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).

SOURCE CODE

       Available at: r.thin source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual