bionic (1) grdlandmask.1gmt.gz
NAME
grdlandmask - "Create a ""wet-dry"" mask grid from shoreline data base"
SYNOPSIS
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.
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][+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.
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.
-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)].
-V[level] (more …)
Select verbosity level [c].
-r (more …)
Set pixel node registration [gridline].
-x[[-]n] (more …)
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 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.
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 “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.
NOTES
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.
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
GSHHS INFORMATION
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.
SEE ALSO
gmt, grdmath, grdclip, psmask, psclip, pscoast
COPYRIGHT
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe