On Tue, Aug 4, 2020 at 11:24 AM Joseph Eisenberg <joseph.eisenb...@gmail.com> wrote:
> This means that the line tagged with natural=coastline is on the inland > side of all marine water, including mangroves, salt marshes, and tidal > channels, as far as possible. It makes sense that in estuaries, the route > of the ways tagged natural=coastline should also extend up to the limit of > marine influence. In some cases this has been taken to mean the limit of > the tides, in others it is the limit of mixing of salt and fresh water. > I agree that's what the Wiki says. The Wiki says a lot of things. In actual practice, in the estuaries of rivers, the 'coastline' is very seldom tagged that far upstream. I return to the example of the Hudson River. The tidal influence extends upstream to Lock and Dam Number One - 248 km from the river mouth. The salt front varies strongly with the season. There can be fresh water in New York Harbor during the spring snowmelt, or salt water at Poughkeepsie (122 km upriver) in a dry summer. (It's also defined somewhat arbitrarily as a conductivity of 510 microsiemens/metre at the surface - but surface salinity is, in most seasons, higher than the salinity at depth because the cold, fresh river water underlies the relatively warm, brackish surface water.) Needless to say, the biome is very different between Albany (always fresh water) and Yonkers (always salt, except for snowmelt events). Oceangoing vessels of up to 9 m draft can ply the river as far as Albany. (In less xenophobic times, vessels of friendly nations could clear customs at Albany.) For pretty much all the rivers in eastern North America, the tidal influence extends to the first dam or waterfall. This usually coincides with what would be the head of navigation if it were not for modern improvements such as locks. Riverports from Augusta, Maine to Macon, Georgia would become 'coastal' cities. That's surely no more the local understanding on the Kennebec or the Ocmulgee than it is on the Elbe! For the Amazon, the situation is even more extreme - the river is tidal for a thousand kilometres from what would be conventionally recognized as the 'coast'. It appears that for most of the world, this rule, if actually implemented - and it is important to stress that it is NOT the way things are mapped at present - would extend the 'coastline' for tens or hundreds of km upstream on most of the first-order rivers of the world. Given the fact that even with today's definition, we frequently go for months without a consistent coastline to give to the renderer, do we want to add tens of thousands more kilometres of 'natural=coastline'? We'd never see a coastline update again! (For this reason, I'm inclined to push the 'coastline' as far toward the sea as sensibly possible, to have as little 'coastline' as possible to get broken, rather than going for months without updates or worse, seeing rendering accidents flood whole continents.) Moreover, I'm somewhat puzzled at Christoph's insistence that 'natural=coastline' have a strict physical definition, and dismiss local understanding as merely political and cultural. In almost all other aspects of OSM, the understanding of the locals is what governs. That understanding is, ipso facto, cultural - but we dismiss it at our peril. Ignoring local understanding is a path to irrelevance. (In another OSM domain, I've seen this sort of nonsense before; I've actually seen someone seriously suggest that a peak should not have its name in OSM unless someone can find a sign with the name on it, because asking locals and consulting reference works is not 'verifiability in the field.') -- 73 de ke9tv/2, Kevin
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