title: Join multiple line segments into one line - GIS
body: <p>I have a map that I have digitised and converted to a vector layer (the rivers only).
  The problem is that the vectorisation has produced a large number of segments for each river, that show up as different features (each may have multiple straight line segments, but they do not cover whole rivers). What I am looking for is a tool to merge into one feature (a polyline, I guess) all the segments whose extremes are within a given distance.
  I am using QGis, and the GRASS plugin. I have tried v.clean.snap, v.build.polylines, but did not yet manage to actually merge the lines. Any help would be very appreciated!</p>