UK coastline data

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Re: UK coastline data

TonyS

Hi

Personally think that High Water Mark and Low Water Mark are very relevant to people and to OSM.

Yeah - tides are a nuisance and can never be predicted with total accuracy and with Global Warming HWM and LWM will change over time. Then there are Highest and Lowest Astronomical Tides, and then tides which increase or decrease according to weather conditions (pressure and wind) (New Orleans tonight is a good example). There are probably a few others which I have forgotten....

Knowing the inter-tidal area at Hunstanton is important, as are those in Morecambe Bay and the River Dee(North Wales/England)  where paths cross the area.

How many beaches are there on the Thames? and what is the inter-tidal ground like - sand, shingle, mud . . . .And what and where  is the access? These questions are what OSM is about.

The OS recognises this and on their maps marks the coastline/MHW with a dense line, but not on non-tidal waters.

OSM needs the equivalent of MLW - as far as I know its not defined (and I do not feel competent to define) - and I think that Borbus is on the good path.

On 13/07/2019 16:04, Colin Smale wrote:

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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Re: UK coastline data

Colin Smale

On 2019-07-13 21:33, Tony Shield wrote:

Hi

Personally think that High Water Mark and Low Water Mark are very relevant to people and to OSM.

Yeah - tides are a nuisance and can never be predicted with total accuracy and with Global Warming HWM and LWM will change over time. Then there are Highest and Lowest Astronomical Tides, and then tides which increase or decrease according to weather conditions (pressure and wind) (New Orleans tonight is a good example). There are probably a few others which I have forgotten....

Knowing the inter-tidal area at Hunstanton is important, as are those in Morecambe Bay and the River Dee(North Wales/England)  where paths cross the area.

How many beaches are there on the Thames? and what is the inter-tidal ground like - sand, shingle, mud . . . .And what and where  is the access? These questions are what OSM is about.

The OS recognises this and on their maps marks the coastline/MHW with a dense line, but not on non-tidal waters.

OSM needs the equivalent of MLW - as far as I know its not defined (and I do not feel competent to define) - and I think that Borbus is on the good path.

What exactly do you mean by MLW not being defined? Do you mean that there is not a robust definition of the concept? Or that it is difficult to establish the exact line of MLW?
 
Another reason to want MLW in OSM: The "Extent of the Realm" is *for the most part* defined as MLWS. This is the limit of the jurisdiction of normal (local) government. Beyond MLWS, the local council no longer has any say - it's the UK laws of the sea, as applicable to territorial waters.
 
I agree that Borbus is doing good things!

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Re: UK coastline data

David Woolley
On 13/07/2019 20:53, Colin Smale wrote:
> Another reason to want MLW in OSM: The "Extent of the Realm" is *for the
> most part* defined as MLWS. This is the limit of the jurisdiction of
> normal (local) government. Beyond MLWS, the local council no longer has
> any say - it's the UK laws of the sea, as applicable to territorial waters.

Low water mark is only a boundary of the realm when it doesn't fall
within a "internal waterway".

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Re: UK coastline data

Devonshire
In reply to this post by Borbus


On Sat, Jul 13, 2019, at 12:47 PM, Borbus wrote:
...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.

Unfortunately, this is one of those fairly pointless discussions that characterises OSM. I know it isn't always possible but I prefer the meaning of words in OSM tags to have the same meaning as in everyday conversation. If you want to add a way on every river in the country describing the mean low water level then fine but that doesn't mean you need to call it coastline.

If tidal beaches, mudflats, marsh areas, etc. are tagged as tidal=yes (which they should be) then all you are doing by adding a way for MLW is describing the part of the river that on average is non-tidal which doesn't add any extra information that isn't already there.

I notice that several people have messed around with the tagging on the Dart over the years so it probably isn't perfect anyway but changing it to coastline certainly isn't the solution.

