Coastline confusion

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Coastline confusion

Sean Blanchflower
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:

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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Re: Coastline confusion

Colin Smale

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:
 
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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Re: Coastline confusion

Tom Hughes-3
On 03/11/2018 15:50, Colin Smale wrote:

> 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.

https://wiki.openstreetmap.org/wiki/Tag:source%3DPGS

Tom

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