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GRDMASK(1) Generic Mapping Tools GRDMASK(1)
NAME
grdmask - Create mask grid files from xy paths.
SYNOPSIS
grdmask pathfiles -Gmask_grd_file] -Ixinc[unit][=|*][/yinc[unit][=|+]]
-Rwest/east/south/north[r] [ -A[m|p] ] [ -F ] [ -H[i][nrec] ] [
-Nout/edge/in ] [ -Ssearch_radius[m|c|k|K] ] [ -V ] [ -:[i|o] ] [
-bi[s|S|d|D[ncol]|c[var1/...]] ] [ -fcolinfo ] [ -m[flag] ]
DESCRIPTION
grdmask can operate in two different modes. 1. It reads one or more xy-
files that each define a closed polygon. The nodes defined by the
specified region and lattice spacing will be set equal to one of three
possible values depending on whether the node is outside, on the
polygon perimeter, or inside the polygon. The resulting mask may be
used in subsequent operations involving grdmath to mask out data from
polygonal areas. 2. The xy-files simply represent data point locations
and the mask is set to the inside or outside value depending on whether
a node is within a maximum distance from the nearest data point. If
the distance specified is zero then only the nodes nearest each data
point are considered "inside".
pathfiles
The name of 1 or more ASCII [or binary, see -b] files holding
the polygon(s) or data points.
-G Name of resulting output mask grid file. (See GRID FILE FORMATS
below).
-I x_inc [and optionally y_inc] is the grid spacing. Optionally,
append a suffix modifier. Geographical (degrees) coordinates:
Append m to indicate arc minutes or c to indicate arc seconds.
If one of the units e, k, i, or n is appended instead, the
increment is assumed to be given in meter, km, miles, or
nautical miles, respectively, and will be converted to the
equivalent degrees longitude at the middle latitude of the
region (the conversion depends on ELLIPSOID). If /y_inc is
given but set to 0 it will be reset equal to x_inc; otherwise it
will be converted to degrees latitude. All coordinates: If = is
appended then the corresponding max x (east) or y (north) may be
slightly adjusted to fit exactly the given increment [by default
the increment may be adjusted slightly to fit the given domain].
Finally, instead of giving an increment you may specify the
number of nodes desired by appending * to the supplied integer
argument; the increment is then recalculated from the number of
nodes and the domain. The resulting increment value depends on
whether you have selected a gridline-registered or pixel-
registered grid; see Appendix B for details. Note: if -Rgrdfile
is used then grid spacing has already been initialized; use -I
to override the values.
-R xmin, xmax, ymin, and ymax specify the Region of interest. For
geographic regions, these limits correspond to west, east,
south, and north and you may specify them in decimal degrees or
in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
and upper right map coordinates are given instead of w/e/s/n.
The two shorthands -Rg and -Rd stand for global domain (0/360
and -180/+180 in longitude respectively, with -90/+90 in
latitude). Alternatively, specify the name of an existing grid
file and the -R settings (and grid spacing, if applicable) are
copied from the grid. For calendar time coordinates you may
either give (a) relative time (relative to the selected
TIME_EPOCH and in the selected TIME_UNIT; append t to -JX|x), or
(b) absolute time of the form [date]T[clock] (append T to
-JX|x). At least one of date and clock must be present; the T
is always required. The date string must be of the form
[-]yyyy[-mm[-dd]] (Gregorian calendar) or yyyy[-Www[-d]] (ISO
week calendar), while the clock string must be of the form
hh:mm:ss[.xxx]. The use of delimiters and their type and
positions must be exactly as indicated (however, input, output
and plot formats are customizable; see gmtdefaults).
OPTIONS
-A If the input data are geographic (as indicated by -fi) then the
sides in the polygons will be approximated by great circle arcs.
When using the -A sides will be regarded as straight lines.
Alternatively, append m to have sides first follow meridians,
then parallels. Or append p to first follow parallels, then
meridians.
