UK coastline data

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UK coastline data

Borbus
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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Re: UK coastline data

Ed Loach-2
You'll probably get comments about import guidelines but I did similar for Tendring about 9 years ago before there were any. I think your use of the word import in this scenario may be misleading as you're not bulk importing the whole coastline but selectively improving sections of coastline by manually improving existing data by using a small subset of available opendata.

If you're using JOSM you can remove excess nodes (which I didn't know at the time and have tried to clear up a bit since).

Coastlines take some care when editing so you don't flood the country; from your post and the lack of any recent issues you've proved you can handle this.

Coastlines change over time - locally a coastal protection scheme added a few fish tailed groynes to MHW so I replaced that short section when the data became available (too recent to trace from imagery).

OSM is a process of continual improvement. I would say if you are doing small areas manually with care rather than bulk importing the whole coastline then carry on doing areas if you're willing to maintain them too.

Best wishes,

Ed

From: Borbus <[hidden email]>
Sent: Thursday, July 11, 2019 8:38:39 PM
To: [hidden email]
Subject: [Talk-GB] UK coastline data
 
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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Re: UK coastline data

Colin Smale
In reply to this post by Borbus

Hi,

Great!

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: https://wiki.openstreetmap.org/wiki/Proposed_features/Tag:waterway%3Dtidal_channel

* 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.

 

Cheers,

Colin

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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Re: UK coastline data

Borbus
On Thu, Jul 11, 2019 at 9:19 PM Colin Smale <[hidden email]> 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:
>   https://wiki.openstreetmap.org/wiki/Proposed_features/Tag:waterway%3Dtidal_channel

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,

Borbus

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Re: UK coastline data

Great Britain mailing list
In reply to this post by Colin Smale
On Thu, Jul 11, 2019 at 9:19 PM Colin Smale <[hidden email]> wrote:

* 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. 

Is this necessarily the case? Couldn't an overhang result in a low water landward of high water? Consider e.g. a sea cave that is flooded at high tide.

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Re: UK coastline data

Colin Smale

Good point. Do you know of one? Let's have a look at how the OS deal with it.

 


On 2019-07-11 22:52, Edward Catmur wrote:

On Thu, Jul 11, 2019 at 9:19 PM Colin Smale <[hidden email]> wrote:

* 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. 

Is this necessarily the case? Couldn't an overhang result in a low water landward of high water? Consider e.g. a sea cave that is flooded at high tide.

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Re: UK coastline data

Colin Smale
In reply to this post by Borbus

On 2019-07-11 22:45, Borbus wrote:

On Thu, Jul 11, 2019 at 9:19 PM Colin Smale <[hidden email]> wrote:
> * 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.
 
Combining them might actually be the right thing to do. As a matter of law the local government jurisdiction extends to MLWS (except where explicitly otherwise defined) so it should be a question of choosing the best data, probably the data from the most recent survey. Don't forget sandbanks and other areas that fall dry at low water - these are also marked by the OS with MLWS and are therefore admin boundaries.
 
> * 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.
 
I couldn't agree more! My vote is to go back to the tidal limit, for exactly that reason.
 
Cheers,
Colin

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Re: UK coastline data

Great Britain mailing list
In reply to this post by Colin Smale
Tricky - it appears to be a rule that all the famous sea caves are accessible by foot at low tide (there's probably a geological reason, like why sea cliffs tend to have a ledge below exposed at low tide). That said, some sea arches have inward-sloping sides - e.g. Stair Hole https://www.openstreetmap.org/node/2128418334 on the 1:25000 the HWM and LWM both appear to follow the outer edge of the arch above while the interior is rendered with the cave/cave entrance symbol.

It's an interesting question how to map sea caves and natural arches - all I've looked at so far have the coastline running along the outer edge of the land above, but OTOH you have natural arches like Rainbow Bridge https://www.openstreetmap.org/way/569676595 mapped as an area natural=rock with Lake Powell running uninterrupted underneath it; and Natural Bridge https://www.openstreetmap.org/node/4325038750 is mapped as two cliffs, not intersecting the creek or path beneath. 

On Thu, Jul 11, 2019 at 9:56 PM Colin Smale <[hidden email]> wrote:

Good point. Do you know of one? Let's have a look at how the OS deal with it.

