[Tagging] Coastline for rivers, estuaries and mangroves?

Christoph Hormann osm at imagico.de
Wed Sep 5 16:03:36 UTC 2018

On Wednesday 05 September 2018, Joseph Eisenberg wrote:
> Specific examples:
> 1) This changeset on the River Dart in southwest England was the
> source of the Help site question:
> https://www.openstreetmap.org/changeset/61959067

The coastline closure there:


is both below the lower limit of the proposal and below the the range i 
can imagine a meaningful coastline closure rule to allow.

I would however be interested in hearing any universal rule that would 
allow this kind of placement based on physically observable criteria 
and that would maintain the coastline as a meaningful geometry on its 

> It looks like quite a large estuary, much wider than the non-tidal
> part of the river upstream.

That is largely not really an estuary but more of a ria.  I have no data 
for this at hand but you can likely see an abrupt change in the 
elevation profile near Totnes where the submerged section of the former 
river valley starts.  So in this case it would make a lot of sense to 
place the coastline closure near the upper end of the tidal section 
because this is much better defined in terms of physical geography.

> 2) The estuaries and mangrove tidal channels in this area:
> https://www.openstreetmap.org/#map=11/-4.8806/136.9339

Here i likewise see no meaningful motivation for the current coastline 
placement - like here:


Poor image quality in the available sources makes identifying the limit 
of the mangroves difficult, you really need to make use of available 
lower resolution open data images in the area for proper maiing here.  
But you can conclude a few thing from the structure of the network of 
channels.  For example


is quite clearly not a river but a tidal channel (there is no river 
feeding it, it is just draining seawater that has entered during 
raising tide).

> I previously changed the coastline to be closer to the river mouths
> in another section of coast to the southeast, but perhaps I should
> change it back? The whole idea of coastline around mangrove swamps is
> most confusing. I don't think the mangroves should be outside of the
> coastline, but where then should it be?

Common practice is to place the coastline at the outer end of the 
mangrove forest.  This is a pragmatic solution because placing it 
inside the mangrove would be non-verifiable.  Of course mapping the 
mangrove is important for the data to be meaningful in this case.

Christoph Hormann

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