-F Force pixel node registration [Default is gridline
registration]. (Node registrations are defined in GMT Cookbook
Appendix B on grid file formats.)
-H Input file(s) has header record(s). If used, the default number
of header records is N_HEADER_RECS. Use -Hi if only input data
should have header records [Default will write out header
records if the input data have them]. Blank lines and lines
starting with # are always skipped.
-N Sets the values that will be assigned to nodes that are outside
the polygons, on the edge, or inside. Values can be any number,
including the textstring NaN [Default is 0/0/1].
-S Set nodes depending on their distance from the nearest data
point. Nodes within radius [0] from a data point are considered
inside. Append m to indicate minutes or c to indicate seconds.
Append k to indicate km (implies -R and -I are in degrees, and
we will use a fast flat Earth approximation to calculate
distance). For more accuracy, use uppercase K if distances
should be calculated along geodesics. However, if the current
ELLIPSOID is spherical then great circle calculations are used.
If -S is not set then we consider the input data to define
closed polygon(s) instead.
-V Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
-bi Selects binary input. Append s for single precision [Default is
d (double)]. Uppercase S or D will force byte-swapping.
Optionally, append ncol, the number of columns in your binary
input file if it exceeds the columns needed by the program. Or
append c if the input file is netCDF. Optionally, append
var1/var2/... to specify the variables to be read. [Default is
2 input columns].
-f Special formatting of input and/or output columns (time or
geographical data). Specify i or o to make this apply only to
input or output [Default applies to both]. Give one or more
columns (or column ranges) separated by commas. Append T
(absolute calendar time), t (relative time in chosen TIME_UNIT
since TIME_EPOCH), x (longitude), y (latitude), or f (floating
point) to each column or column range item. Shorthand -f[i|o]g
means -f[i|o]0x,1y (geographic coordinates).
-m Multiple segment file. Segments are separated by a record whose
first character is flag. [Default is '>'].
GRID FILE FORMATS
By default GMT writes out grid as single precision floats in a COARDS-
complaint netCDF file format. However, GMT is able to produce grid
files in many other commonly used grid file formats and also
facilitates so called "packing" of grids, writing out floating point
data as 2- or 4-byte integers. To specify the precision, scale and
offset, the user should add the suffix =id[/scale/offset[/nan]], where
id is a two-letter identifier of the grid type and precision, and scale
and offset are optional scale factor and offset to be applied to all
grid values, and nan is the value used to indicate missing data. See
grdreformat(1) and Section 4.17 of the GMT Technical Reference and
Cookbook for more information.
When writing a netCDF file, the grid is stored by default with the
variable name "z". To specify another variable name varname, append
?varname to the file name. Note that you may need to escape the
special meaning of ? in your shell program by putting a backslash in
front of it, or by placing the filename and suffix between quotes or
double quotes.
GEOGRAPHICAL AND TIME COORDINATES
When the output grid type is netCDF, the coordinates will be labeled
"longitude", "latitude", or "time" based on the attributes of the input
data or grid (if any) or on the -f or -R options. For example, both
-f0x -f1t and -R 90w/90e/0t/3t will result in a longitude/time grid.
When the x, y, or z coordinate is time, it will be stored in the grid
as relative time since epoch as specified by TIME_UNIT and TIME_EPOCH
in the .gmtdefaults file or on the command line. In addition, the unit
attribute of the time variable will indicate both this unit and epoch.
EXAMPLES
To set all nodes inside and on the polygons coastline_*.xy to 0, and
outside points to 1, do
grdmask coastline_*.xy -R-60/-40/-40/-30 -I 5m -N 1/0/0 -G
land_mask.grd -V
To set nodes within 50 km of data points to 1 and other nodes to NaN,
do
grdmask data.xyz -R-60/-40/-40/-30 -I 5m -N NaN/1/1 -S 50k -G
data_mask.grd -V
SEE ALSO
GMT(1), grdlandmask(1), grdmath(1), grdclip(1), psmask(1), psclip(1)
GMT 4.5.14 1 Nov 2015 GRDMASK(1)