 


On 2019-07-11 22:52, Edward Catmur wrote:

On Thu, Jul 11, 2019 at 9:19 PM Colin Smale <[hidden email]> wrote:

* 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. 

Is this necessarily the case? Couldn't an overhang result in a low water landward of high water? Consider e.g. a sea cave that is flooded at high tide.

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Re: UK coastline data

Devonshire
In reply to this post by Borbus

On Thu, Jul 11, 2019, at 10:41 PM, Borbus wrote:

The Dart cuts the coastline off right at the mouth, which doesn't seem right...


I think the main reason I did that back in the day is that mapping coastline all the way up to Totnes seems extremely non-intuitive. Someone standing on Totnes quay (10 miles inland) is not standing on the coast in any meaningful way.

I don't really care either way but what would be the benefit of changing it to coastline (and slavishly copying the OS is not a benefit) ?

Kevin

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Re: UK coastline data

Gregory Marler
I have been seeing a lot of flooded tiles over the last week. Of course doing a Cntrl+F5 on my browser refreshes the old tiles, so it's not terrible.
Hopefully the current work now is avoiding that happening, as the country "flooded" can look bad to new viewers. It was also annoying when I was out of signal and OsmAnd seemed to have some tiles flooded.

For river coastlines, I believe The Thames is tidal as up as Teddington lock. That would be even more excessive/impractical than Totnes, and have all of central London be "by the sea".

For oddities with tidal marks, I don't know if the "Loe Bar" provides anything interesting. It's a sand bar, on one side is the coast and on the other is "The Loe" freshwater lake.

Thanks for your detailed help.
Gregory.

On Fri, 12 Jul 2019 at 07:46, Devonshire <[hidden email]> wrote:

On Thu, Jul 11, 2019, at 10:41 PM, Borbus wrote:

The Dart cuts the coastline off right at the mouth, which doesn't seem right...


I think the main reason I did that back in the day is that mapping coastline all the way up to Totnes seems extremely non-intuitive. Someone standing on Totnes quay (10 miles inland) is not standing on the coast in any meaningful way.

I don't really care either way but what would be the benefit of changing it to coastline (and slavishly copying the OS is not a benefit) ?

Kevin
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--
Gregory Marler
No More Grapes
07939 689 691

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Re: UK coastline data

Mark Goodge
In reply to this post by Borbus


On 11/07/2019 20:38, Borbus wrote:

> 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.

It's also one of the most useful from a leisure perspective, as a lot of
popular beaches fall primarily or wholly in the intertidal zone. Take,
for example, Hunstanton in Norfolk - at high tide the sea comes all the
way up to the sea wall, and there is no beach as such on the town centre
seafront. But, at low tide, there's a large expanse of sand. And, in
between, there are differing amounts of sand visible!

FWIW, I think that both OSM and OS currently show this correctly, with
the intertidal beach being mapped as sand (as are the sand and mud banks
further south near Heacham and Snettisham). Google, on the other hand,
seems to be ignoring the intertidal area completely and mapping the
coastline according to what is visible on their own aerial imagery (for
a good example of that, zoom into Hunstanton Sea Life Centre on Google
maps and then switch between map and satellite view).

Mark

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Re: UK coastline data

wambacher

Hi,

I don't think you should accept this data.

see: https://wambachers-osm.website/images/osm/snaps_2019/strange_coastline.png_2019/strange_coastline.png

The old coastline (August 2018) is blue and the current coastal line is red.

The affected areas are wetlands, whose "coastline" is very complex. In my opinion, the problem is that this "coastline" is not static, because it is a natural runoff that will never be stable. It will look quite different after several months.

A manually smoothed coastline (better than the one from 2018) would be appropriate.

ym2c from germany.

walter

Translated with www.DeepL.com/Translator


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Re: UK coastline data

wambacher
sorry, wrong link:
https://wambachers-osm.website/images/osm/snaps_2019/strange_coastline.png

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Re: UK coastline data

David Groom
In reply to this post by Devonshire
------ Original Message ------
From: "Devonshire" <[hidden email]>
Sent: 12/07/2019 07:44:55
Subject: Re: [Talk-GB] UK coastline data


On Thu, Jul 11, 2019, at 10:41 PM, Borbus wrote:

The Dart cuts the coastline off right at the mouth, which doesn't seem right...