Kevin


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Re: UK coastline data

Colin Smale
In reply to this post by David Woolley

On 2019-07-13 22:00, David Woolley wrote:

On 13/07/2019 20:53, Colin Smale wrote:
Another reason to want MLW in OSM: The "Extent of the Realm" is *for the most part* defined as MLWS. This is the limit of the jurisdiction of normal (local) government. Beyond MLWS, the local council no longer has any say - it's the UK laws of the sea, as applicable to territorial waters.

Low water mark is only a boundary of the realm when it doesn't fall within a "internal waterway".

Have you got a reference for this, making the link between the boundary of the Realm and the MCA classification of an inland waterway?
 
What could be a consequence of this? Could you illustrate this with an example?
 
The MCA definition of "inland waters" would draw a line across the Thames at Gravesend and across the Dart at Battery Point. These lines don't correspond to any admin boundaries I am aware of.
 
 
 

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Re: UK coastline data

TonyS
In reply to this post by Colin Smale

Hi

I meant that OSM does not have an agreed way of tagging MLWS or MLW.  https://wiki.openstreetmap.org/wiki/Tag:natural%3Dcoastline

Regards

On 13/07/2019 20:53, Colin Smale wrote:

On 2019-07-13 21:33, Tony Shield wrote:

Hi

Personally think that High Water Mark and Low Water Mark are very relevant to people and to OSM.

Yeah - tides are a nuisance and can never be predicted with total accuracy and with Global Warming HWM and LWM will change over time. Then there are Highest and Lowest Astronomical Tides, and then tides which increase or decrease according to weather conditions (pressure and wind) (New Orleans tonight is a good example). There are probably a few others which I have forgotten....

Knowing the inter-tidal area at Hunstanton is important, as are those in Morecambe Bay and the River Dee(North Wales/England)  where paths cross the area.

How many beaches are there on the Thames? and what is the inter-tidal ground like - sand, shingle, mud . . . .And what and where  is the access? These questions are what OSM is about.

The OS recognises this and on their maps marks the coastline/MHW with a dense line, but not on non-tidal waters.

OSM needs the equivalent of MLW - as far as I know its not defined (and I do not feel competent to define) - and I think that Borbus is on the good path.

What exactly do you mean by MLW not being defined? Do you mean that there is not a robust definition of the concept? Or that it is difficult to establish the exact line of MLW?
 
Another reason to want MLW in OSM: The "Extent of the Realm" is *for the most part* defined as MLWS. This is the limit of the jurisdiction of normal (local) government. Beyond MLWS, the local council no longer has any say - it's the UK laws of the sea, as applicable to territorial waters.
 
I agree that Borbus is doing good things!

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Re: UK coastline data

Colin Smale
In reply to this post by Devonshire

On 2019-07-13 22:30, Devonshire wrote:

 
Unfortunately, this is one of those fairly pointless discussions that characterises OSM. I know it isn't always possible but I prefer the meaning of words in OSM tags to have the same meaning as in everyday conversation. If you want to add a way on every river in the country describing the mean low water level then fine but that doesn't mean you need to call it coastline.
 
Coastline would be the high water line, not low water. But your point is valid - equating coastline to MHW could better be called a heuristic instead of a rule. It works most of the time, but we have to accommodate exceptions. All we have to do now, is to define what constitutes an exception.
 
 
 

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Re: UK coastline data

Colin Smale
In reply to this post by TonyS

On 2019-07-13 22:42, Tony Shield wrote:

Hi

I meant that OSM does not have an agreed way of tagging MLWS or MLW.  https://wiki.openstreetmap.org/wiki/Tag:natural%3Dcoastline

That page is about coastline, which is high water, not low water. But you are probably right. In the case of the UK, there are proxies like admin boundaries which help a lot, and the OS have been good enough to survey all this coastal stuff, but there is no way of tagging a line with "boundary=lwm" or whatever. Should it be a tag on a way, similar to the way the coastline is tagged? Or should it be a huge relation, like the admin boundary of the United Kingdom?


