1GRDMASK(1) Generic Mapping Tools GRDMASK(1)
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6 grdmask - Create mask grid files from xy paths.
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9 grdmask pathfiles -Gmask_grd_file] -Ixinc[unit][=|+][/yinc[unit][=|+]]
10 -Rwest/east/south/north[r] [ -A[m|p] ] [ -F ] [ -H[i][nrec] ] [
11 -Nout/edge/in ] [ -Ssearch_radius[m|c|k|K] ] [ -V ] [ -:[i|o] ] [
12 -bi[s|S|d|D[ncol]|c[var1/...]] ] [ -fcolinfo ] [ -m[flag] ]
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15 grdmask can operate in two different modes. 1. It reads one or more xy-
16 files that each define a closed polygon. The nodes defined by the
17 specified region and lattice spacing will be set equal to one of three
18 possible values depending on whether the node is outside, on the poly‐
19 gon perimeter, or inside the polygon. The resulting mask may be used
20 in subsequent operations involving grdmath to mask out data from polyg‐
21 onal areas. 2. The xy-files simply represent data point locations and
22 the mask is set to the inside or outside value depending on whether a
23 node is within a maximum distance from the nearest data point. If the
24 distance specified is zero then only the nodes nearest each data point
25 are considered "inside".
26 pathfiles
28 The name of 1 or more ASCII [or binary, see -b] files holding
29 the polygon(s) or data points.
30 -G Name of resulting output mask grid file. (See GRID FILE FORMATS
32 below).
33 -I x_inc [and optionally y_inc] is the grid spacing. Optionally,
35 append a suffix modifier. Geographical (degrees) coordinates:
36 Append m to indicate arc minutes or c to indicate arc seconds.
37 If one of the units e, k, i, or n is appended instead, the
38 increment is assumed to be given in meter, km, miles, or nauti‐
39 cal miles, respectively, and will be converted to the equivalent
40 degrees longitude at the middle latitude of the region (the con‐
41 version depends on ELLIPSOID). If /y_inc is given but set to 0
42 it will be reset equal to x_inc; otherwise it will be converted
43 to degrees latitude. All coordinates: If = is appended then the
44 corresponding max x (east) or y (north) may be slightly adjusted
45 to fit exactly the given increment [by default the increment may
46 be adjusted slightly to fit the given domain]. Finally, instead
47 of giving an increment you may specify the number of nodes
48 desired by appending + to the supplied integer argument; the
49 increment is then recalculated from the number of nodes and the
50 domain. The resulting increment value depends on whether you
51 have selected a gridline-registered or pixel-registered grid;
52 see Appendix B for details. Note: if -Rgrdfile is used then
53 grid spacing has already been initialized; use -I to override
54 the values.
55 -R xmin, xmax, ymin, and ymax specify the Region of interest. For
57 geographic regions, these limits correspond to west, east,
58 south, and north and you may specify them in decimal degrees or
59 in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
60 and upper right map coordinates are given instead of w/e/s/n.
61 The two shorthands -Rg and -Rd stand for global domain (0/360
62 and -180/+180 in longitude respectively, with -90/+90 in lati‐
63 tude). Alternatively, specify the name of an existing grid file
64 and the -R settings (and grid spacing, if applicable) are copied
65 from the grid. For calendar time coordinates you may either
66 give (a) relative time (relative to the selected TIME_EPOCH and
67 in the selected TIME_UNIT; append t to -JX|x), or (b) absolute
68 time of the form [date]T[clock] (append T to -JX|x). At least
69 one of date and clock must be present; the T is always required.
70 The date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
71 calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
72 string must be of the form hh:mm:ss[.xxx]. The use of delim‐
73 iters and their type and positions must be exactly as indicated
74 (however, input, output and plot formats are customizable; see
75 gmtdefaults).
76
78 -A If the input data are geographic (as indicated by -fi) then the
79 sides in the polygons will be approximated by great circle arcs.
80 When using the -A sides will be regarded as straight lines.
81 Alternatively, append m to have sides first follow meridians,
82 then parallels. Or append p to first follow parallels, then
83 meridians.
