grdmask (1gmt)

grdmask pathfiles -Gmask_grd_file] -Ixinc[m|c][/yinc[m|c]] -Rwest/east/south/north[r] [ -A ] [ -F ] [ -H[nrec] ] [ -L ] [ -M[flag] ] [ -Nout/edge/in ] [ -S[radius][k] ] [ -V ] [ -: ] [ -bi[s][n] ]

grdmask can operate in 2 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 grdfile may be used in subsequent operations involving grdmath 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".

pathfiles

The name of 1 or more ASCII [or binary, see -b] files holding the polygon(s) or data points.

-G

Name of resulting output mask grd file.

-I

x_inc [and optionally y_inc] is the grid spacing. Append m to indicate minutes or c to indicate seconds.

-R

west, east, south, and north specify the Region of interest. To specify boundaries in degrees and minutes [and seconds], use the dd:mm[:ss] format. Append r if lower left and upper right map coordinates are given instead of wesn.

-A

If the input data are geographic (as indicated by -L) then the sides in the polygons will be approximated by great circle arcs. This can be turned off using the -A switch.

-F

Force pixel registration. [Default is grid registration].

-H

Input file(s) has Header record(s). Number of header records can be changed by editing your .gmtdefaults file. If used, GMT default is 1 header record.

-L

Indicates that the x column contains longitudes, which may differ from the regions in -R by [multiples of] 360 degrees [Default assumes no periodicity].

-M

Multiple segment file. Segments are separated by a record whose first character is flag. [Default is '>'].

-N

Sets the values that will be assigned to nodes that are outside the polygons, on the edge, or inside. Values can be any number, including the textstring NaN [Default is 0/0/1].

-S

Set nodes depending on their distance from the nearest data point. Nodes within radius [0] from a data point are considered inside. Append k to indicate map units (e.g., -R -I in degrees and radius in km).

-V

Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].

-bi

Selects binary input. Append s for single precision [Default is double]. Append n for the number of columns in the binary file(s). [Default is 2 input columns].

To set all nodes inside and on the polygons coastline_*.xy to 0, and outside points to 1, do

grdmask coastline_*.xy -R-60/-40/-40/-30 -I5m -N1/0/0 -Gland_mask.grd -V

To set nodes within 50 km of data points to 1 and other nodes to NaN, do

grdmask data.xyz -R-60/-40/-40/-30 -I5m -NNaN/1/1 -S50k -Gdata_mask.grd -V

gmt(1gmt), grdlandmask(1gmt), grdmath(1gmt), grdclip(1gmt), psmask(1gmt), psclip(1gmt)