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I am working with OSM road data for a developing nation. Included in the dataset are various small networks of roads that are orphans or islands i.e. not connected to the main road network. For example, four islands / orphans are evident in the section of roads below.

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My aim here is to have all the roads connected that I can run a Steiner analysis without getting errors from nodes that do not connect.

How can I get QGIS to select these so that I can either delete them or connect them to other sections of road network?

  • Basically you need to clean your network data. I'm more familiar with doing this and the questions already here related to doing it for ArcGIS. You're going to want to look into topology and topology checks. Also dangles (rmdangle) and pseudo-nodes if you want to do Steiner and I'm remembering correctly. I list checks I use for network cleaning in my answer at gis.stackexchange.com/questions/112911 – Chris W Jul 22 '15 at 21:10
  • Thanks for the input @Chris W. I have made various attempts to snap, clean and break the data but still haven't quite got it right and many of the pseudo nodes still exist. I will keep plugging away..... – Tim P Jul 24 '15 at 19:05
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The solution I adopted to this followed on from another response from @Chris W at How do you remove isolated roads from a bigger network? where he use a 'kludge' of a service area. In my case, in QGIS I created a single central point to the network and use v.net alloc for a netwok allocation which (I think) found all connected roads. I used this in the later analysis. For the record, this was still full of pseudonodes even after snapping, breaking and bpol with v.clean.

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