NAME

       d.polar  - Draws polar diagram of angle map such as aspect or flow directions

KEYWORDS

       display, diagram

SYNOPSIS

       d.polar
       d.polar --help
       d.polar   [-x]   map=name    [undef=float]     [output=name]     [--overwrite]    [--help]
       [--verbose]  [--quiet]  [--ui]

   Flags:
       -x
           Plot using Xgraph

       --overwrite
           Allow output files to overwrite existing files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       map=name [required]
           Name of raster angle map

       undef=float
           Pixel value to be interpreted as undefined (different from NULL)

       output=name
           Name for optional EPS output file

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

SEE ALSO

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

XGRAPH REQUIREMENT

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

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

       Last changed: $Date: 2015-09-05 17:56:17 +0200 (Sat, 05 Sep 2015) $

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