[GRASS-user] How to plot a vector field?

[david adam]

Many Thanks!  Guessing would not have worked here...
david

On 06/27/2013 05:07 PM, Hamish wrote:
> David wrote:
>
>> I would like to plot data which consist of a number of points, each of
>> which has a latitude, a longitude, an x-velocity, and a y-velocity.  I
>> want to plot an arrow for each point that starts at the proper latitude
>> and longitude, and has a length and direction that show the velocity.  I
>> used to think of these as vector values, but in GRASS a vector is
>> something else.  What is the proper GRASS terminology for this kind of
>> data?  What modules do this kind of plot?
> Hi,
>
> use the d.barb module from GRASS 6 addons.
>    http://grasswiki.osgeo.org/wiki/AddOns/GRASS_6#d.barb
>
> x and y components of velocity are given using attribute column names
> in the u= and v= parameters when a GRASS vector (i.e. sparse points)
> input= map is given.
>
> "vector maps" mean defined by coordinates, so points, lines, and areas.
> "raster maps" mean defined by 2D or 3D array, so constant grids and pixels.
>
> d.barb will work with both GRASS raster and vector points maps,
> the meaning of the u=, v= parameters changes according to the context.
>
>
> Usage:
>   d.barb [-r] [direction=name] [magnitude=name] [u=name] [v=name]
>     [input=name] [layer=value] [style=string] [color=name] [skip=value]
>     [scale=value] [peak=value] [aspect_type=string]
>     [legend_at=x,y[,x,y,...]] [legend_velo=value[,value,...]]
>     [legend_fontsize=value] [--verbose] [--quiet]
>
> Flags:
>    -r   Rotate direction 180 degrees
>          Useful for switching between atmospheric and oceanographic conventions
>   --v   Verbose module output
>   --q   Quiet module output
>
> Parameters:
>          direction   Raster map (or attribute column) containing velocity direction
>          magnitude   Raster map (or attribute column) containing velocity magnitude
>                  u   Raster map (or attribute column) containing u-component of velocity
>                  v   Raster map (or attribute column) containing v-component of velocity
>              input   Name of input vector map
>              layer   Layer number
>                       A single vector map can be connected to multiple database tables. This number determines which table to use.
>                      default: 1
>              style   Style
>                      options: arrow,barb,small_barb,straw
>                      default: arrow
>              color   Color
>                       Either a standard color name or R:G:B triplet
>                      default: black
>               skip   Draw arrow every Nth grid cell
>                      default: 10
>              scale   Scale factor for arrow rendering
>                      default: 1.0
>               peak   Maximum value for scaling (overrides map's maximum)
>        aspect_type   Direction map aspect type
>                      options: cartesian,compass
>                      default: cartesian
>          legend_at   Screen percentage for legend barb ([0,0] is bottom-left)
>                       Draws a single barb and exits
>                      options: 0-100
>                      default: 10.0,10.0
>        legend_velo   Velocity for legend key arrow
>    legend_fontsize   Font size used in legend
>                      default: 14
>
>
> it's still a work in progress, but ~90% of the features are working.
>
> Hamish
>
>
>