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

NAME

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

KEYWORDS

       display, raster

SYNOPSIS

       d.rast.arrow
       d.rast.arrow help
       d.rast.arrow       [map=string]        [type=string]       [arrow_color=string]       [grid_color=string]
       [x_color=string]    [unknown_color=string]    [skip=integer]     [magnitude_map=string]     [scale=float]
       [--verbose]  [--quiet]

   Parameters:
       map=string
           Name of raster aspect map to be displayed

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

       arrow_color=string
           Color for drawing arrows
           Options:
           red,orange,yellow,green,blue,indigo,violet,white,black,gray,brown,magenta,aqua,grey,cyan,purple
           Default: green

       grid_color=string
           Color for drawing grid or "none"
           Options:
           red,orange,yellow,green,blue,indigo,violet,white,black,gray,brown,magenta,aqua,grey,cyan,purple,none
           Default: gray

       x_color=string
           Color for drawing X's (Null values)
           Options:
           red,orange,yellow,green,blue,indigo,violet,white,black,gray,brown,magenta,aqua,grey,cyan,purple
           Default: black

       unknown_color=string
           Color for showing unknown information
           Options:
           red,orange,yellow,green,blue,indigo,violet,white,black,gray,brown,magenta,aqua,grey,cyan,purple
           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".

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).

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, 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").

EXAMPLE

       Convert U,V velocity component maps into magnitide,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

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

       Last changed: $Date: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $

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       © 2003-2013 GRASS Development Team

GRASS 6.4.3                                                                                 d.rast.arrow(1grass)