GRDLANDMASK(1) Generic Mapping Tools GRDLANDMASK(1)NAMEgrdlandmask - Create "wet-dry" mask grid file from shoreline data base.
SYNOPSISgrdlandmask-Gmask_grd_file] -Ixinc[unit][=|+][/yinc[unit][=|+]]
-Rwest/east/south/north[r] [
-Amin_area[/min_level/max_level][+r|l][ppercent] ] [ -Dresolution[+] ]
[ -F ] [ -Nmaskvalues[o] ] [ -V ]
DESCRIPTIONgrdlandmask reads the selected shoreline database and uses that infor‐
mation to decide which nodes in the specified grid are over land or
over water. The nodes defined by the selected region and lattice spac‐
ing 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 involv‐
ing grdmath to mask out data from land [or water] areas.
-G Name of resulting output mask grid file. (See GRID FILE FORMATS
below).
-I 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 c to indicate arc seconds.
If one of the units e, k, i, or n is appended instead, the
increment is assumed to be given in meter, km, miles, or nauti‐
cal miles, respectively, and will be converted to the equivalent
degrees longitude at the middle latitude of the region (the con‐
version depends on 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 Appendix B for details. Note: if -Rgrdfile is used then
grid spacing has already been initialized; use -I to override
the values.
-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, specify the name of an existing grid file and the
-R settings (and grid spacing, if applicable) are copied from
the grid.
OPTIONS-A Features with an area smaller than min_area in km^2 or of hier‐
archical 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 (requires GSHHS 2.0.1 or
higher). Finally, append +ppercent to exclude polygons whose
percentage area of the corresponding full-resolution feature is
less than percent (requires GSHHS 2.0 or higher). See GSHHS
INFORMATION below for more details.
-D 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 automati‐
cally select a lower resolution should the one requested not be
available [abort if not found]. Note that because the coast‐
lines differ in details a node in a mask file using one resolu‐
tion is not guaranteed to remain inside [or outside] when a dif‐
ferent resolution is selected.
-F Force pixel node registration [Default is gridline registra‐
tion]. (Node registrations are defined in GMT Cookbook Appendix
B on grid file formats.)
-N 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 Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
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 facili‐
tates so called "packing" of grids, writing out floating point data as
2- or 4-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 grdreformat(1)
and Section 4.17 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 spe‐
cial 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.
EXAMPLES
To set all nodes on land to NaN, and nodes over water to 1, using the
high resolution data set, do
grdlandmask -R-60/-40/-40/-30 -Dh -i 5m -N 1/NaN -G land_mask.grd -V
To make a 1x1 degree global grid with the hierarchical levels of the
nodes based on the low resolution data:
grdlandmask-R 0/360/-90/90 -Dl -I 1 -N 0/1/2/3/4 -G levels.grd -V
GSHHS INFORMATION
The coastline database is GSHHS which is compiled from two sources:
World Vector Shorelines (WVS) and CIA World Data Bank II (WDBII). In
particular, all level-1 polygons (ocean-land boundary) are derived from
the more accurate WVS while all higher level polygons (level 2-4, rep‐
resenting land/lake, lake/island-in-lake, and island-in-lake/lake-in-
island-in-lake boundaries) are taken from WDBII. Much processing has
taken place to convert WVS and WDBII 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 poly‐
gon has been determined so that the user may choose not to draw fea‐
tures smaller than a minimum area (see -A); one may also limit the
highest hierarchical level of polygons to be included (4 is the maxi‐
mum). The 4 lower-resolution databases were derived from the full res‐
olution database using the Douglas-Peucker line-simplification algo‐
rithm. The classification of rivers and borders follow that of the
WDBII. See the GMT Cookbook and Technical Reference Appendix K for
further details.
SEE ALSOGMT(1), grdmath(1), grdclip(1), psmask(1), psclip(1), pscoast(1)GMT 4.5.14 1 Nov 2015 GRDLANDMASK(1)
