[Tagging] admin_level on ways

Colin Smale colin.smale at xs4all.nl
Mon Jan 27 12:02:32 UTC 2020

On 2020-01-27 12:43, Martin Koppenhoefer wrote:

> Am Mo., 27. Jan. 2020 um 11:21 Uhr schrieb Colin Smale <colin.smale at xs4all.nl>:
>> However, practically this leeds to ambiguous situations, where for example admin_level=4 is added to islands and might be misinterpreted as administrative "standalone" level 4 entities (with the island name etc.). While a clear separation of administration and coastline could solve this, it would still mean continuous additional maintenance effort due to duplication of already present information.
> I would like to take this opportunity to point out that admin boundaries and coastline are conceptually and geographically distinct, and should almost never coincide. Admin boundaries are typically at the low-water mark, and sometimes miles off shore, whereas the coastline is defined as the high-water line. 
> While I am aware of this, it is not something that is actually reflected in OSM (at least in my area) and is not something I believe we can realistically distinguish (it may be different where high and low tide are significantly different, but if they are very close, as is the case in the mediterranean, it is hard to map). I would not want to request to be able to distinguish high and low water in order to be able to map administrative boundaries (although if you do use different geometry, it is of course fine).

OSM clearly associates coastline with high water:

If the admin boundaries are very close, or even coincident with high
water, I would expect two ways in OSM, possibly overlaying each other,
possibly sharing nodes. Whether they should actually share nodes is
another discussion; the coincidence of coastline and admin boundary is
not by design, but a consequence of our lack of accurate data. That
would suggest they should not share nodes, so they can be updated
independent of each other. 

What does Italian law say about local government jurisdiction over the
foreshore, between high water and low water? What about around
estuaries, does the admin boundary follow the coastline up to the tidal
limit? Do planning laws apply, for example? I understand the largest
tides in the Med are on the African side, up to 2m. Depending on the
slope of the shore, that could give a substantial area of foreshore.
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