# grdvector¶

Plot vector field from two component grids

## Synopsis¶

gmt grdvector compx.nc compy.nc -Jparameters [ -A ] [ -B[p|s]parameters ] [ -Ccpt ] [ -Gfill ] [ -I[x]dx[/dy] ] [ -N ] [ -Qparameters ] [ -Rregion ] [ -S[i|l]scale[unit] ] [ -T ] [ -U[stamp] ] [ -Wpen ] [ -X[a|c|f|r][xshift[u]] ] [ -Y[a|c|f|r][yshift[u]] ] [ -Z ] [ -fflags ] [ -pflags ] [ -ttransp ] [ --PAR=value ]

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

## Description¶

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.

## Required Arguments¶

compx.nc
Contains the x-components of the vector field.
compy.nc
Contains the y-components of the vector field. (See GRID FILE FORMATS below.)
-Jparameters (more …)
Select map projection.

## Optional Arguments¶

-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 frame and axes attributes.
-C[cpt |master[+izinc] |color1,color2[,color3,…]]
Use cpt to assign colors based on vector length. Alternatively, supply the name of a GMT color master dynamic CPT [rainbow] to automatically determine a continuous CPT from the grid’s z-range; you may round up/down the z-range by adding +izinc.. 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 ). If no argument is given to -C then under modern mode we select the current CPT.
-Gfill
Sets color or shade for vector interiors [Default is no fill].
-I[x]dx[/dy]
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].
-N
Do NOT clip vectors at map boundaries [Default will clip].
-Qparameters
Modify vector parameters. For vector heads, append vector head size [Default is 0, i.e., stick-plot]. See VECTOR ATTRIBUTES for specifying additional attributes.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more …)
Specify the region of interest. Specify a subset of the grid.
-S[i|l]scale[unit]
Sets scale for vector plot length in data units per plot distance measurement unit [1]. Append c, i, or p to indicate the measurement unit (cm, inch, or point); if no unit is given we use the default value that is controlled by PROJ_LENGTH_UNIT. Alternatively, use -Sllength[unit] to set a fixed plot length for all vectors. Vectors given via plot unit scaling will plot as straight vectors and their lengths are not affected by map projection and coordinate locations. For geographic data you may alternatively give scale in data units per map distance unit (see Units). Then, your user units are scaled to map distances in the given unit which are projected to plot dimensions. These are geo-vectors that follow great circle paths and their lengths are affected by the map projection and their coordinates. Finally, use -Si if it is simpler to give the reciprocal scale in measurement unit per data unit or km per data unit. To report the minimum, maximum, and mean scaled vector length, use -Vl.
-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[label][+c][+jjust][+odx/dy] (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].

-X[a|c|f|r][xshift[u]]

-Y[a|c|f|r][yshift[u]] (more …)
Shift plot origin.
-Z
The theta grid provided contains azimuths rather than directions (implies -A).
-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 just use -).
-+ 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 all options, then exits.
--PAR=value
Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.

## Units¶

For map distance unit, append unit d for arc degree, m for arc minute, and s for arc second, or e for meter [Default], f for foot, k for km, M for statute mile, n for nautical mile, and u for US survey foot. By default we compute such distances using a spherical approximation with great circles (-jg). You can use -jf to perform “Flat Earth” calculations (quicker but less accurate) or -je to perform exact geodesic calculations (slower but more accurate; see PROJ_GEODESIC for method used).

## 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 1- or 2-byte integers. (more …)

## Vector Attributes¶

Several modifiers may be appended to vector-producing options for specifying 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 a 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, a for arrow [Default], i for tail, A for plain open arrow, and I for plain open tail. Further append l|r to only draw the left or right half-sides 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, a for arrow [Default], i for tail, A for plain open arrow, and I for plain open tail. Further append l|r to only draw the left or right half-sides 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].

+hshape sets the shape of the vector head (range -2/2). Default is controlled by MAP_VECTOR_SHAPE [0].

+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, a for arrow [Default], i for tail, A for plain open arrow, and I for plain open tail. Further append l|r to only draw the left or right half-sides 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 vector plot lengths shorter than norm will have their attributes scaled by length/norm [arrow attributes remains invariant to length]. For Cartesian vectors specify a length in plot units, while for geovectors specify a length in km.

+o[plon/plat] specifies the oblique pole for the great or small circles. Only needed for great circles if +q is given. If no pole is appended then we default to the north pole.

+p[-][pen] sets the vector pen attributes. If pen has a leading - then the head outline is not drawn. [Default pen is half the width of stem pen, 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. See +o to specify a specific pole for the arc [north pole].

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

+t[b|e]trim[unit] will shift the beginning or end point (or both) along the vector segment by the given trim; append suitable unit (c, i, or p). 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 beginning and end. 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 are instead the x, y coordinates of the vector end point.

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.

## Examples¶

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 -pdf gradient


To plot a geographic data sets given the files comp_x.nc and comp_y.nc, using a length 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 -S200k -pdf globe


## Notes¶

Be aware that using -I may lead to aliasing unless your grid is smoothly varying over the new length increments. It is generally better to filter your grids and resample at a larger grid increment and use these grids instead of the originals.