grdlandmask
grdlandmask - Create a “wet-dry” mask grid from shoreline data base
Synopsis
grdlandmask -Gmask_grd_file
-Ixinc[unit][=|+][/yinc[unit][=|+]]
-Rwest/east/south/north[r]
[ -Amin_area[/min_level/max_level][+ag|i|s |S][+r|l][ppercent] ]
[ -Dresolution[+] ]
[ -Nmaskvalues[o] ]
[ -V[level] ] [ -r ]
Note: No space is allowed between the option flag and the associated arguments.
Description
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.
Required Arguments
- -Gmask_grd_file]
- Name of resulting output mask grid file. (See GRID FILE FORMATS below).
- -Ixinc[unit][=|+][/yinc[unit][=|+]]
- 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 GMT File Formats for
details. Note: if -Rgrdfile is used then the grid spacing has
already been initialized; use -I to override the values.
- -R[unit]west/east/south/north[/zmin/zmax][r]
- 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 dimension.
Optional Arguments
- -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.
- -Nmaskvalues[o]
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 formats:
-Nwet/dry.
-Nocean/land/lake/island/pond.
[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 exits.
- --version
- Print GMT version and exit.
- --show-datadir
- Print full path to GMT share directory and exit.
Examples
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.