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grdlandmask - "Create a ""wet-dry"" mask grid from shoreline data base"


grdlandmask mask_grd_file xinc[unit][=|+][/yinc[unit][=|+]] west/east/south/north[r] [
min_area[/min_level/max_level][+ag|i|s |S][+r|l][ppercent] ] [ resolution[+] ] [
maskvalues[o] ] [ [level] ] [ -r ]

Note: No space is allowed between the option flag and the associated arguments.


grdlandmask reads the selected shoreline database and uses that information to decide
which nodes in the specified grid are over land or over water. The nodes defined by the
selected region and lattice spacing will be set according to one of two criteria: (1) land
vs water, or (2) the more detailed (hierarchical) ocean vs land vs lake vs island vs pond.
The resulting mask may be used in subsequent operations involving grdmath to mask out data
from land [or water] areas.


Name of resulting output mask grid file. (See GRID FILE FORMATS below).

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, nautical
mile or US survey foot, respectively, and will be converted to the equivalent
degrees longitude at the middle latitude 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 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 App-file-formats for details.
Note: if -Rgrdfile is used then the grid spacing has already been initialized; use
-I to override the values.

west, east, south, and north specify the region of interest, 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 for grid creation, give
Rcodelon/lat/nx/ny, where code is a 2-character combination of L, C, R (for left,
center, or right) and T, M, B for top, middle, or bottom. e.g., BL for lower left.
This indicates which point on a rectangular region the lon/lat coordinate refers
to, and the grid dimensions nx and ny with grid spacings via -I is used to create
the corresponding region. Alternatively, specify the name of an existing grid file
and the -R settings (and grid spacing, if applicable) are copied from the grid.
Using -Runit expects projected (Cartesian) coordinates compatible with chosen -J
and we inversely project to determine actual rectangular geographic region. For
perspective view (-p), optionally append /zmin/zmax. In case of perspective view
(-p), a z-range (zmin, zmax) can be appended to indicate the third dimension. This
needs to be done only when using the -Jz option, not when using only the -p option.
In the latter case a perspective view of the plane is plotted, with no third


Features with an area smaller than min_area in km^2 or of hierarchical level that
is lower than min_level or higher than max_level will not be plotted [Default is
0/0/4 (all features)]. Level 2 (lakes) contains regular lakes and wide river
bodies which we normally include as lakes; append +r to just get river-lakes or +l
to just get regular lakes. By default (+ai) we select the ice shelf boundary as
the coastline for Antarctica; append +ag to instead select the ice grounding line
as coastline. For expert users who wish to print their own Antarctica coastline
and islands via psxy you can use +as to skip all GSHHG features below 60S or +aS to
instead skip all features north of 60S. Finally, append +ppercent to exclude
polygons whose percentage area of the corresponding full-resolution feature is less
than percent. See GSHHG INFORMATION below for more details.

Selects the resolution of the data set to use ((f)ull, (h)igh, (i)ntermediate,
(l)ow, or (c)rude). The resolution drops off by ~80% between data sets. [Default is
l]. Append + to automatically select a lower resolution should the one requested
not be available [abort if not found]. Alternatively, choose (a)uto to
automatically select the best resolution given the chosen region. Note that
because the coastlines differ in details a node in a mask file using one resolution
is not guaranteed to remain inside [or outside] when a different resolution is

Sets the values that will be assigned to nodes. Values can be any number, including
the textstring NaN. Append o to let nodes exactly on feature boundaries be
considered outside [Default is inside]. Specify this information using 1 of 2



[Default is 0/1/0/1/0 (i.e., 0/1)].

-V[level] (more ...)
Select verbosity level [c].

-r (more ...)
Set pixel node registration [gridline].

-^ or just -
Print a short message about the syntax of the command, then exits (NOTE: on Windows
use just -).

-+ or just +
Print an extensive usage (help) message, including the explanation 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 options, then

Print GMT version and exit.

Print full path to GMT share directory and exit.


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 1- or 2-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
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 "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.


To set all nodes on land to NaN, and nodes over water to 1, using the high resolution data
set, do

gmt grdlandmask -R-60/-40/-40/-30 -Dh -I5m -N1/NaN -Gland_mask.nc -V

To make a 1x1 degree global grid with the hierarchical levels of the nodes based on the
low resolution data:

gmt grdlandmask -R0/360/-90/90 -Dl -I1 -N0/1/2/3/4 -Glevels.nc -V


The coastline database is GSHHG (formerly GSHHS) which is compiled from three sources:
World Vector Shorelines (WVS), CIA World Data Bank II (WDBII), and Atlas of the Cryosphere
(AC, for Antarctica only). Apart from Antarctica, all level-1 polygons (ocean-land
boundary) are derived from the more accurate WVS while all higher level polygons (level
2-4, representing land/lake, lake/island-in-lake, and
island-in-lake/lake-in-island-in-lake boundaries) are taken from WDBII. The Antarctica
coastlines come in two flavors: ice-front or grounding line, selectable via the -A option.
Much processing has taken place to convert WVS, WDBII, and AC data into usable form for
GMT: assembling closed polygons from line segments, checking for duplicates, and
correcting for crossings between polygons. The area of each polygon has been determined
so that the user may choose not to draw features smaller than a minimum area (see -A); one
may also limit the highest hierarchical level of polygons to be included (4 is the
maximum). The 4 lower-resolution databases were derived from the full resolution database
using the Douglas-Peucker line-simplification algorithm. The classification of rivers and
borders follow that of the WDBII. See the GMT Cookbook and Technical Reference Appendix K
for further details.

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