OpenStreetMap
The Challenge of Dynamic Watercourses and Static Admin Lines ππ
Posted by Qwajo OSM on 21 September 2025 in English.One of the trickiest challenges in OSM is when a river (or any linear water feature) doubles as a regional or administrative boundary.
Rivers shift course over time; floods, erosion, and meanders, while boundaries often remain legally fixed. The result? Misalignments, overlapping lines, or confusing gaps on our maps.
For many contributors, editing such overlaps is daunting. Boundaries are sensitive, technically complex, and mistakes can cause big issues. Yet, leaving them mismatched affects disaster planning, legal clarity, and overall map quality.
My Question to Experienced Mappers π
How do you decide whether to follow the legal boundary or the current river course?
Rivers donβt wait. Boundaries donβt move. But as mappers, we can bridge the gap. Iβd love to hear your experiences and solutions!
Qwajo OSM
Discussion
Comment from imagico on 23 September 2025 at 16:00
There are two persistent urban myths w.r.t. boundaries:
Both of these are wrong.
Many (if not most) boundary data sets published by authorities are generalized approximations.
Most international boundaries world wide that follow a physical geography feature (either a river or a watershed divide) are legally (by agreement between the countries involved) tied to that element (Using formulations like: From A to B follows the course/centerline/left side/right side of the river X). Often a more explicit specification of the boundary is done (and demarcated) by a boundary commission, sometimes subject to regular revisions.
What counts in OSM is of course in any case only the line of de facto administration - which is usually well verifiable for international (admin_level 2) boundaries but increasingly non-verifiable for the higher admin levels.
In my experience most mappers treat higher level administrative boundaries as abstract artefacts and ignore them when editing other data and do not connect them to other geometries even if they are functionally tied to them.
Comment from SimonPoole on 23 September 2025 at 16:18
Simply maintain the boundary ways separate from whatever geographic feature potentially coincides with them, with other words donβt share ways in boundary relations and donβt glue (aka share nodes) to the natural features.
Even if the boundary is currently defined by a natural feature, directly or indirectly, it is still less painful to maintain correct borders when they are a separate feature.
Comment from Enock4seth on 28 September 2025 at 12:10
I might have used the same ways for both natural and administrative features but I maintain separate ways for new mappings and also separate existing shared ways.