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Re: UK coastline data

David Woolley
In reply to this post by Colin Smale
On 13/07/2019 21:38, Colin Smale wrote:

> Have you got a reference for this, making the link between the boundary
> of the Realm and the MCA classification of an inland waterway?
> What could be a consequence of this? Could you illustrate this with an
> example?
> The MCA definition of "inland waters" would draw a line across the
> Thames at Gravesend and across the Dart at Battery Point. These lines
> don't correspond to any admin boundaries I am aware of.

The link is actually with territorial boundaries (12 mile limit).

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Re: UK coastline data

Colin Smale

On 2019-07-13 23:07, David Woolley wrote:

On 13/07/2019 21:38, Colin Smale wrote:

Have you got a reference for this, making the link between the boundary of the Realm and the MCA classification of an inland waterway?
What could be a consequence of this? Could you illustrate this with an example?
The MCA definition of "inland waters" would draw a line across the Thames at Gravesend and across the Dart at Battery Point. These lines don't correspond to any admin boundaries I am aware of.

The link is actually with territorial boundaries (12 mile limit).
 
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?

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Re: UK coastline data

TonyS
In reply to this post by Colin Smale

Hi

That page contains

"The Mean low water spring is the position of the lowest tide. There is currently no agreed way of tagging this line in OSM. One way of tagging it is to tag the area between the mean low water spring and OSM coastline as natural=wetland+wetland=tidalflat."

Methinks that MLW or MLWS should be defined in the way that Coastline/MHW is.



On 13/07/2019 21:52, Colin Smale wrote:

On 2019-07-13 22:42, Tony Shield wrote:

Hi

I meant that OSM does not have an agreed way of tagging MLWS or MLW.  https://wiki.openstreetmap.org/wiki/Tag:natural%3Dcoastline

That page is about coastline, which is high water, not low water. But you are probably right. In the case of the UK, there are proxies like admin boundaries which help a lot, and the OS have been good enough to survey all this coastal stuff, but there is no way of tagging a line with "boundary=lwm" or whatever. Should it be a tag on a way, similar to the way the coastline is tagged? Or should it be a huge relation, like the admin boundary of the United Kingdom?


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Re: UK coastline data

Borbus
In reply to this post by Colin Smale
On Sat, Jul 13, 2019 at 9:44 PM Colin Smale <[hidden email]> wrote:
> Coastline would be the high water line, not low water. But your point is
> valid - equating coastline to MHW could better be called a heuristic
> instead of a rule. It works most of the time, but we have to accommodate
> exceptions. All we have to do now, is to define what constitutes an
> exception.

Yes, I agree with this. MHW and MLW are nice because they are
indisputable (assuming accuracy and currency of data). I don't think
anybody would disagree with taking all of the OS MHWS data and tagging
it with "natural=mean_high_water_spring" or something.

The disagreement seems to come over whether those lines coincide with
"natural=coastline" or "natural=water"/"waterway=riverbank". A big
difference with the coastline (and this is especially true for Britain,
of course) is it is actually what defines Britain (the island). I guess
the River Thames is generally considered to be "in Britain".

Could we possibly use administrative boundaries to cut mark the cut off
point between coastline and riverbank then?

That would place the "coastline" at the mouth of the Thames here:
https://www.openstreetmap.org/?mlat=51.5038&mlon=0.6775#map=12/51.4876/0.6736

And the Dart here:
https://www.openstreetmap.org/?mlat=51.5038&mlon=0.6775#map=17/50.38223/-3.59310

--
Borbus.

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Re: UK coastline data

David Woolley
In reply to this post by Colin Smale
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>



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Re: UK coastline data

Mark Goodge


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.

Mark

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Re: UK coastline data

Colin Smale

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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Re: UK coastline data

Borbus
In reply to this post by Borbus
Hello again,

I've finished integrating the VectorMap data all the way along North Norfolk and now the Blackwater estuary too. I did a survey in Maldon before I made changes near the town to make sure nothing was wildly different. It looks like the new Maxar imagery is quite recent in that area and, helpfully, was taken at low tide. I made minor adjustments to the VectorMap data in places where the more recent imagery showed significant differences.

