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 ] [ -M[i|o][flag] ]
11 [ -Q[b|c|l|n][[/]threshold] ] [ -Rwest/east/south/north[r] ] [ -S ] [
12 -V ] [ -Z ] [ -:[i|o] ] [ -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [
13 -f[i|o]colinfo ]
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 [72.0059773539]. (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). Number of header records
44 can be changed by editing your .gmtdefaults4 file. If used, GMT
45 default is 1 header record. Use -Hi if only input data should
46 have header records [Default will write out header records if
47 the input data have them]. Blank lines and lines starting with #
48 are always skipped.
49 -L Boundary condition flag may be x or y or xy indicating data is
51 periodic in range of x or y or both set by -R, or flag may be g
52 indicating geographical conditions (x and y are lon and lat).
53 [Default uses "natural" conditions (second partial derivative
54 normal to edge is zero) unless the grid is automatically recog‐
55 nised as periodic.]
56 -M Multiple segment file. Segment separator is a record beginning
58 with flag. [Default is '>'].
59 -Q Quick mode, use bilinear rather than bicubic interpolation
61 [Default]. Alternatively, select the interpolation mode by
62 adding b for B-spline smooting, c for bicubic interpolation, l
63 for bilinear interpolation or n for nearest-neighbor value.
64 Optionally, append threshold in the range [0,1]. This parameter
65 controls how close to nodes with NaN values the interpolation
66 will go. E.g., a threshold of 0.5 will interpolate about half
67 way from a non-NaN to a NaN node, whereas 0.1 will go about 90%
68 of the way, etc. [Default is 1, which means none of the (4 or
69 16) nearby nodes may be NaN]. -Q0 will just return the value of
70 the nearest node instead of interpolating. This is the same as
71 using -Qn.
72 -R xmin, xmax, ymin, and ymax specify the Region of interest. For
74 geographic regions, these limits correspond to west, east,
75 south, and north and you may specify them in decimal degrees or
76 in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
77 and upper right map coordinates are given instead of w/e/s/n.
78 The two shorthands -Rg and -Rd stand for global domain (0/360
79 and -180/+180 in longitude respectively, with -90/+90 in lati‐
80 tude). For calendar time coordinates you may either give (a)
81 relative time (relative to the selected TIME_EPOCH and in the
82 selected TIME_UNIT; append t to -JX|x), or (b) absolute time of
83 the form [date]T[clock] (append T to -JX|x). At least one of
84 date and clock must be present; the T is always required. The
85 date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
86 calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
87 string must be of the form hh:mm:ss[.xxx]. The use of delim‐
88 iters and their type and positions must be exactly as indicated
89 (however, input, output and plot formats are customizable; see
90 gmtdefaults).
91 -S Suppress the output of interpolated points that result in NaN
93 values.
94 -V Selects verbose mode, which will send progress reports to stderr
96 [Default runs "silently"].
97 -Z Only write out the sampled z-values [Default writes all col‐
99 umns].
100 -: Toggles between (longitude,latitude) and (latitude,longitude)
102 input/output. [Default is (longitude,latitude)].
103 -bi Selects binary input. Append s for single precision [Default is
105 d (double)]. Uppercase S or D will force byte-swapping.
106 Optionally, append ncol, the number of columns in your binary
107 input file if it exceeds the columns needed by the program. Or
108 append c if the input file is netCDF. Optionally, append
109 var1/var2/... to specify the variables to be read. [Default is
110 2 input columns].
111 -bo Selects binary output. Append s for single precision [Default
113 is d (double)]. Uppercase S or D will force byte-swapping.
114 Optionally, append ncol, the number of desired columns in your
115 binary output file. [Default is one more than input].
116 -f Special formatting of input and/or output columns (time or geo‐
118 graphical data). Specify i or o to make this apply only to
119 input or output [Default applies to both]. Give one or more
120 columns (or column ranges) separated by commas. Append T (abso‐
121 lute calendar time), t (relative time in chosen TIME_UNIT since
122 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
123 to each column or column range item. Shorthand -f[i|o]g means
124 -f[i|o]0x,1y (geographic coordinates).
125
127 The ASCII output formats of numerical data are controlled by parameters
128 in your .gmtdefaults4 file. Longitude and latitude are formatted
129 according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted
130 according to D_FORMAT. Be aware that the format in effect can lead to
131 loss of precision in the output, which can lead to various problems
132 downstream. If you find the output is not written with enough preci‐
133 sion, consider switching to binary output (-bo if available) or specify
134 more decimals using the D_FORMAT setting.
135
137 GMT is able to recognize many of the commonly used grid file formats,
138 as well as the precision, scale and offset of the values contained in
139 the grid file. When GMT needs a little help with that, you can add the
140 suffix =id[/scale/offset[/nan]], where id is a two-letter identifier of
141 the grid type and precision, and scale and offset are optional scale
142 factor and offset to be applied to all grid values, and nan is the
143 value used to indicate missing data. See grdreformat(1) and Section
144 4.17 of the GMT Technical Reference and Cookbook for more information.
145 When reading a netCDF file that contains multiple grids, GMT will read,
147 by default, the first 2-dimensional grid that can find in that file. To
148 coax GMT into reading another multi-dimensional variable in the grid
149 file, append ?varname to the file name, where varname is the name of
150 the variable. Note that you may need to escape the special meaning of ?
151 in your shell program by putting a backslash in front of it, or by
152 placing the filename and suffix between quotes or double quotes. See
153 grdreformat(1) and Section 4.18 of the GMT Technical Reference and
154 Cookbook for more information, particularly on how to read splices of
155 3-, 4-, or 5-dimensional grids.
156
158 If an interpolation point is not on a node of the input grid, then a
159 NaN at any node in the neighborhood surrounding the point will yield an
160 interpolated NaN. Bicubic interpolation [default] yields continuous
161 first derivatives but requires a neighborhood of 4 nodes by 4 nodes.
162 Bilinear interpolation [-Q] uses only a 2 by 2 neighborhood, but yields
163 only zeroth-order continuity. Use bicubic when smoothness is impor‐
164 tant. Use bilinear to minimize the propagation of NaNs, or lower
165 threshold.
166
168 To sample the file hawaii_topo.grd along the SEASAT track track_4.xyg
169 (An ASCII table containing longitude, latitude, and SEASAT-derived
170 gravity, preceded by one header record):
171 grdtrack track_4.xyg -Ghawaii_topo.grd -H > track_4.xygt
173 To sample the Sandwell/Smith IMG format file topo.8.2.img (2 minute
175 predicted bathymetry on a Mercator grid) along the lon,lat coordinates
176 given in the file cruise_track.xy, try
177 grdtrack cruise_track.xy -Gtopo.8.2.img,1,1 > obs_and_predicted.d
179
181 GMT(1), surface(1), sample1d(1)
182GMT 4.3.1 15 May 2008 GRDTRACK(1)