Double, double, toil and trouble (how to map Witch's Cauldron?)

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Double, double, toil and trouble (how to map Witch's Cauldron?)

Paul Allen
I've encountered a feature called, in English, "Witch's Cauldron" (also "Witches Cauldron" and "Witch's Pit") and
called, in Welsh, "Pwll y Wrach."  It was mapped by somebody else around 4 years ago and the mapping has
one definite error and a couple of things that may be wrong.  The problem is I'm not sure what the correct
mapping would be.

It's here: <a href="https://www.openstreetmap.org/node/3200691239#map=18/52.07127/-4.77079https://www.openstreetmap.org/node/3200691239#map=18/52.07127/-4.77079">https://www.openstreetmap.org/node/3200691239#map=18/52.07127/-4.77079

It's a complicated geological feature.  I've yet to come across any description of this particular feature that
gives a name for that type of feature, and perhaps there isn't one.  The only way I can describe it is by the
processes that led to it.  It once consisted of a stratum of soft rock at sea level, overlain by harder rock.
Tidal erosion formed a cave by removing the soft rock, leaving a layer of hard rock forming the roof.  Eventually,
part of the roof collapsed.  The result is a hole in the ground with sea water at the bottom, with a tunnel from
the hole to the sea.  Depending upon the state of the tide it's possible to traverse the tunnel from the sea to
the hole (but only in something like a kayak, nothing larger).

It's been mapped as an area of natural=water (no other tags).  In the centre of the water is a node tagged
natural=arch, which is not an arch at all.  The arch is about 25m NW of that node.  OS OpenData StreetView
(available as background imagery in iD and possibly in other editors) shows a thick, grey dashed line
connecting the water in the hole to the coast's high water mark and nearby are the words "Natural Arch."

It currently shows a tributary of nearby river connecting the hole in the ground.  Such an interpretation is
not backed up by the OS or ESRI backgrounds (Bing is too unclear to cast any light on the issue).  Nor
is this backed up by any description of the feature I've found.

So natural=arch is in the wrong place.  Arguably it should be a closed way covering the water passage
underneath, possibly with layer=1.  The tributary that isn't visible in OS should be removed.  And some sort
of water should be mapped (whether it renders or not, just for routeing) under the arch connecting sea to
hole in the ground.  But what sort of water?  And what additional tag to use for the water in the hole?  It's
not really a pond, it's part of the sea.

I can't think of a good way to do it.  The least bad train of thought I had was how it would be mapped if the
arch collapsed.  In that case the HWM would extend inland to encompass the hole in the ground, which might
perhaps be tagged as a cove, because that's what it would be.  So why not do that with a natural=arch over it?

Any better ideas?  If nobody can come up with anything convincing, I'll leave it alone and pretend I never saw
it. :)

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Re: Double, double, toil and trouble (how to map Witch's Cauldron?)

Mateusz Konieczny-3
I would tag it as a waterway in tunnel (though I have no idea about a suitable value) or
as water area with covered=yes and natural=bare_rock area mapped, both with a proper tags.

It also sounds like it is a tourism=attraction .

Sounds a bit similar to https://en.wikipedia.org/wiki/Blue_Hole_(Red_Sea) - though
this one is entirely underwater and of a different origin.

7. Oct 2018 19:11 by [hidden email]:

I've encountered a feature called, in English, "Witch's Cauldron" (also "Witches Cauldron" and "Witch's Pit") and
called, in Welsh, "Pwll y Wrach."  It was mapped by somebody else around 4 years ago and the mapping has
one definite error and a couple of things that may be wrong.  The problem is I'm not sure what the correct
mapping would be.

It's here: <a href="https://www.openstreetmap.org/node/3200691239#map=18/52.07127/-4.77079https://www.openstreetmap.org/node/3200691239#map=18/52.07127/-4.77079" target="_blank" rel="noopener noreferrer">https://www.openstreetmap.org/node/3200691239#map=18/52.07127/-4.77079

It's a complicated geological feature.  I've yet to come across any description of this particular feature that
gives a name for that type of feature, and perhaps there isn't one.  The only way I can describe it is by the
processes that led to it.  It once consisted of a stratum of soft rock at sea level, overlain by harder rock.
Tidal erosion formed a cave by removing the soft rock, leaving a layer of hard rock forming the roof.  Eventually,
part of the roof collapsed.  The result is a hole in the ground with sea water at the bottom, with a tunnel from
the hole to the sea.  Depending upon the state of the tide it's possible to traverse the tunnel from the sea to
the hole (but only in something like a kayak, nothing larger).

