[Tagging] natural=bay as nodes are evil

[Eric Kidd]

When working near the coast of Maine in the US, I  see lots of bays. In
most cases, the ultimate source data for the bay names seems to be various
government maps and databases: GNIS, ancient nautical charts, or whatever.
There's a high degree of agreement between sources: If an island has 4
unnamed coves and 1 named cove in GNIS, then I'll usually find the exact
same data on the nautical charts.

But the key point here is that none of these official sources represent
bays as polygons. GNIS uses a pointssomewhere in the bay. The nautical
charts print the name somewhere in the middle of the bay. Effectively, the
official data really is a point, plus whatever guesswork a human reader
supplies.

The rendering on openstreetmap.org is pretty good: it just prints the bay
name at the marked point, and shows it across a reasonable range of scales.
There are some weird cases with nested bays, but those are weird on the
nautical charts, too.

Merging all of these thousands of official "bay" points into the
surrounding coastal polygons sounds like an editing nightmare. And the data
wouldn't be better—the underlying official sources are all points, anyway.

2014-10-27 16:04 GMT-04:00 Ilpo Järvinen <ilpo.jarvinen at helsinki.fi>:

> On Mon, 27 Oct 2014, Martin Koppenhoefer wrote:
>
> >
> > 2014-10-27 17:37 GMT+01:00 Christoph Hormann <chris_hormann at gmx.de>:
> >
> >       But this is exactly what does not work with a hand mapped
> >       polygon either
> >       since the edge of the bay is not well defined.
> >
> >
> >
> > it will work in most cases, and only give questionable information when
> you
> > are close to the fuzzy end towards the open sea (or another bay). In
> these
> > cases there won't be a correct answer from a human either, because it
> simply
> > isn't clear where that border is.
>
> IMHO, the most controversial thing in this all is that the approach
> Christoph is proposing would require us to not map natural=bay but
> "natural=bay_entry" instead, and that is obviously exactly where the fuzzy
> part is. That is, a mapper would be forced to place bay nodes into the
> place where nobody can say for sure if it's in the bay or not.
>
> Otherwise his algorithm would obviously end up failing because of arbitary
> picked threshold that makes lots of assumptions about the shape of the
> bay. The main assumptions are about width of the bay and depth at the
> nearest coastline that is not in the either extreme of the bay. Consider
> e.g. a very wide bay which has two pockets but at the middle you have some
> penisula extending towards the bay entry and thus also towards the bay
> node. But it would certainly work in many cases just fine like most of
> the algorithms tend to do (of course, assuming we'd map bay_entry instead
> of bay).
>
> --
>  i.
>
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