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- <h2>DESCRIPTION</h2>
- <em>d.polar</em> 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 <em>r.slope.aspect</em>).
- <p>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.
- <p>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).
- <p>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.
- <h2>NOTES</h2>
- If the <b>output</b> parameter is used, the diagram is generated
- as EPS file. If the <b>-x</b> flag is used <em>xgraph</em> is launched.
- Otherwise <em>d.polar</em> will use <em>d.graph</em> to draw the plot
- in the current display frame.
- <p>If <em>d.polar</em> is used on an aspect map generated by
- <em>r.slope.aspect</em>, the <b>undef</b> parameter should be
- set to 0 to distinguish NO DATA (NULL) areas from areas where
- aspect is undefined (i.e. flat areas).
- <p>
- <center>
- <img src="d_polar_aspect.png" alt="Polar diagram of aspect map">
- <br>
- <i>Polar diagram of aspect map</i>
- </center>
- <h2>EXAMPLE</h2>
- In this example, the polar diagram of the 'aspect' angle map in the
- North Carolina sample dataset location is generated:
- <div class="code"><pre>
- g.region raster=aspect -p
- d.polar aspect
- </pre></div>
- <h2>SEE ALSO</h2>
- <em>
- <a href="d.graph.html">d.graph</a>,
- <a href="d.histogram.html">d.histogram</a>,
- <a href="d.rast.arrow.html">d.rast.arrow</a>,
- <a href="r.slope.aspect.html">r.slope.aspect</a>
- </em>
- <h2>XGRAPH REQUIREMENT</h2>
- <a href="http://www.xgraph.org">Xgraph</a> must be installed for the
- <b>-x</b> flag to function.
- <h2>REFERENCES</h2>
- 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
- (<a href="http://dx.doi.org/10.1016/S0166-2481(08)00017-2">DOI</a>)
- <h2>AUTHORS</h2>
- Markus Neteler, ITC-irst, Italy<br>
- Bruno Caprile, ITC-irst, Italy<br>
- Hamish Bowman, Otago University, New Zealand<br>
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