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NAME
d.rast.arrow - Draws arrows representing cell aspect direction for a raster map containing aspect data.
KEYWORDS
display
SYNOPSIS
d.rast.arrow
d.rast.arrow help
d.rast.arrow [-a] map=name [type=string] [arrow_color=string] [grid_color=string] [x_color=string] [unknown_color=string] [skip=integer] [magnitude_map=string] [scale=float] [--verbose] [--quiet]
Flags:
- -a
- Align grids with raster cells
- --verbose
- Verbose module output
- --quiet
- Quiet module output
Parameters:
- map=name
- 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
- Default: green
- grid_color=string
- Color for drawing grid or "none"
- Default: gray
- x_color=string
- Color for drawing X's (null values)
- Default: black
- unknown_color=string
- Color for showing unknown information
- 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
SEE ALSO
d.frame
d.rast
d.rast.edit
d.rast.num
g.region
r.slope.aspect
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: 2010-11-07 14:56:51 -0800 (Sun, 07 Nov 2010) $
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