.TH GRDMASK 1gmt "15 Jul 2011" "GMT 4.5.7" "Generic Mapping Tools"
.SH NAME
grdmask \- Create mask grid files from xy paths.
.SH SYNOPSIS
\fBgrdmask\fP \fIpathfiles\fP \fB\-G\fP\fImask_grd_file\fP] \fB\-I\fP\fIxinc\fP[\fIunit\fP][\fB=\fP|\fB+\fP][/\fIyinc\fP[\fIunit\fP][\fB=\fP|\fB+\fP]] 
\fB\-R\fP\fIwest\fP/\fIeast\fP/\fIsouth\fP/\fInorth\fP[\fBr\fP] [ \fB\-A\fP[\fBm\fP|\fBp\fP] ] [ \fB\-F\fP ] [ \fB\-H\fP[\fBi\fP][\fInrec\fP] ] 
[ \fB\-N\fP\fIout/edge/in\fP ] [ \fB\-S\fP\fIsearch_radius\fP[\fBm|c|k|K\fP] ] 
[ \fB\-V\fP ] [ \fB\-:\fP[\fBi\fP|\fBo\fP] ] [ \fB\-bi\fP[\fBs\fP|\fBS\fP|\fBd\fP|\fBD\fP[\fIncol\fP]|\fBc\fP[\fIvar1\fP\fB/\fP\fI...\fP]] ] [ \fB\-f\fP\fIcolinfo\fP ] [ \fB\-m\fP[\fIflag\fP] ]
.SH DESCRIPTION
\fBgrdmask\fP can operate in two different modes. 1. It reads one or more xy-files that each define a closed polygon.
The nodes defined by the specified region and lattice spacing will be set equal to one of three possible values
depending on whether the node is outside, on the polygon perimeter, or inside the polygon.  The
resulting mask may be used in subsequent operations involving \fBgrdmath\fP to mask out
data from polygonal areas. 2. The xy-files simply represent data point locations and the mask is set to the
inside or outside value depending on whether a node is within a maximum distance from the nearest data point.
If the distance specified is zero then only the nodes nearest each data point are considered "inside".
.TP
\fIpathfiles\fP
The name of 1 or more ASCII [or binary, see \fB\-b\fP] files holding the polygon(s) or data points.
.TP
\fB\-G\fP
Name of resulting output mask grid file.
(See GRID FILE FORMATS below).
.TP
\fB\-I\fP
\fIx_inc\fP [and optionally \fIy_inc\fP] is the grid spacing. Optionally, append a suffix
modifier.  \fBGeographical (degrees) coordinates\fP: Append \fBm\fP to
indicate arc minutes or \fBc\fP to indicate arc seconds.  If one of the units \fBe\fP, \fBk\fP, \fBi\fP,
or \fBn\fP 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 \fBELLIPSOID\fP).  If /\fIy_inc\fP is given but set to 0 it will be reset equal to
\fIx_inc\fP; otherwise it will be converted to degrees latitude.  
\fBAll coordinates\fP: If \fB=\fP is appended then
the corresponding max \fIx\fP (\fIeast\fP) or \fIy\fP (\fInorth\fP) 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 \fInumber of nodes\fP desired by appending \fB+\fP 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 \fB\-R\fP\fIgrdfile\fP is used then
grid spacing has already been initialized; use \fB\-I\fP to override the values.
.TP
\fB\-R\fP
\fIxmin\fP, \fIxmax\fP, \fIymin\fP, and \fIymax\fP specify the Region of interest.  For geographic
regions, these limits correspond to \fIwest, east, south,\fP and \fInorth\fP and you may specify them
in decimal degrees or in [+-]dd:mm[:ss.xxx][W|E|S|N] format.  Append \fBr\fP if lower left and upper right
map coordinates are given instead of w/e/s/n.  The two shorthands \fB\-Rg\fP and \fB\-Rd\fP 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 \fB\-R\fP settings (and grid spacing, if applicable) are copied from the grid.
For calendar time coordinates you may either give (a) relative
time (relative to the selected \fBTIME_EPOCH\fP and in the selected \fBTIME_UNIT\fP; append \fBt\fP to
\fB\-JX\fP|\fBx\fP), or (b) absolute time of the form [\fIdate\fP]\fBT\fP[\fIclock\fP]
(append \fBT\fP to \fB\-JX\fP|\fBx\fP).  At least one of \fIdate\fP and \fIclock\fP
must be present; the \fBT\fP is always required.  The \fIdate\fP string must be of the form [-]yyyy[-mm[-dd]]
(Gregorian calendar) or yyyy[-Www[-d]] (ISO week calendar), while the \fIclock\fP string must be of
the form hh:mm:ss[.xxx].  The use of delimiters and their type and positions must be exactly as indicated
(however, input, output and plot formats are customizable; see \fBgmtdefaults\fP). 
.SH OPTIONS
.TP
\fB\-A\fP
If the input data are geographic (as indicated by \fB\-fi\fP) then the sides in the polygons will be
approximated by great circle arcs.  When using the \fB\-A\fP sides will be regarded as straight lines.
Alternatively, append \fBm\fP to have sides first follow meridians, then parallels. Or append \fBp\FP
to first follow parallels, then meridians.
.TP
\fB\-F\fP
Force pixel node registration [Default is gridline registration].
(Node registrations are defined in \fBGMT\fP Cookbook Appendix B on grid file formats.)
