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).
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").
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
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