I wanted to fix up some other coastal areas I'm familiar with, such as around Folkestone and Dover, but these areas consist of features other than beaches and mud flats that are less clear to me how to map (like "beaches" consisting of large rocks from the cliffs).

There has been a change merged into osm-carto which will, in my opinion, degrade the rendering of many of these tidal areas. You can see the change here, along with my concerns at the bottom (I am georgek): https://github.com/gravitystorm/openstreetmap-carto/issues/3707

Doing this work, and seeing the above changes to osm-carto got me thinking that the time is ripe to sort out how we do water in OSM. There are fundamentally two concerns which are currently conflated in OSM:
* The water area, ie. "you will not be dry if you try to stand here",
* The water boundary, which could be a river/sea bank, a building, a mean high water level, and maybe other things.

The area and the boundary are both important for different reasons and convey very different information. At the moment we handle boundaries very poorly in OSM. This makes actually rendering the boundary very difficult, because it will actually cross water areas in at least two cases: where rivers meet oceans, and where river areas join together.

If these were all done using multipolygons we could separate the concerns completely. So I propose to do this: map areas as multipolygons and boundaries by ways which define multipolygons.

Water boundaries could be any of:
* natural=coastline, this should roughly correspond to the legal boundaries of the country, ie. the coastline is the edge of the country,
* natural=riverbank, for rivers, usually these are managed separately from sea banks, (we can't use the unfortunately named waterway=riverbank, because that actually defines a water area, not a river bank),
* natural=shore, for lakes and anything else?
* anonymous ways, for cases where the bank is not very significant, this can just be mapped as a normal polygon, for example a pond,

These boundaries can coincide with orthogonal tidal features which I also propose:
* tidal:mean_high_water_spring=yes,
* tidal:mean_low_water_spring=yes,
* tidal:... for other levels of interest like astronomical low (for nautical charts),

Note that these can exist where no bank is present, e.g. on beaches.

These boundaries can also contain many other useful pieces of information such as: material of bank, man made or natural, mooring, ownership, management and probably many more things.

It could be quite possible to have a way in which all of the following features coincide:
* natural=coastline,
* tidal:mean_high_water_spring=yes,
* tidal:mean_low_water_spring=yes,
* man_made=sea_bank,
* material=concrete,
* barrier=wall,
* mooring=private,

It's also quite possible to have a completely distinct seabank, high water level and low water level, e.g. in areas of reclaimed land where a man made sea bank is a flood defence and is well above mean high water level.

Sorry for such a long post. I would like to identify any problems with what I'm thinking quickly before I proceed to make proposals for these on the wiki. So if I could ask people to just give a rough thumbs up or thumbs down to this kind of approach I would be grateful.

Thanks, and happy mapping,

Borbus.

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Re: UK coastline data

ael
On Sun, Aug 04, 2019 at 12:34:53PM +0100, Borbus wrote:
> looks like the new Maxar imagery is quite recent in that area and,

Just a quick comment. Parts of the Maxar imagery seem to have significant
offsets. At least I have noticed that it often does not match my
(fairly accurate) gps tracks. And its offsets don't match those for
Maxbox. I think it is marked "beta" presumably in case of these sorts
of problem.

ael


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Re: UK coastline data

Borbus
> Just a quick comment. Parts of the Maxar imagery seem to have significant offsets.

I have noticed that too. The geometry is often wonky with incorrect angles. I'm not sure how that's possible, but comparison with something like Esri reveals it. At the moment I'm using it more for confirming details that aren't present on older imagery rather than trying to get precise geometry from it.

On Sun, Aug 4, 2019 at 8:24 PM ael <[hidden email]> wrote:
On Sun, Aug 04, 2019 at 12:34:53PM +0100, Borbus wrote:
> looks like the new Maxar imagery is quite recent in that area and,

Just a quick comment. Parts of the Maxar imagery seem to have significant
offsets. At least I have noticed that it often does not match my
(fairly accurate) gps tracks. And its offsets don't match those for
Maxbox. I think it is marked "beta" presumably in case of these sorts
of problem.

ael


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