[Tagging] Coastline for rivers, estuaries and mangroves?

Kevin Kenny kevin.b.kenny at gmail.com
Thu Sep 13 18:42:51 UTC 2018

[Off list, I've had my say on list]

> 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).

That's entirely sensible for Great Britain.

And it matches my local situation, except for where the coastline is drawn
- and for the immense scale of the difference.  The Hudson River (and the
problem repeats itself for rivers such as the Delaware and Susquehanna) has
an extremely long estuary. It's really unclear that the English rule makes
sense for it. That's why I've been arguing for leaving room for a modicum
of judgment on the part of the local mappers.

The Hudson is definitely estuarine, with tidal ranges up to a couple of
metres, for its entire lower reach.  The traditional 'mouth' of the river
is an east-west line from the Battery, the southern tip of Manhattan
Island, and this is labeled as 'mile zero' by the boatmen. The river
continues to be tidal, flowing about six hours onshore and six hours
offshore, all through its lower reach. Even in a dry summer, it's virtually
never 'salt' (defined as 100 mg chloride per litre) in surface water beyond
mile 60 (kilometre 97), although denser salt water persists at depth
farther up. The salt front has not, in living memory, retreated past mile
75 (kilometre 121) - above there, it can be thought of as being perennially
fresh water, although continuing to reverse direction above that point.

As I said, the flow is about equally divided in terms of time, but the flow
rates on ebb and flow vary widely - the river water, after all, does
eventually reach the sea. By Haverstraw (about 60 km from the river mouth)
the ebb runs about twice as fast as the flow, and that's as far upstream as
tidal currents present a navigational hazard. (Canoeists and kayakers had
better be aware of the direction of the tide, since few can make progress
upstream paddling against a Hudson River ebb!)

The river remains navigable by moderately-deep-draught vessels all the way
to Albany. A Super-Panamax or larger ship cannot use Albany as a port of
call, but the Port of Albany does see a fair amount of oceangoing traffic.
It can accommodate a ship 289 metres in length, 33.5 metres in width, and
9.5 metres in draught. Its fixed cranes can lift 225 tonnes, and 1000-tonne
barge-mounted cranes are available. Still, it's unquestionably a riverport
- the draught of a ship has to be de-rated for the fresh water, and it's a
long sail up to there. It's chiefly used for bulk cargo from upstate New
York and neighbouring Canada, and for project cargoes such as heavy
electrical equipment from GE Energy in Schenectady.

Most of the lower reach of the Hudson has very little tidal change to its
spatial extent. It's in a narrow valley between two highlands, and both
banks are cliffy.  (The view across the river from Manhattan is quite
spectacular.) The water moves up and down, but has little room to expand
from side to side.

The tidal limit is the 'Federal Dam' (actually a weir by OSM definitions,
since there's a full-width spillway) in Troy, so just as in the UK, we have
a well defined fall line marking the transition from estuary to true river
- at river mile 153 (246 km).  The locals think that extending the
'coastline' upriver nearly 250 km from what is traditionally regarded as
the river mouth is little short of insane. Even if you were to place the
'mouth' of the Severn somewhere near Cardiff, for instance, a comparable
distance up the river would bring you, at the very least, well past
Shrewsbury and perhaps even to the headwaters. Great Britain simply has no
estuaries of nearly that magnitude.

All this leaves me with no clue, in OSM terms, what to call the thing.
Mostly, I don't call it at all. It might answer. :)

On Thu, Sep 13, 2018 at 3:42 AM Joseph Eisenberg <joseph.eisenberg at gmail.com>

> 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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