DragonFly On-Line Manual Pages
GRDLANDMASK(1) Generic Mapping Tools GRDLANDMASK(1)
NAME
grdlandmask - Create "wet-dry" mask grid file from shoreline data base.
SYNOPSIS
grdlandmask -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 ]
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.
-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
nautical miles, respectively, and will be converted to the
equivalent degrees longitude at the middle latitude of the
region (the conversion 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
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 (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
automatically select a lower resolution should the one requested
not be available [abort if not found]. 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.
-F Force pixel node registration [Default is gridline
registration]. (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
facilitates 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
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
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,
representing 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
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(1), grdmath(1), grdclip(1), psmask(1), psclip(1), pscoast(1)
GMT 4.5.14 1 Nov 2015 GRDLANDMASK(1)