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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

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