Line to polygon conversion in GRASS

http://osgeo-org.1803224.n2.nabble.com/converting-lines-to-polygons-td5557002.html

v.in.ogr -o dsn=C:\path\to\shapefile\or\dxf.file output=circles1 type=line
v.clean input=circles1 output=circles2 tool=snap,break,rmdupl thresh=0.2,0,0
v.type input=circles2 output=circles3 type=line,boundary
v.centroids input=circles3 output=circles4 option=add

Be sure to play with the treshold parameter of v.clean