[Tagging] Coastline for rivers, estuaries and mangroves?

Colin Smale colin.smale at xs4all.nl
Mon Sep 3 20:46:55 UTC 2018

Just a reminder that we need a pragmatic, practical definition for OSM.
It has to be either verifiable in situ, preferably in a single visit and
without specialist equipment, knowledge or access, or it needs to be
derivable from openly accessible (and suitably licensed) sources. A
discussion whereby a hundred people contribute their subjective opinions
is unlikely to lead to a *durable* solution for OSM. In this case I
would suggest it would be impossible for a mapper to survey their own
coastline position; it is best left to hydrographers and/or
cartographers to provide the algorithm by which one can define the
correct "coastline". Then we find an open source of this data for our
regions. If this source does not exist, we approximate (based on the
chosen algorithm) until such time as open data is available. 

The nicest thing about standards is of course the wide choice
available.... Which standard do we adopt? Is it not possible that there
are multiple possibilities for the definition of "coastline", and which
one is best for a given use case, can vary according to the wishes of
the party consuming the data? I.e. if we preselect a specific
definition, are we implicitly and unintentionally blocking out other
definitions from representation in OSM, possibly leading to accusations
of "tagging for the renderer," being the single use case which uses the
chosen definition? 

One thing I think does have consensus - the coastline is based on the
High Water Mark (and not Low Water or mid-tide or any other point in the
tidal cycle). This in itself is impossible for a "simple mapper" to
define with any accuracy, so we will have to trust external sources. 

What is unclear, is of course where do we "draw the line", literally and
figuratively, in the case of indented coastlines and river estuaries.
Tidal limit has the advantage of being artificial (dam/weir etc) and
therefore uncontroversial, or at least in most cases readily available,
even if it is just from "common knowledge". So which standard/algorithm
would give the pragmatic, practical definition OSM needs?

On 2018-09-03 21:32, Kevin Kenny wrote:

> It would certainly need to be above Haverstraw - the current there
> http://tbone.biol.sc.edu/tide/tideshow.cgi?site=Haverstraw+%28Hudson+River%29%2C+New+York+Current
> shows significant tidal reversal.  I haven't found a gaging station
> farther upriver that reports tidal currents. Croton Point, where the
> river broadens to form the Tappan Zee, would probably be the lower
> limit. Even that seems unreasonably far upriver.
> The tidal range increases as you move upstream from there. The
> greatest tidal range in the entire river is at Troy. One Native
> American name for the river was "Mahicantuck" which means, more or
> less, "the river flows both ways."
> The estuarine situation will always be hard to deal with, and I think
> we'll simply need to have rough guidelines and then trust the judgment
> of the locals.
> On Mon, Sep 3, 2018 at 2:51 PM Christoph Hormann <osm at imagico.de> wrote: 
> On Monday 03 September 2018, Kevin Kenny wrote: 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. [...] 
> Then you have misunderstood the proposal.
> With the Hudson river obviously the tidal case applies so you have the
> lower limit as:
> With significant tides the coastline should go upstream at least to a
> point where on waterflow is going downstream for a significantly longer
> part of the tidal cycle than it goes upstream due to raising tide.
> This is evidently always below the upper limit (range of tidal
> influence).
> I can't say for sure where i would place the lower limit in case of the
> Hudson - The Narrows is quite definitely too low - but the current
> closure seems fine.
> For low volume tidal rivers (i.e. without a salt wedge and no
> significant influence of the water volume on the ocean salinity) it
> would also be possible to define the lower limit through salinity (not
> in absolute terms but as a fraction of the open ocean salinity in the
> area).
> --
> Christoph Hormann
> http://www.imagico.de/
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