[Tagging] Coastline for rivers, estuaries and mangroves?
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
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.
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
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):
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