[GRASS-user] weird r.thin output

[sidtupper]

I'm trying to digitize a paper map of an island.  I've decomposed a scan of
it into layers, one layer per file.  One of the layers, polygons
representing the high tide line, is a greyscale jpeg file with just two
colours (0, 255).  Imported into GRASS 6.3 (Linux 2.31.6 kernel) and
processed with r.thin, after 429 iterations it becomes a mess of horizonal,
vertical and 45 degree straight lines. 

In case these files would be helpful to see what's going on, I attach them. 
The input file is 
http://n2.nabble.com/file/n4562787/hermit-hightide-2colour.jpg
hermit-hightide-2colour.jpg  and the exported output is 
http://n2.nabble.com/file/n4562787/hightide-thin.png hightide-thin.png .

I hope someone can tell me what I'm doing wrongly.  Thanks.
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