NAME

       r.thin  - Thins non-zero cells that denote linear features in a raster map.

KEYWORDS

       raster, thin

SYNOPSIS

       r.thin
       r.thin help
       r.thin input=name output=name  [iterations=integer]   [--overwrite]  [--verbose]  [--quiet]

   Flags:
       --overwrite
           Allow output files to overwrite existing files

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       input=name
           Name of input raster map

       output=name
           Name for output raster map

       iterations=integer
           Maximal number of iterations
           Default: 200

DESCRIPTION

       r.thin  scans  the named input raster map layer and thins non-zero cells that denote linear features into
       linear features having a single cell width.

       r.thin will thin only the non-zero cells  of  the  named  input  raster  map  layer  within  the  current
       geographic  region  settings.  The cell width of the thinned output raster map layer will be equal to the
       cell resolution of the currently set geographic region.  All of the thinned linear features will have the
       width of a single cell.

       r.thin will create a new output raster data file containing the thinned linear features.  r.thin  assumes
       that  linear  features  are  encoded with positive values on a background of 0's in the input raster data
       file.

NOTE

       r.thin only creates raster map layers.  You will need to run r.to.vect on the  resultant  raster  map  to
       create a vector (v.digit) map layer.

       r.thin  may  create  small  spurs  or  "dangling  lines" during the thinning process.  These spurs may be
       removed (after creating a vector map layer) by v.clean.

       r.thin creates a 0/1 output map.

NOTE

       This code implements  the  thinning  algorithm  described  in  "Analysis  of  Thinning  Algorithms  Using
       Mathematical  Morphology"  by  Ben-Kwei  Jang  and Ronlad T. Chin in Transactions on Pattern Analysis and
       Machine Intelligence, vol. 12, No. 6, June 1990.  The definition Jang  and  Chin  give  of  the  thinning
       process  is "successive removal of outer layers of pixels from an object while retaining any pixels whose
       removal would alter the connectivity or shorten the legs of the sceleton."

       The sceleton is finally thinned when the thinning process converges; i.e.,  "no  further  pixels  can  be
       removed  without  altering the connectivity or shortening the sceleton legs" (p. 541).  The authors prove
       that the thinning process described always converges and produces one-pixel thick sceletons.  The  number
       of  iterations  depends  on  the  original thickness of the object.  Each iteration peels off the outside
       pixels from the object.  Therefore, if the object is <= n pixels thick, the algorithm should converge  in
       <= iterations.

SEE ALSO

       g.region, r.to.vect, v.clean, v.digit, v.build

AUTHOR

       Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory

       The code for finding the bounding box as well as input/output code was written by Mike Baba (DBA Systems,
       1990) and Jean Ezell (USACERL, 1988).

       Last changed: $Date: 2008-05-16 12:09:06 -0700 (Fri, 16 May 2008) $

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       © 2003-2013 GRASS Development Team

GRASS 6.4.3                                                                                       r.thin(1grass)