## 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. An arrow can point in one of eight directions. 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 of containing null data will be marked with an "X".

## NOTES

Arrows are drawn at the size of a cell and cannot be seen if the raster map is relatively close in scale. From a command line mode, the user cannot run d.rast.arrow and then run d.zoom and still see the arrows. The zooming action erases the arrows and the user must run d.rast.arrow again.

GRASS aspect maps are measured in degrees counterclockwise from east. i.e.:

    90  North
    180 West
    270 South
    360 East

They can be created from a raster elevation map with r.slope.aspect.

## SEE ALSO

d.frame
d.rast
d.rast.edit
d.rast.num
d.zoom
g.region
r.mask
r.slope.aspect
parser

## AUTHOR

Chris Rewerts, Agricultural Engineering, Purdue University

Last changed: $Date: 2004/08/25 09:12:23 $