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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)