[Tagging] Coastline for rivers, estuaries and mangroves?

Joseph Eisenberg joseph.eisenberg at gmail.com
Thu Sep 13 07:40:17 UTC 2018

Re: "Data consumers have a right to being able to interpret the data as the
mapper intended"

For data users, it would be most useful if the coastline is in a consistent
position in relation to the sea and land, clearly.

In the past, it was decided that the coastline would represent the high
tide line, and the first OSM mappers generally put the coastline up at the
tidal limit of rivers (which were easy to verify for them, because there is
usually a dam or weir at that location in England).

But perhaps we need to start adding second line to represent the average
low tide, to define the intertidal zone. Right now the only way to see if
an area is in the intertidal zone is if a natural area has been tagged
outside of the coastline. This works for shoals, beaches and wetlands, but
it's a little ambiguous. If we start mapping the low tide line, this will
clearly show the interidal zone. This outer line could also be defined to
cut across rivers and estuaries at the mouth, similar to how political
baselines are defined (however, it would not be arbitarily defined like the
baseline). The part of the river between the OSM coastline and the "low
tide line" would represent an estuary.

Thinking about the limits of the intertidal zone would make it clear that
the high tide line (=coastline in OSM) should not be a tangent across the
mouth of an estuary. A brackish estuary is a boundary environment between
marine and fresh water, just like the intertidal zone is transitional
between land and sea.

The high tide line or coastline then would define the largest extent of the
marine environment and the minimum extent of land plus inland waters. The
low tide line could, in contrast, show the smallest extent of the marine
environment and the largest possible definition of inland waters.

In this case, I would think it would make sense to start mapping water
areas for estuaries. Even though the water would be outside the coastline
(and therefore already could be rendered with no problems), mapping the
area of estuaries and intertidal waters would clearly define the size of
estuaries and the extent of this zone.

I know that in the past, others have said that defining estuaries would
just create 2 limit problems: you would have to decide where the estuary
turns into river and where it turns into sea, while setting the coastline
is just one decision. But this would be a compromise that lets everyone get
their preferred data, and allows 3 color of lakes / rivers / sea. In fact,
a renderer could choose to render estuaries the same as river, or the same
as sea, or even as a gradient between the two, because they would be
defined. Right now, with varying locations of the coast it's not feasible
to render rivers and sea differently with OSM data, and it's not possible
to measure the size of estuaries directly.

Would these advantages be worth the extra work of tracing a second line
along the coast?

Joseph Eisenberg

On Tue, Sep 11, 2018 at 4:34 PM Colin Smale <colin.smale at xs4all.nl> wrote:

> On 2018-09-11 08:27, Graeme Fitzpatrick wrote:
> We will need to be a little pragmatic, because OSM mappers are never going
> to be able to do a proper survey of the coastline
> I agree, but we also can't easily say where the tidal limit reaches?
> In most cases the state mapping or hydrography agency will know. They have
> the gear, the knowledge and the mandate to make that determination.
> but that is a separate issue to the COASTLINE discussion.
> Maybe, but personally, I still think that the river banks shouldn't be
> marked as coastline, & that the coastline should cut across the river at
> the coast, so I guess we may agree to continue disagreeing :-)
> I guess so, but what is at stake here is not getting you or me to change
> our minds, but to define what the word means in an OSM context. Mappers may
> also disagree about the definition of "highway" (including or excluding the
> grassy bits?) but IMHO data consumers have a right to being able to
> interpret the data as the mapper intended. If different mappers use a tag
> in differing ways, how is the consumer to know the intention? Having
> differing conventions for each country is just about doable, but if
> individual mappers all have their own definitions, the data becomes less
> valuable. There is much discussion and debate about selected tagging
> topics, but the only thing that really counts is the result, conclusion,
> consensus etc that should come out of it. Unfortunately it rarely does, and
> that saddens me.  OSM is broadening its reach to more and more parts of the
> world, and that is good, but there needs to be equal effort put in to the
> depth of data and the quality (consistency) of the data.
> Cheers,
> Colin
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