I think the main reason I did that back in the day is that mapping coastline all the way up to Totnes seems extremely non-intuitive. Someone standing on Totnes quay (10 miles inland) is not standing on the coast in any meaningful way.
I agree, I long ago made the same point regarding the River Thames as it passes through London.  
David



I don't really care either way but what would be the benefit of changing it to coastline (and slavishly copying the OS is not a benefit) ?

Kevin

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Re: UK coastline data

Borbus
In reply to this post by Devonshire
On Fri, Jul 12, 2019 at 7:46 AM Devonshire <[hidden email]> wrote:
> I think the main reason I did that back in the day is that mapping
> coastline all the way up to Totnes seems extremely
> non-intuitive. Someone standing on Totnes quay (10 miles inland) is not
> standing on the coast in any meaningful way.

Does that matter, though? The way many things in OSM are tagged is quite
arbitrary. What if "coastline" just means "mean high water level"? A tag
for MHWL seems much more useful than "you would probably consider this
the coast rather than a river bank".

> I don't really care either way but what would be the benefit of changing
> it to coastline (and slavishly copying the OS is not a benefit) ?

The benefit is we don't have to arbitrarily draw the line somewhere. The
tidal limit is well-defined so it's easy to be consistent.

Borbus.

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Re: UK coastline data

Borbus
In reply to this post by Mark Goodge
On Fri, Jul 12, 2019 at 9:17 AM Mark Goodge <[hidden email]> wrote:
> It's also one of the most useful from a leisure perspective, as a lot of
> popular beaches fall primarily or wholly in the intertidal zone. Take,
> for example, Hunstanton in Norfolk - at high tide the sea comes all the
> way up to the sea wall, and there is no beach as such on the town centre
> seafront. But, at low tide, there's a large expanse of sand. And, in
> between, there are differing amounts of sand visible!

Hunstantson was one of the beaches I updated in my edits. The
data is now correct, but the carto layer doesn't actually show
the coastline (the MHW level) unfortunately.

Interesting that you mention the sand and mud further south. At
some point between Hunstanton and Snettisham the intertidal zone
changes from "beach" to "tidal flat". I've actually continued
the "beach" all the way down to Snettisham with the "tidal flats"
starting beyond the nature reserve. This is quite an important
distinction, because if you go to those areas expecting a beach
but get a tidal flat you'll be in for a (probably unpleasant)
surprise.

Any idea where to draw the line here?

There's a similar situation on the other side between Skegness
and Gibraltar Point.

Borbus.

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Re: UK coastline data

Borbus
In reply to this post by wambacher
On Fri, Jul 12, 2019 at 12:40 PM <[hidden email]> wrote:
> The old coastline (August 2018) is blue and the current coastal line
> is red.

The blue shows what I was talking about earlier where some of the
coastline was at MLWL and some was all the way up at the sea wall
(exceptionally high tide).

> The affected areas are wetlands, whose "coastline" is very complex. In
> my opinion, the problem is that this "coastline" is not static,
> because it is a natural runoff that will never be stable. It will look
> quite different after several months.

I am familiar with this area. Those areas are salt marshes and much more
stable than you might think. The main change that has happened over the
past ten years in this area is the high water marks have receded. This
is reflected in the most recent OS data that I've used. Those inlets are
deep tidal channels and definitely should be on the map. A smoothed
coastline would be a huge loss in this area.

The bits that would change a lot more are the low water levels because
this land is comprised of soft mud and sand. Unfortunately none of the
aerial imagery we have available cover these vast tidal flats. Most cut
off at the "coastline" (high water level).

Borbus.

On Fri, Jul 12, 2019 at 12:40 PM <[hidden email]> wrote:

Hi,

I don't think you should accept this data.

see: https://wambachers-osm.website/images/osm/snaps_2019/strange_coastline.png_2019/strange_coastline.png

The old coastline (August 2018) is blue and the current coastal line is red.

The affected areas are wetlands, whose "coastline" is very complex. In my opinion, the problem is that this "coastline" is not static, because it is a natural runoff that will never be stable. It will look quite different after several months.

A manually smoothed coastline (better than the one from 2018) would be appropriate.

ym2c from germany.

walter

Translated with www.DeepL.com/Translator

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Re: UK coastline data

Devonshire
In reply to this post by Borbus


On Fri, Jul 12, 2019, at 8:05 PM, Borbus wrote:.
Does that matter, though? The way many things in OSM are tagged is quite
arbitrary. What if "coastline" just means "mean high water level"? A tag
for MHWL seems much more useful than "you would probably consider this
the coast rather than a river bank".

