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NAME

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

KEYWORDS

       display, map annotations, raster, arrow

SYNOPSIS

       d.rast.arrow
       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]

   Flags:
       -a
           Align grids with raster cells

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

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

       type=string
           Type of existing raster aspect map
           Options: grass, compass, drainage, agnps, answers
           Default: grass

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

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

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

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

       skip=integer
           Draw arrow every Nth grid cell
           Default: 1

       magnitude_map=string
           Raster map containing values used for arrow length

       scale=float
           Scale factor for arrows (magnitude map)
           Default: 1.0

DESCRIPTION

       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".

       When  specifying  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).

NOTES

       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 counterclockwise from east, and AGNPS  and  Drainage  type  aspect
       maps  will be displayed in D8 representation, i.e. the eight multiples of 45 degrees. Cell
       values are 1 to 8 clockwise from north for AGNPS and 1 to 8  counterclockwise  from  north
       east for Drainage. See r.watershed for more details about the Drainage aspect.

       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").

EXAMPLE

       Convert   U,V   velocity  component  maps  into  magnitude,direction  maps  for  use  with
       d.rast.arrow:
       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
       Sea wind speed (magnitude) and direction shown in the Tasmanian Sea

SEE ALSO

        d.frame, d.rast, d.rast.edit, d.rast.num, g.region, r.slope.aspect, r.watershed

AUTHORS

       Original author
       Chris Rewerts
       Agricultural Engineering,
       Purdue University
       Magnitude and 360 arrow code
       Hamish Bowman
       Department of Marine Science,
       University of Otago, New Zealand
       Align grids with raster cells and Drainage aspect type
       Huidae Cho

SOURCE CODE

       Available at: d.rast.arrow source code (history)

       Accessed: Tuesday Jun 27 11:12:34 2023

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       © 2003-2023 GRASS Development Team, GRASS GIS 8.3.0 Reference Manual