1GRDMASK(1) GMT GRDMASK(1)
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6 grdmask - Create mask grid from polygons or point coverage
7
9 grdmask pathfiles -Gmask_grd_file
10 -Iincrement
11 -Rregion [ -A[m|p|x|y] ] [ -N[z|Z|p|P]values ] [
12 -Ssearch_radius[unit] ] [ -V[level] ] [ -bibinary ] [ -dinodata ] [
13 -eregexp ] [ -fflags ] [ -ggaps ] [ -hheaders ] [ -iflags ] [ -nflags ]
14 [ -r ] [ -x[[-]n] ] [ -:[i|o] ]
15 Note: No space is allowed between the option flag and the associated
17 arguments.
18
20 grdmask can operate in two different modes. 1. It reads one or more
21 pathfiles that each define a closed polygon. The nodes defined by the
22 specified region and lattice spacing will be set equal to one of three
23 possible values depending on whether the node is outside, on the poly‐
24 gon perimeter, or inside the polygon. The resulting mask may be used in
25 subsequent operations involving grdmath to mask out data from polygonal
26 areas. 2. The pathfiles simply represent data point locations and the
27 mask is set to the inside or outside value depending on whether a node
28 is within a maximum distance from the nearest data point. If the dis‐
29 tance specified is zero then only the nodes nearest each data point are
30 considered "inside".
31
33 pathfiles
34 The name of 1 or more ASCII [or binary, see -bi] files holding
35 the polygon(s) or data points.
36 -Gmask_grd_file]
38 Name of resulting output mask grid file. (See GRID FILE FORMATS
39 below).
40 -Ixinc[unit][+e|n][/yinc[unit][+e|n]]
42 x_inc [and optionally y_inc] is the grid spacing. Optionally,
43 append a suffix modifier. Geographical (degrees) coordinates:
44 Append m to indicate arc minutes or s to indicate arc seconds.
45 If one of the units e, f, k, M, n or u is appended instead, the
46 increment is assumed to be given in meter, foot, km, Mile, nau‐
47 tical mile or US survey foot, respectively, and will be con‐
48 verted to the equivalent degrees longitude at the middle lati‐
49 tude of the region (the conversion depends on PROJ_ELLIPSOID).
50 If y_inc is given but set to 0 it will be reset equal to x_inc;
51 otherwise it will be converted to degrees latitude. All coordi‐
52 nates: If +e is appended then the corresponding max x (east) or
53 y (north) may be slightly adjusted to fit exactly the given
54 increment [by default the increment may be adjusted slightly to
55 fit the given domain]. Finally, instead of giving an increment
56 you may specify the number of nodes desired by appending +n to
57 the supplied integer argument; the increment is then recalcu‐
58 lated from the number of nodes and the domain. The resulting
59 increment value depends on whether you have selected a grid‐
60 line-registered or pixel-registered grid; see App-file-formats
61 for details. Note: if -Rgrdfile is used then the grid spacing
62 has already been initialized; use -I to override the values.
63 -Rxmin/xmax/ymin/ymax[+r][+uunit] (more ...)
65 Specify the region of interest.
66
68 -A[m|p|x|y]
69 If the input data are geographic (as indicated by -f) then the
70 sides in the polygons will be approximated by great circle arcs.
71 When using the -A sides will be regarded as straight lines.
72 Alternatively, append m to have sides first follow meridians,
73 then parallels. Or append p to first follow parallels, then
74 meridians. For Cartesian data, points are simply connected,
75 unless you append x or y to construct stair-case paths whose
76 first move is along x or y, respectively.
77 -N[z|Z|p|P]values
79 Sets the out/edge/in that will be assigned to nodes that are
80 outside the polygons, on the edge, or inside. Values can be any
81 number, including the textstring NaN [Default is 0/0/1].
82 Optionally, use Nz to set polygon insides to the z-value
83 obtained from the data (either segment header -Zzval, -Lheader
84 or via -aZ=name); use -NZ to consider the polygon boundary as
85 part of the inside. Alternatively, use -Np to use a running num‐
86 ber as polygon ID; optionally append start of the sequence [0].
87 Here, -NP includes the polygon perimeter as inside. Note:
88 -Nz|Z|p|P cannot be used in conjunction with -S; they also all
89 optionally accept /out [0].
90 -Ssearch_radius[unit]
92 Set nodes to inside, on edge, or outside depending on their dis‐
93 tance to the nearest data point. Nodes within radius [0] from
94 the nearest data point are considered inside; append a distance
95 unit (see UNITS). If radius is given as z then we instead read
96 individual radii from the 3rd input column. Unless Cartesian
97 data, specify the unit of these radii by appending it after -Sz.
98 If -S is not set then we consider the input data to define one
99 or more closed polygon(s) instead.
100 -V[level] (more ...)
102 Select verbosity level [c].
103 -bi[ncols][t] (more ...)
105 Select native binary input. [Default is 2 input columns].
106 -dinodata (more ...)
108 Replace input columns that equal nodata with NaN.
109 -e[~]"pattern" | -e[~]/regexp/[i] (more ...)
111 Only accept data records that match the given pattern.
112 -f[i|o]colinfo (more ...)
114 Specify data types of input and/or output columns.
115 -g[a]x|y|d|X|Y|D|[col]z[+|-]gap[u] (more ...)