> I don't really care either way but what would be the benefit of changing
> it to coastline (and slavishly copying the OS is not a benefit) ?

The benefit is we don't have to arbitrarily draw the line somewhere. The
tidal limit is well-defined so it's easy to be consistent.

Just because the coastline follows MLW as it goes around the coast doesn't mean it needs to follow every tidal waterway inland. That doesn't follow at all.

To achieve what you want you would need to add yet another way inside of the riverbank and intertidal areas which seems like a fair bit of effort to do for every river for no real benefit to map users whatsoever. Then you need to get it all to render right where you have tidal mud banks, etc. in the centre of the river.

Kevin


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Re: UK coastline data

Borbus
On Fri, Jul 12, 2019 at 9:11 PM Devonshire <[hidden email]> wrote:
> Just because the coastline follows MLW as it goes around the coast
> doesn't mean it needs to follow every tidal waterway inland. That
> doesn't follow at all.

Why not? What is the meaning of "coastline"?

The Dart is one example of where it seems obvious where to "draw the
line" by taking a cursory glance at aerial imagery, but does this line
have any bearing on reality?

My feeling is that the natural=coastline tag is a misnomer and it should
really just be called "mean_high_water_level" or
"mean_high_water_spring" (I'm still unsure about whether OS show MHWL or
MHWS, I thought it was MHWL, which is between mean high water spring and
mean high water neap).

Is there a meaning to "coastline" that makes it distinct from any other
high water level that can't be expressed with other tags? (Other tags
could be water salinity, presence of beaches, dunes, cliffs etc. that
are real physical features).

> To achieve what you want you would need to add yet another way inside of
> the riverbank and intertidal areas which seems like a fair bit of effort
> to do for every river for no real benefit to map users whatsoever. Then
> you need to get it all to render right where you have tidal mud banks,
> etc. in the centre of the river.

That data is included with the OS tidal waters data. It's not much more
effort to use it and it's very useful data for many people. People use
maps for many different things. Rendering is not a problem. Carto
handles it just fine already. But it does expect the intertidal zone to
be between a "coastline" and the edge of a tidalflat.

--
Borbus

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Re: UK coastline data

Colin Smale

On 2019-07-13 13:35, Borbus wrote:

On Fri, Jul 12, 2019 at 9:11 PM Devonshire <[hidden email]> wrote:
> Just because the coastline follows MLW as it goes around the coast
> doesn't mean it needs to follow every tidal waterway inland. That
> doesn't follow at all.

Why not? What is the meaning of "coastline"?

The Dart is one example of where it seems obvious where to "draw the
line" by taking a cursory glance at aerial imagery, but does this line
have any bearing on reality?

My feeling is that the natural=coastline tag is a misnomer and it should
really just be called "mean_high_water_level" or
"mean_high_water_spring" (I'm still unsure about whether OS show MHWL or
MHWS, I thought it was MHWL, which is between mean high water spring and
mean high water neap).
 
The data included with Boundary-Line would appear to be mean high water (springs) according to the User Guide and Technical Specification, although in some places it is referred to as the High Water Mark and High Water Line.
 
Is there a meaning to "coastline" that makes it distinct from any other
high water level that can't be expressed with other tags? (Other tags
could be water salinity, presence of beaches, dunes, cliffs etc. that
are real physical features).
 
Salinity is too variable to be useful. My vote is to stick to MHWS, or whatever the prevailing law states as the edge of the land.
 
How about creating an OSM tidal prediction model? Then we could take all the WGS84 elevations that are near the coast in OSM, and make our own model, and make it open source. How hard can it be? (PS I know exactly how hard it would be, but it would be a typical OSM attitude to reject existing standards and roll our own)
 
Just for completeness, even MHWS is not the limit of where the water comes to. It's a mean value, averaged over a long period; statistically, half the high tides at spring tide will encroach further landward than MHWS. Every tide is different. But you have to draw the line somewhere.
 
When is our coastline fit for purpose? It seems to be a rendering hint, to colour one side of the line "blue" and the other side various colours. Do we need a rendering hint to separate the sea from an estuary? It might also be said to form a useful polygon to allow the dry bits of the world to be excised from the global database in a convenient way. What do we want here?
 
 
 

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