d.rast.arrow(1grass) GRASS GIS User's Manual d.rast.arrow(1grass)
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 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)
Accessed: unknown
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© 2003-2022 GRASS Development Team, GRASS GIS 7.8.7 Reference Manual
GRASS 7.8.7 d.rast.arrow(1grass)
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