.grd
166 To translate the gridline-registered file surface.grd to pixel regis‐
168 tration while keeping the same region and grid interval:
169 grdsample surface.grd -T -Gpixel.grd
171
173 GMT(1), grdedit(1), grdfft(1), grdfilter(1)
174GMT 4.3.1 15 May 2008 GRDSAMPLE(1)