[Talk-GB] UK coastline data

Colin Smale colin.smale at xs4all.nl
Sun Jul 14 09:59:22 UTC 2019

On 2019-07-14 08:49, Mark Goodge wrote:

> On 14/07/2019 00:39, David Woolley wrote: On 13/07/2019 22:21, Colin Smale wrote: So what was your point again about internal waterways? The "extent of the realm" is not the 12-mile limit, it is ±MLW, isn't it? 
> Assuming it is mapped correctly, this is an example of an administrative boundary that is outside the low water mark: <https://www.openstreetmap.org/way/174971215>

Yes, and that's probably a good example of where "the coast" crosses an
estuary rather than continuing up it.

After all, if MLW was always the admin boundary, then most of the Thames
through London would be outside local government control. In reality, of
course, it's part of the GLA and partitioned between various London
Boroughs. Pragmatically, admin boundaries cross the MLW where
appropriate to maintain meaningful local government areas.

I don't know if there is an official formula for when admin boundaries
do actually do this. Looking at boundary maps, it appears to be the
principle that if opposite banks of the estuary are close enough, the
admin boundaries cross the estuary at that point and then run up the
centre (or thereabouts) of the river if the river itself is a boundary
(which it often is). But I don't know what amounts to "close enough". On
the Thames, it's just to the west of Southend, as illustrated in the
above link. On the  Severn, it's just downstream of the Severn Bridge.
On the Humber, it crosses from Spurn Point. But admin boundaries don't
cross The Wash, and in Scotland admin boundaries don't cross the Forth
until upstream of Kincardine. So it seems to be based on what is locally
appropriate rather than a rigid measurement. Which is something you
can't map simply by observation; you have to know what the actual
consensus is. 

It's a government decision (within certain rules), but I am still
searching for the legislation that defines these specific lines. I am
working on the assumption that they are somehow connected with the
baselines used to delineate areas of Internal Waters and Territorial
Sea, but I am not entirely confident. 

This transposes the UNCLOS rules into UK legislation, but does not give
the actual locations of the Baseline: 

This shows the limit of the Territorial Sea, which, interestingly, does
not always correspond to the OSM data: 

This shows graphically where the straight baselines have been applied: 

I note that the limits are much further seaward than I was expecting; in
the Thames estuary the line is roughly from Herne Bay to Clacton, for
example. This doesn't align with any admin boundaries I know of.
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