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PROGRAM:

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

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