thinning polygons

[Scott Miller]

Good day,

I've been working with a stream network map layer that was
originally created in ARC/INFO.  The streams were digitized
at a very fine resolution, and most of the streams appear as 
polygonal features, even those with a width of < 2m.  I would
like to create a generalized data layer from this by 
running a vector down the middle of the polygons, but prefer 
to avoid the brute-force approach of digitizing each stream
section.

I have tried running the r.thin routine in GRASS on the raster
map of the streams, with minimal success; the streams become
disjointed, and a discontinuous stream system is the result.
An additional problem is the presence of side-channels; the
confluences are at the edge of the polygons, but should be 
moved towards the center of the stream in the center of a 
resultant vector map.

My question is: is there an automated method for running a 
vector down the center of a given polygonal feature, or 
collapsing a polygonal feature to a given width?  If not in
GRASS, I have some familiarity with ARC, as well.

Thanks in advance,

Scott

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Scott Miller			e-mail: miller at tucson.ars.ag.gov
USDA - ARS SWRC			phone:  (602) 670-6481 ext. 342
2000 E. Allen Rd.		fax:  (602) 670-5550
Tucson, AZ  85721
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