# grdvector¶

Plot vector field from two component grids

## Synopsis¶

**gmt grdvector** *compx.nc* *compy.nc* **-J***parameters*
[ **-A** ]
[ **-B**[**p**|**s**]*parameters* ]
[ **-C***cpt* ]
[ **-G***fill* ]
[ **-I**[**x**]*dx*[/*dy*] ]
[ **-N** ] [ **-Q***parameters* ]
[ **-R***region* ]
[ **-S**[**i**|**l**]*scale* ]
[ **-T** ]
[ **-U**[*stamp*] ]
[ **-W***pen* ]
[ **-X**[**a**|**c**|**f**|**r**][*xshift*] ]
[ **-Y**[**a**|**c**|**f**|**r**][*yshift*] ]
[ **-Z** ]
[ **-f**flags ]
[ **-p**flags ]
[ **-t**transp ]
[ **--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. (See Grid File Formats).

*compy.nc*Contains the y-components of the vector field. (See Grid File Formats).

**-J***parameters*Specify the projection. (See full description) (See cookbook summary) (See projections table).

## Optional Arguments¶

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

**-B**[**p**|**s**]*parameters*Set map boundary frame and axes attributes. (See full description) (See cookbook information).

**-C**[*cpt*|*master*[**+h**[*hinge*]][**+i***zinc*][**+u**|**U***unit*] |*color1,color2*[,*color3*,…]]Name of a CPT file. Alternatively, supply the name of a GMT color master dynamic CPT [

*turbo*, but we use*geo*for @earth_relief and*srtm*for @srtm_relief data] to automatically determine a continuous CPT from the grid’s*z*-range; you may round the range to the nearest multiple of*zinc*by adding**+i***zinc*. If given a GMT Master soft-hinge CPT (see Of Colors and Color Legends) then you can enable the hinge at data value*hinge*[0] via**+h**, whereas for hard-hinge CPTs you can adjust the location of the hinge [0]. For other CPTs, you may convert their*z*-values from meter to another distance unit (append**+U***unit*) or from another unit to meter (append**+u***unit*), with*unit*taken from**e**|**f**|**k**|**M**|**n**|**u**. Yet another option is to specify**-C***color1*,*color2*[,*color3*,…] to build a linear continuous CPT from those colors automatically. In this case*color***n**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.

**-G***fill*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].

**-Q***parameters*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***xmin*/*xmax*/*ymin*/*ymax*[**+r**][**+u***unit*]Specify the region of interest. Specify a subset of the grid. (See full description) (See cookbook information).

**-S**[**i**|**l**]*scale*Sets scale for vector plot lengths in data units per plot distance measurement unit. Append

**c**,**i**, or**p**to indicate the desired plot distance measurement unit (cm, inch, or point); if no unit is given we use the default value that is controlled by PROJ_LENGTH_UNIT. Vector lengths converted via plot unit scaling will plot as straight Cartesian 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 vector magnitudes (in data units) are scaled to map*distances*in the given distance unit, and finally projected onto the Earth to give*plot*dimensions. These are geo-vectors that follow great circle paths and their lengths may be affected by the map projection and their coordinates. Finally, use**-Si**if it is simpler to give the reciprocal scale in plot length or distance units per data unit. To report the minimum, maximum, and mean data and plot vector lengths of all vectors plotted, use**-V**. Alternatively, use**-Sl***length*to set a fixed plot length for all vectors.

**-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**][**+j***just*][**+o***dx*/*dy*]Draw GMT time stamp logo on plot. (See full description) (See cookbook information).

**-V**[*level*]Select verbosity level [

**w**]. (See full description) (See cookbook information).

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

**-X**[**a**|**c**|**f**|**r**][*xshift*]Shift plot origin. (See full description) (See cookbook information).

**-Y**[**a**|**c**|**f**|**r**][*yshift*]Shift plot origin. (See full description) (See cookbook information).

**-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*]][**+w***lon0*/*lat0*[/*z0*]][**+v***x0*/*y0*] (more …)Select perspective view.

**-t***transp*[/*transp2*] (more …)Set transparency level(s) in percent.

**-^**or just**-**Print a short message about the syntax of the command, then exit (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 exit.

**-?**or no argumentsPrint a complete usage (help) message, including the explanation of all options, then exit.

**--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**) using the authalic radius
(see PROJ_MEAN_RADIUS). 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).

## 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 beginning point (**b**) to the
end point (**e**) of a line segment:

+aanglesets the angle \(\theta\) of the vector head apex [30].

+bplaces a vector head at thebeginning of the vector path [none]. Optionally, appendtfor a terminal line,cfor a circle,afor arrow [Default],ifor tail,Afor plain open arrow, andIfor plain open tail. Further appendl|rto only draw the left or right half-sides of this head [both sides].

+eplaces a vector head at theend of the vector path [none]. Optionally, appendtfor a terminal line,cfor a circle,afor arrow [Default],ifor tail,Afor plain open arrow, andIfor plain open tail. Further appendl|rto only draw the left or right half-sides of this head [both sides].

+g[fill] sets the vector head fill [Default fill is used, which may be no fill]. Turn off vector head fill by not appending afill. Some modules have a separate-Gfilloption and if used will select the fill as well.

+hshapesets the shape of the vector head (range -2/2). Default is controlled by MAP_VECTOR_SHAPE [default is theme dependent]. A zero value produces no notch (e.g., the dashed line in the figure). Positive values moves the notch toward the head apex while a negative value moves away. The example above uses+h0.5.

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

+mplaces a vector head at the mid-point the vector path [none]. Appendforrfor forward or reverse direction of the vector [forward]. Optionally, appendtfor a terminal line,cfor a circle,afor arrow [Default],ifor tail,Afor plain open arrow, andIfor plain open tail. Further appendl|rto only draw the left or right half-sides of this head [both sides]. Cannot be combined with+bor+e.

+nnormscales down vector attributes (pen thickness, head size) with decreasing length, where vector plot lengths shorter thannormwill 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+qis given. If no pole is appended then we default to the north pole.

+p[pen] sets the vector pen attributes. If nopenis appended then the head outline is not drawn. [Default pen is half the width of stem pen, and head outline is drawn]. Above, we used+p2p,orange. The vector stem attributes are controlled by-W.

+qmeans the inputdirection,lengthdata instead represent thestartandstopopening angles of the arc segment relative to the given point. See+oto specify a specific pole for the arc [north pole].

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

+t[b|e]trimwill shift the beginning or end point (or both) along the vector segment by the giventrim; append suitable unit (c,i, orp). If the modifiersb|eare not used thentrimmay 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:

+jjustdetermines how the inputx,ypoint relates to the vector. Choose frombeginning [default],end, orcenter.

+smeans the inputangle,lengthare instead the \(x_e, y_e\) coordinates of the vector end point.

Finally, Cartesian vectors may take these modifiers:

+zscaleexpects input \(\Delta x, \Delta y\) vector components and uses thescaleto convert to polar coordinates with length in given unit.

**Note**: Vectors were completely redesigned for GMT5 which separated the vector head (a polygon)
from the vector stem (a line). In GMT4, the entire vector was a polygon and it could only
be a straight Cartesian vector. Yes, the old GMT4 vector shape remains accessible if
you specify a vector (**-Sv**|**V**) using the GMT4 syntax, explained here: *size*, if present, will
be interpreted as \(t_w/h_l/h_w\) or *tailwidth/headlength/halfheadwidth* [Default is 0.075c/0.3c/0.25c (or
0.03i/0.12i/0.1i)]. By default, arrow attributes remain invariant to the length of the
arrow. To have the size of the vector scale down with decreasing size, append **+n***norm*,
where vectors shorter than *norm* will have their attributes scaled by *length*/*norm*.
To center the vector on the balance point, use **-Svb**; to align point with the vector head,
use **-Svh**; to align point with the vector tail, use **-Svt** [Default]. To give the
head point’s coordinates instead of direction and length, use **-Svs**. Upper case
**B**, **H**, **T**, **S** will draw a double-headed vector [Default is single head].

