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d.rast.arrow(1)		    GRASS GIS User's Manual	       d.rast.arrow(1)

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

KEYWORDS
       display,	map annotations, raster

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=nameA [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  run-
       ning  d.rast.arrow.  The	second method involves setting the map parame-
       ter 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  con-
       taining	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	magni-
       tude  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  de-
       bug  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 counter-
       clockwise 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	 coun-
       terclockwise from north east for	Drainage. See r.watershed for more de-
       tails about the Drainage	aspect.

       GRASS aspect maps are measured using Cartesian conventions, i.e.	in de-
       grees 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)

       Main  index | Display index | Topics index | Keywords index | Graphical
       index | Full index

       A(C) 2003-2020 GRASS Development	Team, GRASS GIS	7.8.3 Reference	Manual

GRASS 7.8.3						       d.rast.arrow(1)

NAME | KEYWORDS | SYNOPSIS | DESCRIPTION | NOTES | EXAMPLE | SEE ALSO | AUTHORS | SOURCE CODE

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