raster-to-vector conversion of rivers

[Ralf Kunkel]

We try to convert a couple of raster maps created outside of GRASS to GRASS.
Some of these maps contain line-based information like rivers. Converting
these line maps using s.in.ascii, s.to.rast (or s.menu conversion) followed
by r.poly gives closed polygons around the raster elements. We didn't found
another possibility for this type of conversion until know. PLEASE HELP...

Thanks in advance
Ralf Kunkel