1GRDTRACK(1) Generic Mapping Tools GRDTRACK(1)
2
6 grdtrack - Sampling of a 2-D grid file along 1-D trackline (a sequence
7 of x,y points)
8
10 grdtrack xyfile -Ggrdfile [ -H[i][nrec] ] [ -Lflag ] [
11 -Q[b|c|l|n][[/]threshold] ] [ -Rwest/east/south/north[r] ] [ -S ] [ -V
12 ] [ -Z ] [ -:[i|o] ] [ -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [
13 -f[i|o]colinfo ] [ -m[i|o][flag] ]
14
16 grdtrack reads a grid file (or a Sandwell/Smith IMG file) and a table
17 (from file or standard input) with (x,y) positions in the first two
18 columns (more columns may be present). It interpolates the grid at the
19 positions in the table and writes out the table with the interpolated
20 values added as a new column. A bicubic [Default], bilinear, B-spline
21 or nearest-neighbor (see -Q) interpolation is used, requiring boundary
22 conditions at the limits of the region (see -L).
23 xyfile This is an ASCII (or binary, see -b) file where the first 2 col‐
25 umns hold the (x,y) positions where the user wants to sample the
26 2-D data set.
27 -G grdfile is a 2-D binary grid file with the function f(x,y). If
29 the specified grid is in Sandwell/Smith Mercator format you must
30 append a comma-separated list of arguments that includes a scale
31 to multiply the data (usually 1 or 0.1), the mode which stand
32 for the following: (0) Img files with no constraint code,
33 returns data at all points, (1) Img file with constraints coded,
34 return data at all points, (2) Img file with constraints coded,
35 return data only at constrained points and NaN elsewhere, and
36 (3) Img file with constraints coded, return 1 at constraints and
37 0 elsewhere, and optionally the max latitude in the IMG file
38 [80.738]. (See GRID FILE FORMAT below.)
39
41 No space between the option flag and the associated arguments.
42 -H Input file(s) has header record(s). If used, the default number
44 of header records is N_HEADER_RECS. Use -Hi if only input data
45 should have header records [Default will write out header
46 records if the input data have them]. Blank lines and lines
47 starting with # are always skipped.
48 -L Boundary condition flag may be x or y or xy indicating data is
50 periodic in range of x or y or both set by -R, or flag may be g
51 indicating geographical conditions (x and y are lon and lat).
52 [Default uses "natural" conditions (second partial derivative
53 normal to edge is zero) unless the grid is automatically recog‐
54 nised as periodic.]
55 -Q Quick mode, use bilinear rather than bicubic interpolation
57 [Default]. Alternatively, select the interpolation mode by
58 adding b for B-spline smoothing, c for bicubic interpolation, l
59 for bilinear interpolation or n for nearest-neighbor value.
60 Optionally, append threshold in the range [0,1]. This parameter
61 controls how close to nodes with NaN values the interpolation
62 will go. E.g., a threshold of 0.5 will interpolate about half
63 way from a non-NaN to a NaN node, whereas 0.1 will go about 90%
64 of the way, etc. [Default is 1, which means none of the (4 or
65 16) nearby nodes may be NaN]. -Q0 will just return the value of
66 the nearest node instead of interpolating. This is the same as
67 using -Qn.
68 -R xmin, xmax, ymin, and ymax specify the Region of interest. For
70 geographic regions, these limits correspond to west, east,
71 south, and north and you may specify them in decimal degrees or
72 in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
73 and upper right map coordinates are given instead of w/e/s/n.
74 The two shorthands -Rg and -Rd stand for global domain (0/360
75 and -180/+180 in longitude respectively, with -90/+90 in lati‐
76 tude). Alternatively, specify the name of an existing grid file
77 and the -R settings (and grid spacing, if applicable) are copied
78 from the grid. For calendar time coordinates you may either
79 give (a) relative time (relative to the selected TIME_EPOCH and
80 in the selected TIME_UNIT; append t to -JX|x), or (b) absolute
81 time of the form [date]T[clock] (append T to -JX|x). At least
82 one of date and clock must be present; the T is always required.
83 The date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
84 calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
85 string must be of the form hh:mm:ss[.xxx]. The use of delim‐
86 iters and their type and positions must be exactly as indicated
87 (however, input, output and plot formats are customizable; see
88 gmtdefaults).
89 -S Suppress the output of interpolated points that result in NaN
91 values.
92 -V Selects verbose mode, which will send progress reports to stderr
94 [Default runs "silently"].
