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

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