grdmask(1) GMT grdmask(1)
NAME
grdmask - Create mask grid from polygons or point coverage
SYNOPSIS
grdmask pathfiles -Gmask_grd_file
-Iincrement
-Rregion [ -A[m|p|x|y] ] [ -N[z|Z|p|P]values ] [
-Ssearch_radius[unit] ] [ -V[level] ] [ -bibinary ] [ -dinodata ] [
-eregexp ] [ -fflags ] [ -ggaps ] [ -hheaders ] [ -iflags ] [ -nflags ]
[ -r ] [ -x[[-]n] ] [ -:[i|o] ]
Note: No space is allowed between the option flag and the associated
arguments.
DESCRIPTION
grdmask can operate in two different modes. 1. It reads one or more
pathfiles 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 polygonal
areas. 2. The pathfiles 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 dis-
tance specified is zero then only the nodes nearest each data point are
considered ainsidea.
REQUIRED ARGUMENTS
pathfiles
The name of 1 or more ASCII [or binary, see -bi] files holding
the polygon(s) or data points.
-Gmask_grd_file]
Name of resulting output mask grid file. (See GRID FILE FORMATS
below).
-Ixinc[unit][+e|n][/yinc[unit][+e|n]]
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 s to indicate arc seconds.
If one of the units e, f, k, M, n or u is appended instead, the
increment is assumed to be given in meter, foot, km, Mile, nau-
tical mile or US survey foot, respectively, and will be con-
verted to the equivalent degrees longitude at the middle lati-
tude of the region (the conversion depends on PROJ_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 coordi-
nates: If +e 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 +n to
the supplied integer argument; the increment is then recalcu-
lated from the number of nodes and the domain. The resulting
increment value depends on whether you have selected a grid-
line-registered or pixel-registered grid; see App-file-formats
for details. Note: if -Rgrdfile is used then the grid spacing
has already been initialized; use -I to override the values.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more a|)
Specify the region of interest.
OPTIONAL ARGUMENTS
-A[m|p|x|y]
If the input data are geographic (as indicated by -f) 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. For Cartesian data, points are simply connected,
unless you append x or y to construct stair-case paths whose
first move is along x or y, respectively.
-N[z|Z|p|P]values
Sets the out/edge/in 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].
Optionally, use Nz to set polygon insides to the z-value
obtained from the data (either segment header -Zzval, -Lheader
or via -aZ=name); use -NZ to consider the polygon boundary as
part of the inside. Alternatively, use -Np to use a running num-
ber as polygon ID; optionally append start of the sequence [0].
Here, -NP includes the polygon perimeter as inside. Note:
-Nz|Z|p|P cannot be used in conjunction with -S; they also all
optionally accept /out [0].
-Ssearch_radius[unit]
Set nodes to inside, on edge, or outside depending on their dis-
tance to the nearest data point. Nodes within radius [0] from
the nearest data point are considered inside; append a distance
unit (see UNITS). If radius is given as z then we instead read
individual radii from the 3rd input column. Unless Cartesian
data, specify the unit of these radii by appending it after -Sz.
If -S is not set then we consider the input data to define one
or more closed polygon(s) instead.
-V[level] (more a|)
Select verbosity level [c].
-bi[ncols][t] (more a|)
Select native binary input. [Default is 2 input columns].
-dinodata (more a|)
Replace input columns that equal nodata with NaN.
-e[~]^<i>apattern^<i>a | -e[~]/regexp/[i] (more a|)
Only accept data records that match the given pattern.
-f[i|o]colinfo (more a|)
Specify data types of input and/or output columns.
-g[a]x|y|d|X|Y|D|[col]z[+|-]gap[u] (more a|)
Determine data gaps and line breaks.
-h[i|o][n][+c][+d][+rremark][+rtitle] (more a|)
Skip or produce header record(s).
-icols[+l][+sscale][+ooffset][,^<i>a|] (more a|)
Select input columns and transformations (0 is first column).
-n[b|c|l|n][+a][+bBC][+tthreshold]
Append +bBC to set any boundary conditions to be used, adding g
for geographic, p for periodic, or n for natural boundary condi-
tions. For the latter two you may append x or y to specify just
one direction, otherwise both are assumed. [Default is geo-
graphic if grid is geographic].
-r (more a|)
Set pixel node registration [gridline].
-x[[-]n] (more a|)
Limit number of cores used in multi-threaded algorithms (OpenMP
required).
-^ or just -
Print a short message about the syntax of the command, then
exits (NOTE: on Windows just use -).
-+ or just +
Print an extensive usage (help) message, including the explana-
tion of any module-specific option (but not the GMT common
options), then exits.
-? or no arguments
Print a complete usage (help) message, including the explanation
of all options, then exits.
UNITS
For map distance unit, append unit d for arc degree, m for arc minute,
and s for arc second, or e for meter [Default], f for foot, k for km, M
for statute mile, n for nautical mile, and u for US survey foot. By
default we compute such distances using a spherical approximation with
great circles. Prepend - to a distance (or the unit is no distance is
given) to perform aFlat Eartha calculations (quicker but less accurate)
or prepend + to perform exact geodesic calculations (slower but more
accurate).
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 apackinga of grids, writing out floating point
data as 1- or 2-byte integers. To specify the precision, scale and off-
set, the user should add the suffix =ID[+sscale][+ooffset][+ninvalid],
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 invalid is the value used to indicate missing
data. See grdconvert and Section grid-file-format 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 aza. 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
alongitudea, alatitudea, or atimea 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 -R90w/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 gmt.conf file or on the command line. In addition, the unit
attribute of the time variable will indicate both this unit and epoch.
SAVE STORAGE SPACE
Since most uses of grdmask revolves around creating mask grids that
hold just a few integer values (and perhaps NaN), we choose to write
them to disk as byte grids by appending the suffix =nb to the desired
grid filename. Some situations may store integers that exceed the
range available in a byte and for those we specify a short integer grid
with =ns. For larger integers you may consider =ni, otherwise use the
default float grid format.
EXAMPLES
To set all nodes inside and on the polygons coastline_*.xy to 0, and
outside points to 1, do
gmt grdmask coastline_*.xy -R-60/-40/-40/-30 -I5m -N1/0/0 -Gland_mask.nc=nb -V
To set nodes within 50 km of data points to 1 and other nodes to NaN,
do
gmt grdmask data.xyz -R-60/-40/-40/-30 -I5m -NNaN/1/1 -S50k -Gdata_mask.nc=nb -V
To assign polygon IDs to the gridnodes using the insides of the poly-
gons in plates.gmt, based on the attribute POL_ID, do
gmt grdmask plates.gmt -R-40/40/-40/40 -I2m -Nz -Gplate_IDs.nc=ns -aZ=POL_ID -V
Same exercise, but instead compute running polygon IDs starting at 100,
do
gmt grdmask plates.gmt -R-40/40/-40/40 -I2m -Np100 -Gplate_IDs.nc=ns -V
SEE ALSO
gmt(1), grdlandmask(1), grdmath(1), grdclip(1), psmask(1), psclip(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 grdmask(1)
gmt5 5.4.2 - Generated Thu Jun 29 07:38:10 CDT 2017