## DESCRIPTION

d.polar calculates and displays a polar diagram of an angle raster map such as aspect, wind direction, or flow. The input angle map orientation must be counter-clockwise (CCW) from east, and the angle map units must be degrees. This refers to the standard orientation of GRASS (e.g., see r.slope.aspect).

The radius of the outer circle is defined by the total number of pixels in the map. If the polar diagram does not reach the outer circle, no data (NULL) pixels were found in the map.

The vector in the diagram indicates a prevailing direction, its length the strength of such direction. In case all angle vectors are oriented along the same direction, the length for the vector is maximal (in fact it is equal to length of the most populated bin of the underlying histogram).

As a side effect, the quality of the angle map can be derived from the diagram. Strong spikes may indicate an over-representation of the related angle(s) which happens in particular if integer maps are used.

## NOTES

If the output parameter is used, the diagram is generated as EPS file. If the -x flag is used xgraph is launched. Otherwise d.polar will use d.graph to draw the plot in the current display frame.

If d.polar is used on an aspect map generated by r.slope.aspect, the undef parameter should be set to 0 to distinguish NO DATA (NULL) areas from areas where aspect is undefined (i.e. flat areas).

Polar diagram of aspect map

## EXAMPLE

In this example, the polar diagram of the 'aspect' angle map in the North Carolina sample dataset location is generated:

    g.region raster=aspect -p
    d.polar aspect

## XGRAPH REQUIREMENT

Xgraph must be installed for the -x flag to function.

## SEE ALSO

d.graph, d.histogram, d.rast.arrow, r.slope.aspect

## REFERENCES

J. Hofierka, H. Mitasova, and M. Neteler (2009): Terrain parameterization in GRASS. In T. Hengl and H.I. Reuter, editors, Geomorphometry: concepts, software, applications. Elsevier (DOI)

## AUTHORS

Markus Neteler, ITC-irst, Italy
Bruno Caprile, ITC-irst, Italy
Hamish Bowman, Otago University, New Zealand