GRDMASK(1) Generic Mapping Tools GRDMASK(1)NAMEgrdmask - Create mask grid files from xy paths.
SYNOPSISgrdmask 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] ]
DESCRIPTIONgrdmask 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 poly‐
gon perimeter, or inside the polygon. The resulting mask may be used
in subsequent operations involving grdmath to mask out data from polyg‐
onal 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 nauti‐
cal miles, respectively, and will be converted to the equivalent
degrees longitude at the middle latitude of the region (the con‐
version 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 lati‐
tude). 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 delim‐
iters 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 registra‐
tion]. (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 dis‐
tance). For more accuracy, use uppercase K if distances should
be calculated along geodesics. However, if the current ELLIP‐
SOID 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 geo‐
graphical 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 (abso‐
lute 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 facili‐
tates 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 spe‐
cial 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 ALSOGMT(1), grdlandmask(1), grdmath(1), grdclip(1), psmask(1), psclip(1)GMT 4.5.14 1 Nov 2015 GRDMASK(1)
