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grdvector - Plot vector field from two component grids


grdvector compx.nc compy.nc -Jparameters [ ] [ [p|s]parameters ] [ cpt ] [ fill ] [
[x]dx[/dy] ] [ ] [ ] [ ] [ ] [ parameters ] [ region ] [ [i|l]scale ] [ ] [
[just/dx/dy/][c|label] ] [ pen ] [ x_offset ] [ y_offset ] [ ] [ -ccopies ] [ -f<flags> ]
[ -p<flags> ] [ -t<transp> ]

Note: No space is allowed between the option flag and the associated arguments.


grdvector reads two 2-D grid files which represents the x- and y-components of a vector
field and produces a vector field plot by drawing vectors with orientation and length
according to the information in the files. Alternatively, polar coordinate r, theta grids
may be given instead.


Contains the x-components of the vector field.

Contains the y-components of the vector field. (See GRID FILE FORMATS below.)

-Jparameters (more ...)
Select map projection.


-A The grid files contain polar (r, theta) components instead of Cartesian (x, y)
[Default is Cartesian components].

-B[p|s]parameters (more ...)
Set map boundary intervals.

Use cpt to assign colors based on vector length. Alternatively, supply the name of
a GMT color master CPT [rainbow] and let grdvector automatically determine a
16-level continuous CPT from the grid's z-range. Yet another option is to specify
-Ccolor1,color2[,color3,...] to build a linear continuous cpt from those colors
automatically. In this case colorn can be a r/g/b triplet, a color name, or an
HTML hexadecimal color (e.g. #aabbcc ).

-Gfill Sets color or shade for vector interiors [Default is no fill].

Only plot vectors at nodes every x_inc, y_inc apart (must be multiples of original
grid spacing). Append m for arc minutes or s for arc seconds. Alternatively, use
-Ix to specify the multiples multx[/multy] directly [Default plots every node].

-K (more ...)
Do not finalize the PostScript plot.

-N Do NOT clip vectors at map boundaries [Default will clip].

-O (more ...)
Append to existing PostScript plot.

-P (more ...)
Select "Portrait" plot orientation.

Modify vector parameters. For vector heads, append vector head size [Default is 0,
i.e., stick-plot]. See VECTOR ATTRIBUTES for specifying additional attributes.

-R[unit]xmin/xmax/ymin/ymax[r] (more ...)
Specify the region of interest. Specify a subset of the grid.

Sets scale for Cartesian vector length in data units per distance measurement unit
[1]. Append c, i, or p to indicate the measurement unit (cm, inch,or point).
Prepend l to indicate a fixed length for all vectors. For Geographic data, give
scale in data units per km. Use -Si if it is simpler to give the reciprocal scale
in measurement unit per data unit or km per data unit.

-T Means the azimuths of Cartesian data sets should be adjusted according to the signs
of the scales in the x- and y-directions [Leave alone]. This option can be used to
convert vector azimuths in cases when a negative scale is used in one of both
directions (e.g., positive down).

-U[just/dx/dy/][c|label] (more ...)
Draw GMT time stamp logo on plot.

-V[level] (more ...)
Select verbosity level [c].

-Wpen Set pen attributes used for vector outlines [Default: width = default, color =
black, style = solid].


-Y[a|c|f|r][y-shift[u]] (more ...)
Shift plot origin.

-Z The theta grid provided contains azimuths rather than directions (requires -A).

-ccopies (more ...)
Specify number of plot copies [Default is 1].

-f[i|o]colinfo (more ...)
Specify data types of input and/or output columns.

-p[x|y|z]azim/elev[/zlevel][+wlon0/lat0[/z0]][+vx0/y0] (more ...)
Select perspective view.

-t[transp] (more ...)
Set PDF transparency level in percent.

-^ or just -
Print a short message about the syntax of the command, then exits (NOTE: on Windows
use just -).

-+ or just +
Print an extensive usage (help) message, including the explanation 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 options, then

Print GMT version and exit.