.TP
\fB\-H\fP
Input file(s) has header record(s).  If used, the default number of header records is \fBN_HEADER_RECS\fP.
Use \fB\-Hi\fP if only input data should have header records [Default will write out header records if the
input data have them]. Blank lines and lines starting with # are always skipped.
.TP
\fB\-N\fP
Sets the values that will be assigned to nodes that are \fIout\fPside the polygons,
on the \fIedge\fP, or \fIin\fPside.  Values can be any number, including the
textstring NaN  [Default is 0/0/1].
.TP
\fB\-S\fP
Set nodes depending on their distance from the nearest data point.  Nodes within \fIradius\fP [0] from
a data point are considered inside.  Append \fBm\fP to indicate minutes
or \fBc\fP to indicate seconds.  Append \fBk\fP to indicate km (implies \fB\-R\fP and \fB\-I\fP are in degrees, and
we will use a fast flat Earth approximation to calculate distance).  For more accuracy, use uppercase \fBK\fP
if distances should be calculated along geodesics.
However, if the current \fBELLIPSOID\fP is spherical then great circle calculations are used.
If \fB\-S\fP is not set then we consider the input data to define closed polygon(s) instead.
.TP
\fB\-V\fP
Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].
.TP
\fB\-bi\fP
Selects binary input.
Append \fBs\fP for single precision [Default is \fBd\fP (double)].
Uppercase \fBS\fP or \fBD\fP will force byte-swapping.
Optionally, append \fIncol\fP, the number of columns in your binary input file
if it exceeds the columns needed by the program.
Or append \fBc\fP if the input file is netCDF. Optionally, append \fIvar1\fP\fB/\fP\fIvar2\fP\fB/\fP\fI...\fP to
specify the variables to be read.
[Default is 2 input columns].
.TP
\fB\-f\fP
Special formatting of input and/or output columns (time or geographical data).
Specify \fBi\fP or \fBo\fP to make this apply only to input or output [Default applies to both].
Give one or more columns (or column ranges) separated by commas.
Append \fBT\fP (absolute calendar time), \fBt\fP (relative time in chosen \fBTIME_UNIT\fP since \fBTIME_EPOCH\fP),
\fBx\fP (longitude), \fBy\fP (latitude), or \fBf\fP (floating point) to each column
or column range item.  Shorthand \fB\-f\fP[\fBi\fP|\fBo\fP]\fBg\fP means \fB\-f\fP[\fBi\fP|\fBo\fP]0\fBx\fP,1\fBy\fP
(geographic coordinates).
.TP
\fB\-m\fP
Multiple segment file.  Segments are separated by a record whose first character is \fIflag\fP.
[Default is '>'].
.SH GRID FILE FORMATS
By default \fBGMT\fP writes out grid as single precision floats in a COARDS-complaint netCDF file format.
However, \fBGMT\fP 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
\fB=\fP\fIid\fP[\fB/\fP\fIscale\fP\fB/\fP\fIoffset\fP[\fB/\fP\fInan\fP]], where \fIid\fP is a two-letter identifier of the grid type and precision, and \fIscale\fP and \fIoffset\fP are optional scale factor
and offset to be applied to all grid values, and \fInan\fP is the value used to indicate missing data.
See \fBgrdreformat\fP(1) and Section 4.17 of the GMT Technical Reference and Cookbook for more information.
.P
When writing a netCDF file, the grid is stored by default with the variable name "z". To specify another variable name \fIvarname\fP,
append \fB?\fP\fIvarname\fP to the file name.  Note that you may need to escape the special meaning of \fB?\fP in your shell program
by putting a backslash in front of it, or by placing the filename and suffix between quotes or double quotes.
.SH GEOGRAPHICAL AND TIME COORDINATES
When the output grid type is netCDF, the coordinates will be labeled "longitude", "latitude", or "time" based on the
attributes of the input data or grid (if any) or on the
\fB\-f\fP or \fB\-R\fP options. For example, both \fB\-f0x\fP \fB\-f1t\fP and \fB\-R\fP90w/90e/0t/3t will result in a longitude/time
grid. When the x, y, or z coordinate is time, it will be stored in the grid as relative time since epoch as 
specified by \fBTIME_UNIT\fP and \fBTIME_EPOCH\fP in the \.gmtdefaults file or on the command line.
In addition, the \fBunit\fP attribute of the time variable will indicate both this unit and epoch.
.SH EXAMPLES
To set all nodes inside and on the polygons coastline_*.xy to 0, and outside points
to 1, do
.sp
\fBgrdmask\fP coastline_*.xy \fB\-R\fP-60/-40/-40/-30 \fB\-I\fP5\fBm\fP \fB\-N\fP1/0/0 \fB\-G\fPland_mask.grd \fB\-V\fP
.br
.sp
To set nodes within 50 km of data points to 1 and other nodes to NaN, do
.sp
\fBgrdmask\fP data.xyz \fB\-R\fP-60/-40/-40/-30 \fB\-I\fP5\fBm\fP \fB\-N\fPNaN/1/1 \fB\-S\fP50\fBk\fP \fB\-G\fPdata_mask.grd \fB\-V\fP
.SH "SEE ALSO"
.IR GMT (1),
.IR grdlandmask (1),
.IR grdmath (1),
.IR grdclip (1),
.IR psmask (1),
.IR psclip (1)