grdlandmask(1) GMT grdlandmask(1)
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][pper-
cent] ] [ -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 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 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, nau-
tical mile or US survey foot, respectively, and will be con-
verted to the equivalent degrees longitude at the middle lati-
tude 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 coordi-
nates: 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 recalcu-
lated from the number of nodes and the domain. The resulting
increment value depends on whether you have selected a grid-
line-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
[A+-]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 (Carte-
sian) 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 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. By default (+ai)
we select the ice shelf boundary as the coastline for Antarc-
tica; append +ag to instead select the ice grounding line as
coastline. For expert users who wish to print their own Antarc-
tica 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 fea-
ture 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 automati-
cally 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 consid-
ers 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 for-
mats:
-Nwet/dry.
-Nocean/land/lake/island/pond.
[Default is 0/1/0/1/0 (i.e., 0/1)].
-V[level] (more a|)
Select verbosity level [c].
-r (more a|)
Set pixel node registration [gridline].
-x[[-]n] (more a|)
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 explana-
tion 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 apackinga of grids, writing out floating point
data as 1- or 2-byte integers. To specify the precision, scale and off-
set, 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 aza. 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.
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 cor-
recting 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 data-
base using the Douglas-Peucker line-simplification algorithm. The clas-
sification of rivers and borders follow that of the WDBII. See the GMT
Cookbook and Technical Reference Appendix K for further details.
SEE ALSO
gmt(1), grdmath(1), grdclip(1), psmask(1), psclip(1), pscoast(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 grdlandmask(1)
gmt5 5.4.2 - Generated Wed Jun 28 20:21:03 CDT 2017