Re: [Talk-GB] 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  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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Re: [Talk-GB] 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: [Talk-GB] UK coastline data

[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: [Talk-GB] 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: 
> 

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: 
https://www.legislation.gov.uk/uksi/2014/1353/made 

This shows the limit of the Territorial Sea, which, interestingly, does
not always correspond to the OSM data: 
https://data.admiralty.co.uk/portal/apps/sites/#/marine-data-portal/items/2bd3ea1bbdc04e83bb9fb54644723d20?geometry=-16.516%2C49.719%2C16.048%2C54.439


This shows graphically where the straight baselines have been applied: 
https://data.admiralty.co.uk/portal/apps/sites/#/marine-data-portal/items/0b5d3be5acba43578230019a4f2df95d


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: [Talk-GB] 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: 



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: [Talk-GB] UK coastline data

[David Woolley]

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: 





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Re: [Talk-GB] UK coastline data

[Borbus]

On Sat, Jul 13, 2019 at 9:44 PM Colin Smale  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=0.6775#map=12/51.4876/0.6736

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

-- 
Borbus.
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Re: [Talk-GB] UK coastline data

[Tony Shield]

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: [Talk-GB] 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: [Talk-GB] UK coastline data

[David Woolley]

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: [Talk-GB] UK coastline data

[Colin Smale]

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: [Talk-GB] UK coastline data

[Colin Smale]

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: [Talk-GB] UK coastline data

[Tony Shield]

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: [Talk-GB] UK coastline data

[Colin Smale]

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: [Talk-GB] UK coastline data

[Devonshire]

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: [Talk-GB] 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: [Talk-GB] 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: [Talk-GB] UK coastline data

[Tony Shield]

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 > 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: [Talk-GB] UK coastline data

[Colin Smale]

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

> On Fri, Jul 12, 2019 at 9:11 PM Devonshire  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
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Re: [Talk-GB] UK coastline data

[Borbus]

On Fri, Jul 12, 2019 at 9:11 PM Devonshire  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.

-- 
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Re: [Talk-GB] UK coastline data

[Devonshire]

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: [Talk-GB] UK coastline data

[Borbus]

On Fri, Jul 12, 2019 at 12:40 PM  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  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
>
> --
> My projects:
>
> Admin Boundaries of the World 
> Missing Boundaries
> 
> Emergency Map 
> Postal Code Map (Germany only) 
> Fools (QA for zipcodes in Germany) 
> Postcode Boundaries of Germany
> 
> ___
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>
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Re: [Talk-GB] UK coastline data

[Borbus]

On Fri, Jul 12, 2019 at 9:17 AM Mark Goodge  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: [Talk-GB] UK coastline data

[Borbus]

On Fri, Jul 12, 2019 at 7:46 AM Devonshire  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: [Talk-GB] UK coastline data

[David Groom]

-- Original Message --
From: "Devonshire" 
To: talk-gb@openstreetmap.org
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: [Talk-GB] UK coastline data

[wambacher]

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

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Re: [Talk-GB] 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

-- 
My projects:

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Missing Boundaries

Emergency Map 
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Re: [Talk-GB] UK coastline data

[Mark Goodge]

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: [Talk-GB] 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  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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>


-- 
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No More Grapes
07939 689 691
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Re: [Talk-GB] UK coastline data

[Devonshire]

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: [Talk-GB] UK coastline data

[Edward Catmur via Talk-GB]

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  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  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: [Talk-GB] UK coastline data

[Colin Smale]

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

> On Thu, Jul 11, 2019 at 9:19 PM Colin Smale  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. 

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Re: [Talk-GB] 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  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: [Talk-GB] UK coastline data

[Edward Catmur via Talk-GB]

On Thu, Jul 11, 2019 at 9:19 PM Colin Smale  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: [Talk-GB] UK coastline data

[Borbus]

On Thu, Jul 11, 2019 at 9:19 PM Colin Smale  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: [Talk-GB] UK coastline data

[Colin Smale]

Hi, 

Great! 

Don't worry about having "too many nodes" - the OS data is already
generalised a bit (I think they target 1:1) 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, 
> 
> 

Re: [Talk-GB] UK coastline data

[Ed Loach]

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 
Sent: Thursday, July 11, 2019 8:38:39 PM
To: talk-gb@openstreetmap.org
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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[Talk-GB] 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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