[Tagging] Coastline for rivers, estuaries and mangroves?

Kevin Kenny kevin.b.kenny at gmail.com
Mon Sep 3 18:08:45 UTC 2018

Imagico's proposal is perhaps objective, but surely doesn't match
perception in my part of the world. It seems odd that the 'coastline'
must extend upward to https://www.openstreetmap.org/way/90929525 - but
that is, according to Imagico's definitions, simultaneously the lowest
and highest permissible limit. The locals would be astonished to call
the whole lower Hudson River 'ocean,' even though it is tidal.
Salinity varies; the point at which chloride concentration of 100 mg/l
is observed varies seasonally by over 100 km. There is an observable
salinity gradient through the entire estuary, but anywhere north of,
say, Hyde Park is 'fresh' water by any reasonable definition, even in
a dry summer. Is it reasonable to draw 'coastline' on fresh water?
On Mon, Sep 3, 2018 at 12:05 PM Christoph Hormann <osm at imagico.de> wrote:
> 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
> https://www.openstreetmap.org/relation/3111444
> 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 -
> see:
> http://www.imagico.de/map/coastline_quality4_en.php
> 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
> geometrically).
> > 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):
> https://www.openstreetmap.org/way/463191729
> https://www.openstreetmap.org/way/474230093
> --
> Christoph Hormann
> http://www.imagico.de/
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