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]   [eps=string]   [--verbose]  [--quiet]

   Flags:
       -x
           Plot using Xgraph

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       map=name
           Name of raster angle map

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

       eps=string
           Name of 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 eps parameter is used, the diagram is generated as EPS file. If the -x flag is used
       xgraph is lauched.  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 rast=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.

REFERENCE

       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: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $

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