84 -F Force pixel node registration [Default is gridline registra‐
86 tion]. (Node registrations are defined in GMT Cookbook Appendix
87 B on grid file formats.)
88 -H Input file(s) has header record(s). If used, the default number
90 of header records is N_HEADER_RECS. Use -Hi if only input data
91 should have header records [Default will write out header
92 records if the input data have them]. Blank lines and lines
93 starting with # are always skipped.
94 -N Sets the values that will be assigned to nodes that are outside
96 the polygons, on the edge, or inside. Values can be any number,
97 including the textstring NaN [Default is 0/0/1].
98 -S Set nodes depending on their distance from the nearest data
100 point. Nodes within radius [0] from a data point are considered
101 inside. Append m to indicate minutes or c to indicate seconds.
102 Append k to indicate km (implies -R and -I are in degrees, and
103 we will use a fast flat Earth approximation to calculate dis‐
104 tance). For more accuracy, use uppercase K if distances should
105 be calculated along geodesics. However, if the current ELLIP‐
106 SOID is spherical then great circle calculations are used.
107 -V Selects verbose mode, which will send progress reports to stderr
109 [Default runs "silently"].
110 -bi Selects binary input. Append s for single precision [Default is
112 d (double)]. Uppercase S or D will force byte-swapping.
113 Optionally, append ncol, the number of columns in your binary
114 input file if it exceeds the columns needed by the program. Or
115 append c if the input file is netCDF. Optionally, append
116 var1/var2/... to specify the variables to be read. [Default is
117 2 input columns].
118 -f Special formatting of input and/or output columns (time or geo‐
120 graphical data). Specify i or o to make this apply only to
121 input or output [Default applies to both]. Give one or more
122 columns (or column ranges) separated by commas. Append T (abso‐
123 lute calendar time), t (relative time in chosen TIME_UNIT since
124 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
125 to each column or column range item. Shorthand -f[i|o]g means
126 -f[i|o]0x,1y (geographic coordinates).
127 -m Multiple segment file. Segments are separated by a record whose
129 first character is flag. [Default is '>'].
130
132 By default GMT writes out grid as single precision floats in a COARDS-
133 complaint netCDF file format. However, GMT is able to produce grid
134 files in many other commonly used grid file formats and also facili‐
135 tates so called "packing" of grids, writing out floating point data as
136 2- or 4-byte integers. To specify the precision, scale and offset, the
137 user should add the suffix =id[/scale/offset[/nan]], where id is a two-
138 letter identifier of the grid type and precision, and scale and offset
139 are optional scale factor and offset to be applied to all grid values,
140 and nan is the value used to indicate missing data. See grdreformat(1)
141 and Section 4.17 of the GMT Technical Reference and Cookbook for more
142 information.
143 When writing a netCDF file, the grid is stored by default with the
145 variable name "z". To specify another variable name varname, append
146 ?varname to the file name. Note that you may need to escape the spe‐
147 cial meaning of ? in your shell program by putting a backslash in front
148 of it, or by placing the filename and suffix between quotes or double
149 quotes.
150
152 When the output grid type is netCDF, the coordinates will be labeled
153 "longitude", "latitude", or "time" based on the attributes of the input
154 data or grid (if any) or on the -f or -R options. For example, both
155 -f0x -f1t and -R90w/90e/0t/3t will result in a longitude/time grid.
156 When the x, y, or z coordinate is time, it will be stored in the grid
157 as relative time since epoch as specified by TIME_UNIT and TIME_EPOCH
158 in the .gmtdefaults file or on the command line. In addition, the unit
159 attribute of the time variable will indicate both this unit and epoch.
160
162 To set all nodes inside and on the polygons coastline_*.xy to 0, and
163 outside points to 1, do
164 grdmask coastline_*.xy -R-60/-40/-40/-30 -I5m -N1/0/0 -Gland_mask.grd
166 -V
167 To set nodes within 50 km of data points to 1 and other nodes to NaN,
169 do
170 grdmask data.xyz -R-60/-40/-40/-30 -I5m -NNaN/1/1 -S50k -Gdata_mask.grd
172 -V
173
175 GMT(1), grdlandmask(1), grdmath(1), grdclip(1), psmask(1), psclip(1)
176GMT 4.5.6 10 Mar 2011 GRDMASK(1)