[Tagging] coastline v. water

Andy Townsend ajt1047 at gmail.com
Wed Nov 25 16:46:58 UTC 2020


Recently, Chesapeake Bay (the largest estuary in the United States with a surface area of over 10,000 sqkm) has been changed from "natural=coastline" tagging to form a large "natural=water;water=lagoon" multipolygon instead. The area has also been split into the bay itself, the Pocomore Sound, the Tangier Sound, and other smaller bodies.

Current coastline:https://overpass-turbo.eu/s/10xZ  (zoom in or out and re-query as desired).

Previous coastline as of 2020-06-01:https://overpass-turbo.eu/s/10y1  (again, zoom in or out and rerun the query).

As a consequence, the world-wide coastline processing is stuck. Discussions have happened here on this list, as well as on talk-us and on the changeset itself:https://www.openstreetmap.org/changeset/94093155

Among the reasons for this change, the following have been mentioned:

* polygon allows better labelling
* polygon allows better geocoding for points on bay water surface
* bay is not really "sea" hence coastline is incorrect
* natural=water tagging allows for quicker turnaround times to see your edits on the map
* local mappers should decide how they want stuff tagged

Opponents of the change have said, among other things:

* natural=coastline does not mean "literal" coastline
* a major change like this should be discussed thoroughly before executing
* large polygon hampers editing+QA
* boundaries between water polygons, or between water polygons and sea, are arbitrary and not verifiable

The possible solutions to this issue are:

* accept current situation as correct and resume world-wide coastline processing based on this as a new "known good" state
* revert the change wholesale and request prior discussion and consensus in the community
* any mixture of the above

Following internal discussion within the DWG, we propose the following:

* the polygons that have been created will not be removed

* the land-side members of the polygons for Chesapeake Bay, Tangier Sound, Pocomore Sound and potentially others that have been created as part of this  operation will be given back their natural=coastline tags

In addition, currentlyhttps://www.openstreetmap.org/relation/11884052  is mapped as "natural=water; water=lagoon" which does not match the wiki definition athttps://wiki.openstreetmap.org/wiki/Tag:water=lagoon  .  Perhapshttps://wiki.openstreetmap.org/wiki/Tag:natural%3Dbay  would be a better tag?  This is of course an entirely separate discussion to where the OSM concept "natural=coastline" should go - we don't propose to change the tagging on relation
11884052 now but it probably does need looking at.

This is not intended as a definitive solution for all times, just as a stop-gap measure until a consensus is found and, once it has, tools have been amended where necessary. Future community discussion may still lead to the removal of the coastline tagging, or to the removal of the polygons and their replacement by a label point.  For now we're just trying to get to a place where other people around the world can make valid coastline edits and see their changes go live.  The current impasse over Chesapeake Bay is currently stopping that.

Are the local mappers willing to help implement this? If not, the DWG will do it so that normal coastline processing elsewhere can resume.  We apologise in advance to anyone who thinks that this is an incorrect decision, but unfortunately sometimes a decision between one of two outcomes (neither of which is universally popular) has to be made.  In such cases the DWG often reverts to the "status quo ante", and we think that makes sense here too.

Best Regards,

Andy Townsend, on behalf of the Data Working Group

On 18/11/2020 20:19, Eric H. Christensen via Tagging wrote:
> After a few days of much work, a recent collaborative project to turn the Chesapeake Bay from a nothing space outlined by natural=coastline to what we considered to be a more accurate relation of natural=water, we've received some negative feedback.
> The difference of opinion seems to lie in the definition of what we're mapping.  The use of coastline is for "seas"[0] while the use of water is for "inland areas of water"[1].  Even though the Chesapeake Bay is tidal, there is no question that it is an inland waterway (it is completely surrounded by land except for the mouth at its southeast side).  The idea of using coastlines for basically creating an edge between the land and the nothingness of the ocean makes sense when, as far as the eye can see it's only water.
> Now, some of the feedback that has been presented[2] is that because it is tidal it is part of the sea.  I have pointed out that many rivers and streams (and ditches!) are tidal; does that make them part of the sea?  I would not think so.  In fact, there are named seas on this planet that are not even connected to other water formations (the tiniest, according to the National Geographic, is the Sea of Marmara which has an area just less than 12,950 sq km, larger than the Chesapeake Bay).
> But, tagging the Chesapeake Bay, and its tributaries, as "water" brings several benefits to the map and the users.  First, it helps identify the sections of water that exist in these areas (this can't really be done with node points as there is no way to define start and end points of an area).  There are many defined bays, rivers, and streams that make up the greater Chesapeake Bay area.  What one may see as one large mass of water is actually many smaller defined segments each with their own history.  Second, we can speed up any updates (fixes) to outlines of the polygons that happen in these water areas without having to wait for the entire Earth's coastlines to be re-rendered.  I suspect having less coastline to render would also speed up the rendering of coastlines as well?
> I would like for the tagging community to clarify the different between "water" and "coastline" and when to use each.  The definition on water seems to say to use it on inland water but there seems to be, at least, and open interpretation of the word "sea" for coastline that is dragging many inland waters into that category.
> Thanks,
> Eric "Sparks" Christensen
> [0] https://wiki.openstreetmap.org/wiki/Tag:natural%3Dcoastline
> [1] https://wiki.openstreetmap.org/wiki/Tag:natural%3Dwater
> [2] https://www.openstreetmap.org/changeset/94093155#map=10/37.1620/-76.1581
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