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

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: 2011-11-08 12:29:50 +0100 (Tue, 08 Nov 2011) $ Full index © 2003-2014 GRASS Development Team