[Tagging] Coastline for rivers, estuaries and mangroves?

Joseph Eisenberg joseph.eisenberg at gmail.com
Fri Sep 7 03:02:26 UTC 2018

I've now edited the coastline in the area mentioned. I have now added
natural=coastline along all the ways forming the edge of the mangroves and
open water.

Further west, I moved the administrative boundary off of the coastline of
internal waterways, positioning it near the low water line / baseline,
because I believe this is closer to the official Indonesian definition for
Kabupaten (admin level 6) boundaries, and it no longer creates separate
polygons around each patch of mangroves.

This brought up another issue. I did not want to delete the natural=water
areas, so I changed them to multipolygons (since I had to break the closed
ways to make a proper coastline) and marked salt=yes, removing
natural=river from the areas that might better be described as tidal
channels. I considered using water=tidal or water=salt, but both of these
tags seem to have limited use and an unclear definition; JOSM suggested

But I am uncertain what to do with the waterway=river in the case of tidal
channels and the complex connections between rivers in these mangrove
areas. A search of taginfo did not find an alternative tag, although
river=tidal is in use. I think there should still be a waterway midline for
the large tidal channels in the mangroves which can be used by boats or
even ships, to help navigation software. (Many of these channels were
actually created by flowing river water; the rivers in this area meander
strongly and often change the location of the mouth, as can be seen by
comparing the current situation to 100 year old Dutch maps)

Perhaps waterway=river with tidal=yes or river=tidal is the best option to
prevent tag fragmentation? Or is river=tidal_channel preferable? The
problem is determining the direction of water flow when two channels
connect. Besides tidal, is there a better tag to imply two-way water flow
depending on the tidal cycle?


> Message: 3
> Date: Wed, 5 Sep 2018 18:03:36 +0200
> From: Christoph Hormann <osm at imagico.de>
> To: "Tag discussion, strategy and related tools"
>         <tagging at openstreetmap.org>
> Subject: Re: [Tagging] Coastline for rivers, estuaries and mangroves?
> Message-ID: <201809051803.36467.osm at imagico.de>
> Content-Type: text/plain;  charset="utf-8"
> 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:
> https://www.openstreetmap.org/way/216482240
> 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
> own.
> > 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:
> https://www.openstreetmap.org/way/614052686
> 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
> https://www.openstreetmap.org/relation/7301266
> 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
> http://www.imagico.de/
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