95 -Z Only write out the sampled z-values [Default writes all col‐
97 umns].
98 -: Toggles between (longitude,latitude) and (latitude,longitude)
100 input/output. [Default is (longitude,latitude)].
101 -bi Selects binary input. Append s for single precision [Default is
103 d (double)]. Uppercase S or D will force byte-swapping.
104 Optionally, append ncol, the number of columns in your binary
105 input file if it exceeds the columns needed by the program. Or
106 append c if the input file is netCDF. Optionally, append
107 var1/var2/... to specify the variables to be read. [Default is
108 2 input columns].
109 -bo Selects binary output. Append s for single precision [Default
111 is d (double)]. Uppercase S or D will force byte-swapping.
112 Optionally, append ncol, the number of desired columns in your
113 binary output file. [Default is one more than input].
114 -f Special formatting of input and/or output columns (time or geo‐
116 graphical data). Specify i or o to make this apply only to
117 input or output [Default applies to both]. Give one or more
118 columns (or column ranges) separated by commas. Append T (abso‐
119 lute calendar time), t (relative time in chosen TIME_UNIT since
120 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
121 to each column or column range item. Shorthand -f[i|o]g means
122 -f[i|o]0x,1y (geographic coordinates).
123 -m Multiple segment file(s). Segments are separated by a special
125 record. For ASCII files the first character must be flag
126 [Default is '>']. For binary files all fields must be NaN and
127 -b must set the number of output columns explicitly. By default
128 the -m setting applies to both input and output. Use -mi and
129 -mo to give separate settings to input and output.
130
132 The ASCII output formats of numerical data are controlled by parameters
133 in your .gmtdefaults4 file. Longitude and latitude are formatted
134 according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted
135 according to D_FORMAT. Be aware that the format in effect can lead to
136 loss of precision in the output, which can lead to various problems
137 downstream. If you find the output is not written with enough preci‐
138 sion, consider switching to binary output (-bo if available) or specify
139 more decimals using the D_FORMAT setting.
140
142 GMT is able to recognize many of the commonly used grid file formats,
143 as well as the precision, scale and offset of the values contained in
144 the grid file. When GMT needs a little help with that, you can add the
145 suffix =id[/scale/offset[/nan]], where id is a two-letter identifier of
146 the grid type and precision, and scale and offset are optional scale
147 factor and offset to be applied to all grid values, and nan is the
148 value used to indicate missing data. See grdreformat(1) and Section
149 4.17 of the GMT Technical Reference and Cookbook for more information.
150 When reading a netCDF file that contains multiple grids, GMT will read,
152 by default, the first 2-dimensional grid that can find in that file. To
153 coax GMT into reading another multi-dimensional variable in the grid
154 file, append ?varname to the file name, where varname is the name of
155 the variable. Note that you may need to escape the special meaning of ?
156 in your shell program by putting a backslash in front of it, or by
157 placing the filename and suffix between quotes or double quotes. See
158 grdreformat(1) and Section 4.18 of the GMT Technical Reference and
159 Cookbook for more information, particularly on how to read splices of
160 3-, 4-, or 5-dimensional grids.
161
163 If an interpolation point is not on a node of the input grid, then a
164 NaN at any node in the neighborhood surrounding the point will yield an
165 interpolated NaN. Bicubic interpolation [default] yields continuous
166 first derivatives but requires a neighborhood of 4 nodes by 4 nodes.
167 Bilinear interpolation [-Q] uses only a 2 by 2 neighborhood, but yields
168 only zeroth-order continuity. Use bicubic when smoothness is impor‐
169 tant. Use bilinear to minimize the propagation of NaNs, or lower
170 threshold.
171
173 To sample the file hawaii_topo.grd along the SEASAT track track_4.xyg
174 (An ASCII table containing longitude, latitude, and SEASAT-derived
175 gravity, preceded by one header record):
176 grdtrack track_4.xyg -Ghawaii_topo.grd -H > track_4.xygt
178 To sample the Sandwell/Smith IMG format file topo.8.2.img (2 minute
180 predicted bathymetry on a Mercator grid) along the lon,lat coordinates
181 given in the file cruise_track.xy, try
182 grdtrack cruise_track.xy -Gtopo.8.2.img,1,1 > obs_and_predicted.d
184
186 GMT(1), surface(1), sample1d(1)
187GMT 4.5.6 10 Mar 2011 GRDTRACK(1)