grdlandmask - "Create a ""wet-dry"" mask
grid from shoreline data base"
grdlandmask -Gmask_grd_file
-Iincrement
-Rregion [
-Amin_area[/min_level/max_level][+ag|i|s
|S][+r|l][ppercent] ] [
-Dresolution[+] ] [ -N ] [
-Nmaskvalues ] [ -V[level] ] [ -r ] [
-x[[-]n] ]
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.
- -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, 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 +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 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.
- -Rwest/east/south/north[/zmin/zmax][+r][+uunit]
- 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. Appending +uunit 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
dimension.
- -Amin_area[/min_level/max_level][+ag|i|s|S][+r|l][+ppercent]
- 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.
- -Dresolution[+]
- 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 selected.
- -E
- Indicate that nodes that fall exactly on a polygon boundary should be
considered to be outside the polygon [Default considers them to be
inside].
- -Nmaskvalues
- Sets the values that will be assigned to nodes. Values can be any number,
including the textstring NaN. Also select -E to let nodes exactly
on feature boundaries be considered outside [Default is inside]. Specify
this information using 1 of 2 formats:
-Nwet/dry.
-Nocean/land/lake/island/pond.
[Default is 0/1/0/1/0 (i.e., 0/1)].
- -^ 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 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 all
options, then exits.
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[+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 "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.
A grid produced by grdlandmask is a categorical dataset. As
such, one has to be careful not to interpolate it with standard methods,
such as splines. However, if you make a map of this grid using a map
projection the grid will be reprojected to yield a rectangular matrix in the
projected coordinates. This interpolation is done using splines by default
and thus may yield artifacts in your map. We recommend you use grdimage
-nn to instead use a nearest neighbor interpolation for such
cases.
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.
gmt, grdmath, grdclip, psmask, psclip, pscoast
2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F.
Wobbe