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Hi Edward,
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I have actually been quietly working on a new page about UK Boundaries: <a href="https://wiki.openstreetmap.org/wiki/User:Csmale/ukboundaries">https://wiki.openstreetmap.org/wiki/User:Csmale/ukboundaries</a>
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Please take a look; would you let me know what you think?
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Regarding what you see in Norfolk: The OS provides (in Boundary-Line) both admin boundaries, which (with a few exceptions) align to MHWS, and explicit MHWS data, which in general corresponds to the OSM definition of "coastline," although we like to cut across estuaries in spite of the actual MHW line sometimes going far inland. It is logically quite possible that these flats dry on certain extreme tides (LAT), but are still covered at MLWS. Also these sandbanks shift around, so what dried at low water some time ago may never dry today. Aerial imagery is only a snapshot and we don't know at what state of the tide the image was taken; as always, we need to exercise care when we interpret these images. In addition, the OS excludes certain small rocks and islands. This is from their Boundary-Line user guide:
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<em>The following rules govern the representation in Boundary-Line of offshore islands and rocks in the sea:</em>
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<em>• offshore rocks and islands will be shown if 0.4 hectares or more in area at high tide;</em>
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<em>• offshore islands with buildings will always be shown, irrespective of size;</em>
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<em>Offshore islands or rocks will not be shown if they are:</em>
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<em>• beyond the defined extent of the realm, irrespective of size; or</em>
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<em>• covered at high tide.</em>
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<em>If an offshore island or rock is shown, it will be contained within all the relevant administrative unit collections. </em>
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I try to curate the admin boundaries, and I take OS Boundary-Line as my primary source. On occasions I will update the coastline using the OS data as well, but the vast majority of coastline data is still tagged as source=PGS. Logically, the admin boundary cannot be landward of the coastline (although they can be coincident). Where the old PGS coastline clashes badly with the more recent MLWS data, I sometimes update the coastline to make the data internally consistent.
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Coming back to your link about Udal Law in Orkney and Shetland: it looks like that is about ownership and title, not government jurisdiction. These are different concepts; you might own your garden, but you may need permission from your council if you want to build on it.
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Regards,
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Colin
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On 08/19/2021 8:40 AM Edward Bainton <bainton.ete@gmail.com> wrote:
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Thanks, Colin - that's interesting.
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I'm more familiar with the Wash, but there the shapes look much less convoluted: off Lewis there are multiple islands of jurisdiction where shoals must appear, but <a href="https://www.openstreetmap.org/?mlat=52.8578&mlon=0.3591#map=12/52.8578/0.3591&layers=N">in Norfolk</a> the jurisdiction is shown only as continuous with the mainland, despite some quite large shoals nearby.
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Do you know if there's a wiki article on this? I've found <a href="https://wiki.openstreetmap.org/wiki/United_Kingdom_boundaries">https://wiki.openstreetmap.org/wiki/United_Kingdom_boundaries</a> which declares itself out of date. There's also <a href="https://wiki.openstreetmap.org/wiki/United_Kingdom#Documentation">https://wiki.openstreetmap.org/wiki/United_Kingdom#Documentation</a> that suggests the page would be <a href="https://wiki.openstreetmap.org/wiki/Boundaries_in_the_United_Kingdom">https://wiki.openstreetmap.org/wiki/Boundaries_in_the_United_Kingdom</a> and that's empty, too.
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If we haven't got a page yet, I might start one.
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On Thu, 19 Aug 2021 at 06:58, Colin Smale <<a href="mailto:colin.smale@xs4all.nl">colin.smale@xs4all.nl</a>> wrote:
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Hi Edward,
<br>That is exactly the reason. Local authority jurisdiction extends to low water (MLW or MLWS), the coastline is defined as high water MHW or MHWS).
<br>In England and Wales I know it's defined as MLWS but I have a niggling memory of the legal definition being subtly different in Scotland.
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<br>I can't recall seeing LAT begging used outside of a shipping context before, but I will be sure to look into your link, thanks!
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<br>Regards, Colin
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On 19 August 2021 00:11:01 CEST, Edward Bainton <<a target="_blank" href="mailto:bainton.ete@gmail.com" rel="noopener">bainton.ete@gmail.com</a>> wrote:
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Hi all
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I've just seen that the boundary of Comhairle nan Eilean Sar ("Council of the Western Isles"?) is a very odd shape:
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<a target="_blank" href="https://www.openstreetmap.org/?mlat=57.5057&mlon=-7.3120#map=13/57.5056/-7.3121&layers=N" rel="noopener">https://www.openstreetmap.org/?mlat=57.5057&mlon=-7.3120#map=13/57.5056/-7.3121&layers=N</a>
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Is there a simple explanation? Is the local authority boundary line Mean Low Water Springs, perhaps, and the OSM coastline is MHWS?
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(I don't know if it's relevant, but <a target="_blank" href="https://en.wikipedia.org/wiki/Udal_law" rel="noopener">Udal law</a> in Orkney and Shetland law has lowest astronomical tide as the limit of land ownership. Perhaps there's a Norse influence here, too? But I'm guessing probably more prosaic.)
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Thanks
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