d.rast.arrow - GRASS manual
Author: GRASS Development Team
Description: d.rast.arrow: Draws arrows representing cell aspect direction for a raster map containing aspect data.
Keywords: display, map annotations, raster, arrow

## 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, terraflow
  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 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`".

When specifying 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 counterclockwise 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 counterclockwise from north east for Drainage. See r.watershed for more details about the Drainage aspect. Terraflow (same as ArcGIS) type aspect map will use a power-of-two encoding clockwise from 1 for east to 128 for north east. See r.terraflow for more details about the Terraflow encoding.

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 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, r.terraflow

## 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)

Latest change: Thursday Feb 20 12:48:50 2025 in commit: 1633be4dc57dd1819a0f6ccd8ea4bc2b8bbec864

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© 2003-2026 GRASS Development Team, GRASS 8.5.1dev Reference Manual