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       d.rast.arrow   -  Draws  arrows  representing  cell  aspect  direction  for  a  raster map
       containing aspect data.


       display, map annotations, raster


       d.rast.arrow --help
       d.rast.arrow   [-a]    map=name     [type=string]      [color=name]      [grid_color=name]
       [null_color=name]      [unknown_color=name]      [skip=integer]     [magnitude_map=string]
       [scale=float]   [--help]  [--verbose]  [--quiet]  [--ui]

           Align grids with raster cells

           Print usage summary

           Verbose module output

           Quiet module output

           Force launching GUI dialog

       map=name [required]
           Name of raster aspect map to be displayed

           Type of existing raster aspect map
           Options: grass, compass, agnps, answers
           Default: grass

           Color for drawing arrows
           Either a standard color name or R:G:B triplet
           Default: green

           Color for drawing drawing grid
           Either a standard color name, R:G:B triplet, or "none"
           Default: gray

           Color for drawing null values (X symbol)
           Either a standard color name, R:G:B triplet, or "none"
           Default: black

           Color for showing unknown information (? symbol)
           Either a standard color name, R:G:B triplet, or "none"
           Default: red

           Draw arrow every Nth grid cell
           Default: 1

           Raster map containing values used for arrow length

           Scale factor for arrows (magnitude map)
           Default: 1.0


       d.rast.arrow is designed to help users better visualize surface water flow  direction,  as
       indicated  in  an aspect raster map layer.  There are two ways to specify the aspect layer
       the program is to use.  The first is to display the  aspect  map  layer  on  the  graphics
       monitor before running d.rast.arrow.  The second method involves setting the map parameter
       to the name of the desired aspect map.  This allows the arrows to be drawn over any  other
       maps already displayed on the graphics monitor.

       d.rast.arrow  will  draw  an arrow over each displayed cell to indicate in which direction
       the cell slopes. If the aspect layer has a category value denoting locations of  "unknown"
       aspect,  d.rast.arrow  draws  a  question  mark over the displayed cells of that category.
       Cells containing null data will be marked with an "X".  You can disable  drawing  of  null
       data and unknown aspect values by setting its color to "none".

       If  you  specify  the  magnitude_map  option,  arrow  lengths  denoting  magnitude will be
       extracted from the cell values of the specified map. In this case the tail  of  the  arrow
       will  be  centered  on  the source cell.  You may adjust the overall scale using the scale
       option.  d.rast.arrow will ignore NULL and negative magnitudes, and will warn you  if  the
       debug  level is set at 5 or higher. Be aware. If your application uses negative values for
       magnitude, you can use r.mapcalc to prepare the magnitude map to suit your needs (absolute
       value, inverted direction and so on).


       By default, arrows are drawn at the size of a cell and cannot be seen if the raster map is
       relatively close in scale. You can use the skip option to draw arrows every n-th  cell  in
       both  directions  if you are working with relatively high resolutions. It may be useful to
       disable the grid in this case, which is accomplished by setting its color to "none".

       For GRASS and Compass type aspect maps, the cell values of the aspect map  will  determine
       the  corresponding  direction  in 360 degrees. ANSWERS type aspect maps will be plotted in
       multiples  of  15  degrees,  and  AGNPS  type  aspect  maps  will  be  displayed   in   D8
       representation, i.e. the eight multiples of 45 degrees.

       GRASS   aspect   maps   are   measured   using  Cartesian  conventions,  i.e.  in  degrees
       counterclockwise from east. e.g.:
       90  North
       180 West
       270 South
       0,360 East
       They can be created from a raster elevation map with r.slope.aspect.

       Compass type aspect maps are measured in degrees clockwise from north.

       This module uses oceanographic conventions, i.e. arrows point downslope or direction "to",
       as opposed to atmospheric conventions (direction "from").


       Convert   U,V   velocity  component  maps  into  magnitide,direction  maps  for  use  with
       r.mapcalc "magnitude = sqrt(U_map^2 + V_map^2)"
       r.mapcalc "direction = atan(U_map, V_map)"
       d.rast.arrow map=direction type=grass magnitude_map=magnitude skip=3 grid=none




       Original author
       Chris Rewerts
       Agricultural Engineering,
       Purdue University
       Magnitude and 360 arrow code
       Hamish Bowman
       Department of Marine Science,
       University of Otago, New Zealand

       Last changed: $Date: 2014-11-25 15:36:43 +0100 (Tue, 25 Nov 2014) $

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