Print full path to GMT share directory and exit.


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 1- or 2-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. In case
the two characters id is not provided, as in =/scale than a id=nf is assumed. When
reading grids, the format is generally automatically recognized. If not, the same suffix
can be added to input grid file names. See grdconvert and Section grid-file-format of the
GMT Technical Reference and Cookbook for more information.

When reading a netCDF file that contains multiple grids, GMT will read, by default, the
first 2-dimensional grid that can find in that file. To coax GMT into reading another
multi-dimensional variable in the grid file, append ?varname to the file name, where
varname is the name of the variable. 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. The ?varname suffix can also be used
for output grids to specify a variable name different from the default: "z". See
grdconvert and Sections modifiers-for-CF and grid-file-format of the GMT Technical
Reference and Cookbook for more information, particularly on how to read splices of 3-,
4-, or 5-dimensional grids.


Several modifiers may be appended to the vector-producing options to specify the placement
of vector heads, their shapes, and the justification of the vector. Below, left and right
refers to the side of the vector line when viewed from the start point to the end point of
the segment:
+aangle sets the angle of the vector head apex [30].

+b places a vector head at the beginning of the vector path [none]. Optionally, append
t for a terminal line, c for a circle, or a for arrow [Default]. Further append l|r to
only draw the left or right side of this head [both sides].

+e places a vector head at the end of the vector path [none]. Optionally, append t for
a terminal line, c for a circle, or a for arrow [Default]. Further append l|r to only
draw the left or right side of this head [both sides].

+g-|fill turns off vector head fill (if -) or sets the vector head fill [Default fill
is used, which may be no fill].

+l draws half-arrows, using only the left side of specified heads [both sides].

+m places a vector head at the mid-point the vector path [none]. Append f or r for
forward or reverse direction of the vector [forward]. Optionally, append t for a
terminal line, c for a circle, or a for arrow head [Default]. Further append l|r to
only draw the left or right side of this head [both sides]. Cannot be combined with +b
or +e.

+nnorm scales down vector attributes (pen thickness, head size) with decreasing length,
where vectors shorter than norm will have their attributes scaled by length/norm [arrow
attributes remains invariant to length].

+oplon/plat specifies the oblique pole for the great or small circles. Only needed for
great circles if +q is given.

+p[-][pen] sets the vector pen attributes. If pen has a leading - then the head outline
is not drawn. [Default pen is used, and head outline is drawn]

+q means the input angle, length data instead represent the start and stop opening
angles of the arc segment relative to the given point.

+r draws half-arrows, using only the right side of specified heads [both sides].

t[b|e]trim will shift the beginning or end point (or both) along the vector segment by
the given trim; append suitable unit. If the modifiers b|e are not used then trim may
be two values separated by a slash, which is used to specify different trims for the
two ends. Positive trims will shorted the vector while negative trims will lengthen it
[no trim].

In addition, all but circular vectors may take these modifiers:
+jjust determines how the input x,y point relates to the vector. Choose from beginning
[default], end, or center.

+s means the input angle, length is instead the x, y coordinates of the vector end

Finally, Cartesian vectors may take these modifiers:
+zscale[unit] expects input dx,dy vector components and uses the scale to convert to
polar coordinates with length in given unit.


To draw the vector field given by the files r.nc and theta.nc on a linear plot with scale
5 cm per data unit, using vector rather than stick plot, scale vector magnitudes so that
10 units equal 1 inch, and center vectors on the node locations, run

gmt grdvector r.nc theta.nc -Jx5c -A -Q0.1i+e+jc -S10i > gradient.ps

To plot a geographic data sets given the files com_x.nc and comp_y.nc, using a scale of
200 km per data unit and only plot every 3rd node in either direction, try

gmt grdvector comp_x.nc comp_y.nc -Ix3 -JH0/20c -Q0.1i+e+jc -S200 > globe.ps

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