[Talk-GB] UK coastline data

Colin Smale colin.smale at xs4all.nl
Thu Jul 11 20:19:31 UTC 2019



Don't worry about having "too many nodes" - the OS data is already
generalised a bit (I think they target 1:10000) so it could be a lot
"worse". I spend a lot of time curating the admin boundaries;
occasionally I will update a bit of coastline from OS data when I am "in
the area". 

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. 

Danger lurks in a few areas: 

* 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. 

* Where the coastline is essentially vertical (harbour walls, steep
cliffs) MHWS and MLWS can coincide in OS data (sharing nodes but not
ways), but of course low water can never be landward of high water.
Structures like piers that are built out above the water can fall
outside of the low water line, and therefore also outside the admin
boundary. It is what it is. 

* 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. 

* 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:

* 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.



On 2019-07-11 21:38, Borbus wrote:

> Hi, 
> I've recently done an import of coastline data from OS VectorMap into OSM around The Wash. I did this because I'm interested in coastal regions and the coastline was a complete mess in that area. I'm sure it's similar in other parts of GB as well. 
> The mess often happens because mappers don't necessarily know what a "coastline" is (I didn't before I researched it). For land-based maps the coastline that is shown is generally shown is mean high water level. The other "coastline" that is also shown on land-based maps is the mean lower water level. The bit between these lines is the intertidal zone. This is admittedly a bit less interesting, but it's certainly useful when there are causeways and other features in the intertidal zone. The actual high and low tides can be higher or lower than the means. The tide varies throughout the month and the highest highs and lowest lows are called spring tides. Nautical charts will show the lowest low, not mean low. 
> This seems like quite difficult data to obtain so using OS seems to be the obvious choice here. I'm pleased with how the import went in The Wash. It integrated well with the existing OSM data around the coastline. It's certainly a lot easier to integrate than groundwater but it does require a lot of manual processing. 
> But before I start importing other areas (I'm looking at the Blackwater estuary next), I want to discuss it with others because I'm concerned that the way I've done it could negatively impact other mappers. 
> The data as it comes is essentially the two coastlines as described above: MHW and MLW. The MHW can just replace the existing coastline in OSM. It adds many, many more nodes to the coastlines, and possibly more ways too. The MLW along with MHW then can form multipolygons containing the intertidal zone, which is mapped as a wetland=tidalflat. 
> Using the coastline to make multipolygons means the coastline is broken up into many, many small ways. One concern is that the GB island multipolygon will become very hard to maintain. On my computer JOSM is very slow to operate when I load this multipolygon. 
> So before I continue I'd like to give people the chance to tell me to stop and, if necessary, suggest a better way to do this import. Or maybe people wouldn't like to see this import done at all. Personally I think there is value in integrating the data but some may disagree. 
> Happy mapping, 
> Borbus. 
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