[Tagging] Coastline for rivers, estuaries and mangroves?

Christoph Hormann osm at imagico.de
Mon Sep 3 16:04:30 UTC 2018

On Monday 03 September 2018, Joseph Eisenberg wrote:
> In my case, I've been debating whether to change the tagging of the
> coastline in southwestern New Guinea (Papua, Indonesia) where many
> large tropical rivers meet the Arafura sea among mangroves. The heavy
> rainfall in this area means that the rivers have a pronounced
> current. But the water is brackish and certainly tidally-influenced
> far inland. Right now, it seems odd that many patches of mangroves
> are made into "islands" by the use of natural=coastline, though
> locally they would be considered part of the larger landmass of New
> Guinea.

The sitaution at a mangrove coast is slightly different from elsewhere 
because the mangrove forest is typically mapped as land w.r.t. the 
coastline which makes the tidal channels in between look like wide 
rivers - which is however often misleading.

For example


is not a river despite being tagged as a riverbank polygon.

This misunderstanding of the nature of mangrove coasts misinterpreting 
wide tidal channels as large rivers has led for example for some time 
in West Africa to a massive shortening of the coastline and a large 
fraction of virtual closing segments in the total coastline lenth - 


This is now mostly fixed but the lure especially of armchair mappers to 
map this way is still there.

> See: https://www.openstreetmap.org/#map=11/-4.8806/136.9339 ; the
> coastline is quite noticable because it has also been tagged as the
> administrative boundary.

The administrative boundaries are generally a bad place to position the 
coastline because they are typically defined very differently from how 
OSM defines the coastline (and are also mostly very inaccurate 

> What will be most helpful for data users and map renderers? Should
> the coastline extend inland many kilometers, to where the mangroves
> end? This will create a large number of apparent islands, and small
> rivers will be entirely part of the "ocean" beyond the coastline.
> Should it be down the the mouth of the river, to keep the coastline
> as compact as possible?

The coastline should be (a) a meaningful geometry on its own, i.e. the 
virtual parts of it (closing segments at river mouths) should be short 
and (b) be as easy to verify for the mapper as possible.

Technically it is also important that the range of acceptable coastline 
positions is not too large so mappers do not move around the coastline 
a lot just to scratch their personal itch.

> What about huge estuaries like the Saint Lawrence and the Rio De La
> Plata? Should there be a vote on Imagico's proposal, or a new
> proposal?

I would be glad if anyone wants to reactivate the proposal or comes up 
with simpler rules for where to close the coastline.  But IMO a hard 
requirement for this would be that these are physically based rules 
rooted in the observable reality and not based on political or other 
purely abstract considerations.

Some newer examples of problematic closure placements (in addition to 
the ones in the proposal):


Christoph Hormann

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