Hi Joseph,
thanks for bringing up this issue up here. As one of the parties involved, I'll
try to sort out the several issues I see regarding the coastline and then try
to answer your questions. Sorry, for the long post, but I believe there is
quite some context needed to understand the relevant issues here.
1. Where is the coastline?
You can have lots of different definitions of what a "coastline" is and based
on those definitions the coastline is in different places. That's okay, because
OSM has free tagging. Everybody can invent new tags and you can tag the
coastline according to definition X here and according to definition Y there.
When OSM started, people looked for a reasonable easy to apply and useful
definition that would work everywhere. And of course we didn't invent this
definition from scratch, but we oriented ourselves on what other people and
other maps have been doing for a long time.
You have basically three kinds of things involved here: The "sea", the "land",
and "inland water". I think I don't have to explain what the difference between
"land" and the other categories are, you just have to figure out where to draw
the line (OSM uses mean high water line), but thats essentially just details.
The difference between "sea" and "inland water" is a bit more fuzzy, but not
really that complicated. "Sea" is usually salty and usually tidal, "inland
water" is usually fresh water and not tidal. On river mouths you have to decide
where to draw the line and that's something local mappers can decide based on
local discussions and sometimes it comes down to their gut feeling whether the
water area is more river-like or more sea-like. There is a lof of room here to
discuss how to interpret the rules in a specific case, but that doesn't change
the rules themselves.
2. Why is it important to agree on one definition?
The definition is not something which can be decided locally, because
coastlines are a worldwide phenomenon and they show up on basically all maps.
Most maps, the first thing they do is draw the coastline (or the land or sea
area, respectively). On small zoom levels where you don't see much detail, the
one thing that you always see is the coastline. And when large water areas are
not tagged as coastline, you see the difference. I don't particularly care
whether you put the line between the sea and the river a few meters or
kilometers up or down the river. That's certainly something where we are in
that fuzzy area which local mappers can agree on and decide. But the bay at the
mouth of the Rio de la Plata and the Chesapeake Bay are way beyond anything we
can see as inland water by the agreed upon definition. And they are both
clearly visible on a world map.
This is not only important for maps, but even more so for other uses. If you
create statistics based on how many square kilometers of sea there is or
something like it, you don't want this definition to change under you.
After I started a discussion of the problems with the recent coastline changes
in the Chesapeake Bay (see https://www.openstreetmap.org/changeset/94093155)
these points have also been discussed on the tagging mailing lists (starting
here:
https://lists.openstreetmap.org/pipermail/tagging/2020-November/056310.html)
and I believe my opinion here agrees with what everybody said their in answer
to Erics posts.
3. Why is it so difficult to change that definition?
OSM has used this definition for a long time. Thousands of maps and other uses
of the data became to depend on this definition. I believe we can not change it
now in any substantial way, because we would break the data for many many uses.
We have free tagging, so we can always amend the data. We do this all the time,
leaving the definition of an existing tag as it is but have extra tags to more
clearly define some details. This doesn't change anything for existing users
and map-creators and other users who want more control over the details can use
the extra tags. So if you want to invent some tags and put them on a
multipolygon for a bay so that the bay can be labelled properly, that's totally
okay in my opinion. You are *adding* to the map that way. But you can't take
away something that has already been agreed on.
In the end there are essential and often used tags, like the coastline or, say,
the "highway" tag, that are by now essentially unchangeable. Unfortunately we
don't have a mechanism in OSM to make larger changes to our data schema in an
ordered and controlled fashion, that would allow these kinds of changes. But
we, as community, do have a responsibility towards the users of OSM to not
"break their world".
4. Coastline Processing
Now we are getting to the stuff Joseph is talking about: There is an extra issue
involved in any discussion of the coastline that makes everything slightly more
complicated and that many newer people might not know about.
The way we process the natural=coastline tag for generating maps, or any other
use, is different