117 Determine data gaps and line breaks.
118 -h[i|o][n][+c][+d][+rremark][+rtitle] (more ...)
120 Skip or produce header record(s).
121 -icols[+l][+sscale][+ooffset][,...] (more ...)
123 Select input columns and transformations (0 is first column).
124 -n[b|c|l|n][+a][+bBC][+tthreshold]
126 Append +bBC to set any boundary conditions to be used, adding g
127 for geographic, p for periodic, or n for natural boundary condi‐
128 tions. For the latter two you may append x or y to specify just
129 one direction, otherwise both are assumed. [Default is geo‐
130 graphic if grid is geographic].
131 -r (more ...)
133 Set pixel node registration [gridline].
134 -x[[-]n] (more ...)
136 Limit number of cores used in multi-threaded algorithms (OpenMP
137 required).
138 -^ or just -
140 Print a short message about the syntax of the command, then
141 exits (NOTE: on Windows just use -).
142 -+ or just +
144 Print an extensive usage (help) message, including the explana‐
145 tion of any module-specific option (but not the GMT common
146 options), then exits.
147 -? or no arguments
149 Print a complete usage (help) message, including the explanation
150 of all options, then exits.
151
153 For map distance unit, append unit d for arc degree, m for arc minute,
154 and s for arc second, or e for meter [Default], f for foot, k for km, M
155 for statute mile, n for nautical mile, and u for US survey foot. By
156 default we compute such distances using a spherical approximation with
157 great circles. Prepend - to a distance (or the unit is no distance is
158 given) to perform "Flat Earth" calculations (quicker but less accurate)
159 or prepend + to perform exact geodesic calculations (slower but more
160 accurate).
161
163 By default GMT writes out grid as single precision floats in a
164 COARDS-complaint netCDF file format. However, GMT is able to produce
165 grid files in many other commonly used grid file formats and also
166 facilitates so called "packing" of grids, writing out floating point
167 data as 1- or 2-byte integers. To specify the precision, scale and off‐
168 set, the user should add the suffix =ID[+sscale][+ooffset][+ninvalid],
169 where ID is a two-letter identifier of the grid type and precision, and
170 scale and offset are optional scale factor and offset to be applied to
171 all grid values, and invalid is the value used to indicate missing
172 data. See grdconvert and Section grid-file-format of the GMT Technical
173 Reference and Cookbook for more information.
174 When writing a netCDF file, the grid is stored by default with the
176 variable name "z". To specify another variable name varname, append
177 ?varname to the file name. Note that you may need to escape the special
178 meaning of ? in your shell program by putting a backslash in front of
179 it, or by placing the filename and suffix between quotes or double
180 quotes.
181
183 When the output grid type is netCDF, the coordinates will be labeled
184 "longitude", "latitude", or "time" based on the attributes of the input
185 data or grid (if any) or on the -f or -R options. For example, both
186 -f0x -f1t and -R90w/90e/0t/3t will result in a longitude/time grid.
187 When the x, y, or z coordinate is time, it will be stored in the grid
188 as relative time since epoch as specified by TIME_UNIT and TIME_EPOCH
189 in the gmt.conf file or on the command line. In addition, the unit
190 attribute of the time variable will indicate both this unit and epoch.
191
193 A grid produced by grdmask is a categorical dataset. As such, one has
194 to be careful not to interpolate it with standard methods, such as
195 splines. However, if you make a map of this grid using a map projec‐
196 tion the grid will be reprojected to yield a rectangular matrix in the
197 projected coordinates. This interpolation is done using splines by
198 default and thus may yield artifacts in your map. We recommend you use
199 grdimage -nn to instead use a nearest neighbor interpolation for such
200 cases.
201
203 Since most uses of grdmask revolves around creating mask grids that
204 hold just a few integer values (and perhaps NaN), we choose to write
205 them to disk as byte grids by appending the suffix =nb to the desired
206 grid filename. Some situations may store integers that exceed the
207 range available in a byte and for those we specify a short integer grid
208 with =ns. For larger integers you may consider =ni, otherwise use the
209 default float grid format.
210
212 To set all nodes inside and on the polygons coastline_*.xy to 0, and
213 outside points to 1, do
214 gmt grdmask coastline_*.xy -R-60/-40/-40/-30 -I5m -N1/0/0 -Gland_mask.nc=nb -V
216 To set nodes within 50 km of data points to 1 and other nodes to NaN,
218 do
219 gmt grdmask data.xyz -R-60/-40/-40/-30 -I5m -NNaN/1/1 -S50k -Gdata_mask.nc=nb -V
221 To assign polygon IDs to the gridnodes using the insides of the poly‐
223 gons in plates.gmt, based on the attribute POL_ID, do
224 gmt grdmask plates.gmt -R-40/40/-40/40 -I2m -Nz -Gplate_IDs.nc=ns -aZ=POL_ID -V
226 Same exercise, but instead compute running polygon IDs starting at 100,
228 do
229 gmt grdmask plates.gmt -R-40/40/-40/40 -I2m -Np100 -Gplate_IDs.nc=ns -V
231
233 gmt, grdlandmask, grdmath, grdclip, psmask, psclip
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236 2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
2375.4.5 Feb 24, 2019 GRDMASK(1)