NAME

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

KEYWORDS

       raster, geometry

SYNOPSIS

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

   Flags:
       --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:
       input=name [required]
           Name of input raster map

       output=name [required]
           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-NULL cells that denote  linear
       features  into  linear  features having a single cell width. Raster lines often need to be
       thinned (skeletonizing raster features) to  a  single  pixel  width  before  they  can  be
       transformed to vector data.

       r.thin  will  thin  only the non-NULL (no data) raster 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
       NULL’s in the input raster data file, hence it creates a NULL/1 output map.

NOTES

       r.thin only creates raster map layers. In order to create a vector map, the user will need
       to run r.to.vect on the resultant raster map.

       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 (rmdangle tool).

       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.

EXAMPLE

       To  vectorize  the  raster  map  streams_derived in the North Carolina sample dataset that
       represents the stream network derived from the 10m resolution DEM by r.watershed, run:
       g.region raster=elevation -p
       # create flow accumulation map
       r.watershed elevation=elevation accumulation=accum_50K thresh=50000
       # extract streams from flow accumulation map
       r.mapcalc "streams_from_flow = if(abs(accum_50K) > 1000, 1, null())"
       # skeletonize map
       r.thin streams_from_flow out=streams_thin
       d.mon wx0
       d.rast streams_from_flow
       d.erase
       d.rast streams_thin

       Raster feature thinning (skeletonizing)

       The resulting map cabe optionally vectorized:
       r.to.vect streams_thin output=streams_thin type=line
       # visualize
       d.rast accum_50K
       d.vect streams_thin color=red width=2

       Vectorized stream network after thinning extracted from flow accumulation map

SEE ALSO

        g.region, r.to.vect, v.clean, wxGUI vector digitizer, 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: 2015-04-20 16:09:08 +0200 (Mon, 20 Apr 2015) $

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