OpenStreetMap

Fixing broken riverbanks

Posted by PlaneMad on 24 February 2017 in English (English)

Stumbled upon a dried up Nile on the map, which seems to be a jumble of large multipolygons.

This might require the expertise of an expert with the JOSM relation editor which can be a time consuming process.

It would be great if someone could make a simple howto tutorial for fixing such scenarios to help fix so many other big rivers that are broken this way.

Location: Ed Dueim, an-Nil al-Abyad, Sudan

Comment from imagico on 24 February 2017 at 15:17

There is no universal recipe to fix this kind of problem. There are many different kinds of mistakes that occur in broken multipolygons and each of them requires a different approach.

Things that are helpful usually with with riverbank polygons:

  • make the individual polygons small - don't make the mistake of fixing things by merging everything into one big polygon.
  • decide on which area your polygon is meant to cover along the river
  • throw out all members outside this area
  • throw out duplicate members (which are in the relation several times)
  • assemble a closed outer ring, if necessary drawing new ways or splitting ways
  • add all islands within as inner rings
  • make sure all tags are on the relation and none are left on the member ways (unless they apply only to them)
  • create additional polygons using members previously removed as necessary

This is all a matter of exercise and training but even with a lot of experience this is still tedious work.

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Comment from mustafakamil on 2 March 2017 at 05:12

I'm from Sudan, I am having the same problem in riverbanks of the Nile because the Nile in flood season comes out of the main stream

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