[Talk-GB] UK coastline data

Borbus borbus at gmail.com
Thu Jul 11 20:45:37 UTC 2019

On Thu, Jul 11, 2019 at 9:19 PM Colin Smale <colin.smale at xs4all.nl> wrote:
> I would recommend you don't refer to "the two coastlines" as this will
> just lead to confusion. The one true coastline is the high water line,
> taken to be MHWS (in England and Wales). The low water mark is also
> useful because that is where the jurisdiction of local authorities
> normally ends.

Oh yeah, the reason I wrote about the "other coastline" was because I
think it sometimes does cause confusion. In the area I was looking the
mapped coastline was sometimes the MHW, sometimes the MLW, and sometimes
it was mapped at the sea wall which in that case would be an
exceptionally high tide or storm surge. But yes, the coastline should
only be the MHW.

> * Coastal admin boundaries (the "Extent of the Realm") are usually MLWS,
>   but there are such things as "seaward extensions" which extend the
>   "realm" further into the water. Check out for example Brighton Marina,
>   Torbay, City of Bristol.

I have noticed the boundaries often correspond with MLW. I have tried to
leave the boundaries alone even when they overlap with the MLW because I
thought combining them might be confusing.

> * Where the "coastline" crosses the mouth of a river or estuary, there
>   has been lots of discussion about this in the past, as usual without a
>   clear definitive verdict. The OS data will take you upstream to the
>   tidal limit of rivers, which sometimes gives results which some people
>   find undesirable. Example: River Dart in Devon.

Yes, this was something I meant to ask as well. Often the coastlines
cross the rivers at completely arbitrary points. Thinking about it too
much brings up the famous coastline paradox. Mapping it right back to
the tidal limit does seem like the only way that isn't arbitrary. The
Dart cuts the coastline off right at the mouth, which doesn't seem right
at all to me. It would be good to be consistent.

> * The OS MHWS data will also place tidal inlets outside the coastline;
>   there is a proposal/vote underway which seems to confirm this, but
>   existing data might not:

Yes, it does on the data I've been looking at. But this seems correct to
me for the same reason as the tidal extent of rivers.

> * My personal opinion is that the OS data is likely to be professionally
>   curated, and is probably the most accurate source we are ever going to
>   get. In many places you might conclude that it is wrong, when
>   comparing it to aerial imagery. However we will never know the tidal
>   conditions at the time of the imagery. The coastline, and the
>   low-water mark more so, is subject to change over the course of time,
>   and OS doesn't resurvey coastal boundaries very often (although they
>   seem to do it every few years). I would recommend adding the date of
>   the OS data to the OSM coastline, to aid future updates.

Yes, indeed. I regret not adding the version to the data I imported. I
suppose it could be determined from the date it was added to OSM. It
should be quite easy to keep it up to date, though. The "replace
geometry" plugin in JOSM is very useful for this.

Happy mapping,

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openstreetmap.org/pipermail/talk-gb/attachments/20190711/ffb0f783/attachment.html>

More information about the Talk-GB mailing list