Hi Sean,
The coastline ways with source=PGS are really old and inaccurate. I am
not sure of their exact provenance but I think they were traced from
some primitive aerial imagery.
HWM data is available as part of OS Boundary-Line. It shows the "island"
("Inner Trial Bank") is (just) within the coastline, so it is surrounded
by (I guess) salt marsh which in this case is above the HWM (i.e. it
does not completely flood). On this basis the "island" should not be
tagged as natural=coastline.
Having said that, because of the nature of the geography of the area it
is possible for coastlines to move, and they are only resurveyed every
few years. So the age of the data source can also be very relevant.
But I see your point about the OS maps, which seem a little
inconsistent. If you zoom in, the foreshore becomes visible, and this
island is indeed depicted as an island. Zoom out and the foreshore is
flooded, including this island.
Colin
On 2018-11-03 16:18, Sean Blanchflower wrote:
> Hi all,
> I've been contacted by an OSM user confused about what to do on the coast of
> The Wash.
>
> Basically the two ways:
> https://www.openstreetmap.org/way/2961222
> https://www.openstreetmap.org/way/35834999
>
> are both natural=coastline there, but the former, an "island" is inside the
> latter so can't really be coastline.
>
> However, the latter (the main coastline) in this area is inaccurate, and
> should really follow the Mean High Water level more precisely.
>
> i) The MHW on OS OpenData StreetView runs around the 'island' to the north,
> meaning the island should be removed, or at least relabelled...
> ii) ...but the 'coastline' on OS LandRanger maps (which should also be the
> MHW, shouldn't it?) runs well to the south of the 'island'.
>
> So I'm confused. I think it's clear that the main coastline needs refining,
> but it's not clear where to.
>
> S.
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