## Examples¶

**Note**: Below are some examples of valid syntax for this module.
The examples that use remote files (file names starting with `@`

)
can be cut and pasted into your terminal for testing.
Other commands requiring input files are just dummy examples of the types
of uses that are common but cannot be run verbatim as written.

**Note**: Since many GMT plot examples are very short (i.e., one module call between the
**gmt begin** and **gmt end** commands), we will often present them using the quick
modern mode GMT Modern Mode One-line Commands syntax, which simplifies such short scripts.

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

## Vector scaling and unit effects¶

The scale given via **-S** may require some consideration. As explained in **-S**,
it is specified in data-units per plot or distance unit. The plot or distance unit
chosen will affect the type of vector you select. In all cases, we first compute
the magnitude *r* of the user’s data vectors at each selected node from the *x* and *y*
components (unless you are passing *r*, *theta* grids directly with **-A**). These
magnitudes are given in whatever data units they come with. Let us pretend our data
grids record secular changes in the Earth’s magnetic horizontal vector field in units
of nTesla/year, and that at a particular node the magnitude is 28 nTesla/year (in some
direction). If you specify the scale using plot distance units (**c**|**i**|**p**)
then you are selecting *Cartesian* vectors. Let us further pretend that you selected
**-S**10c as your scale option. That means you want 10 nTesla/year to equate to a
1 cm plot length. Internally, we convert this scale to a plot scale of 1/10 = 0.1 cm
per nTesla/year. Given our vector magnitude of 28 nTesla/year, we multiply it by our
plot scale and finally obtain a vector length of 2.8 cm, which is then plotted.
The user’s data units do not enter of course, i.e., they always cancel [Likewise, if
we had used **-S**25i (25 nTesla/year per inch) the plot scale would be (1/25) = 0.04
inch per nTesla/year and the vector plot lengths would be 28 * 0.04 inch = 1.12 inch].
If we now wished to plot a 10 nTesla/year reference vector in the map legend we would
plot one that is 10 times 0.1 cm = 1 cm long since the scale length is *constant*
regardless of map projection and location. A 10 nTesla/year vector will be 1 cm anywhere.

Let us contrast this behavior with what happens if we use a geographic distance unit
instead, say **-S**0.5k (0.5 nTesla/year per km). Internally, this becomes a map scale of
2 km per nTesta/year. Given our node magnitude of 28 nTesla/year, the vector length will
be 28 x 2 km = 56 km. Again, the user’s data unit do not enter. Now, that vector length
of 56 km must be projected onto the Earth, and because of map distortions, a 56 km vector
will be mapped to a length on the plot that is a function of the user’s map projection,
the map scale, and possibly the location on the map. E.g., a 56 km vector due east at
Equator on a Mercator map would seem to equal ~0.5 degree longitude but at 60 north it
would be more like ~1 degree longitude. A consequence of this effect is that a user
who wants to add a 10 nTesla/year reference vector to a legend faces the same problem we do
when we wish to draw a 100 km map scale on a map: the plotted length usually will depend
on latitude and hence that reference scale is only useful around that latitude.

This brings us to the inverse scale option, **-Si***length*. This variant is useful
when providing the inverse of the scale is simpler. In the Cartesian case above, we
could instead give **-Si**0.1c which would directly imply a plot scale of 0.1 cm per
nTesla/year. Likewise, for geographic distances we could give **-Si**2k for 2 km per
nTesla/year scale as well. As the **-Si** argument increases, the plotted vector length
increases as well, while for plain **-S** the plot length decreases with increasing scale.

## 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.