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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       © 2003-2013 GRASS Development Team

GRASS 6.4.3                                                                                      d.polar(1grass)