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I have a vector map that I converted from a raster, representing the rivers in the map. However, because the colours in the map were very diffuse, the resulting vector map has many interrupted lines. Is there a way to extend the lines until they touch another, and then join the touching ones into a single line? I am using QGIS with GRASS.


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Raster map vectorisation is a pain in the ass. There is no easy way to do this with the software that is not designed for this purpose. I suggest you to consider to try out this free tool for vectorisation. It will be able to do that things that you are asking about.

Another solution to your issue is to process your initial raster to the binary raster where only rivers will be left and there won't be any gaps between rivers. You may do it with GIMP, but you will need to preserve spatial reference beforehand.

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