It's been mapped as an area of natural=water (no other tags).  In the centre of the water is a node tagged
natural=arch, which is not an arch at all.  The arch is about 25m NW of that node.  OS OpenData StreetView
(available as background imagery in iD and possibly in other editors) shows a thick, grey dashed line
connecting the water in the hole to the coast's high water mark and nearby are the words "Natural Arch."

It currently shows a tributary of nearby river connecting the hole in the ground.  Such an interpretation is
not backed up by the OS or ESRI backgrounds (Bing is too unclear to cast any light on the issue).  Nor
is this backed up by any description of the feature I've found.

So natural=arch is in the wrong place.  Arguably it should be a closed way covering the water passage
underneath, possibly with layer=1.  The tributary that isn't visible in OS should be removed.  And some sort
of water should be mapped (whether it renders or not, just for routeing) under the arch connecting sea to
hole in the ground.  But what sort of water?  And what additional tag to use for the water in the hole?  It's
not really a pond, it's part of the sea.

I can't think of a good way to do it.  The least bad train of thought I had was how it would be mapped if the
arch collapsed.  In that case the HWM would extend inland to encompass the hole in the ground, which might
perhaps be tagged as a cove, because that's what it would be.  So why not do that with a natural=arch over it?

Any better ideas?  If nobody can come up with anything convincing, I'll leave it alone and pretend I never saw
it. :)

--
Paul


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Re: Double, double, toil and trouble (how to map Witch's Cauldron?)

Paul Allen
On Sun, Oct 7, 2018 at 6:26 PM Mateusz Konieczny <[hidden email]> wrote:
I would tag it as a waterway in tunnel (though I have no idea about a suitable value)

For me, a tunnel is man-made, not natural.  The tunnel=* I see in the wiki seem (at quick glance)
to all be man-made.

or as water area with covered=yes and natural=bare_rock area mapped, both with a proper tags.

Better, although natural=arch seems like a better fit, even if it's undocumented.  But that still
doesn't deal with the water under the arch and in the basin.

It also sounds like it is a tourism=attraction .

A very minor attraction.  I was intending to add that but it didn't seem to pose any problems so I
didn't mention it.

Sounds a bit similar to https://en.wikipedia.org/wiki/Blue_Hole_(Red_Sea) - though
this one is entirely underwater and of a different origin.

Not a good match.  But it led me into a twist maze of Wikipedia pages and eventually I found
https://en.wikipedia.org/wiki/Sea_cave  So now I know that technically it is a collapsed littoral
cave or collapsed sea cave.  There's one in Oregon called Devil's Punchbowl which has been
turned into a proper tourist attraction: https://www.openstreetmap.org/#map=19/44.74739/-124.06529
Whoever mapped that didn't deal with the water aspect in any way but did add something I hadn't
thought of: the cliff edge around it.  Without the water it just looks like a hilly area with a depression.

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Re: Double, double, toil and trouble (how to map Witch's Cauldron?)

ael-3
In reply to this post by Paul Allen
On Sun, Oct 07, 2018 at 06:11:38PM +0100, Paul Allen wrote:
> I've encountered a feature called, in English, "Witch's Cauldron" (also
> "Witches Cauldron" and "Witch's Pit") and
> called, in Welsh, "Pwll y Wrach."  It was mapped by somebody else around 4
> years ago and the mapping has
> one definite error and a couple of things that may be wrong.  The problem
> It currently shows a tributary of nearby river connecting the hole in the

Is that the stream tagged with source = npe?  Just because you can't
see it from your armchair, doesn't mean that it isn't there. Although
I admit npe is often a bit approximate, and it may have dried up.
Maybe the NLS 2 1/2" historic map might shed more light.

Just ATM, I can't recall the right terminology, but these partially
collapsed sea caves are quite common, and I am sure I have seen them
with a dedicated tag. No doubt someone else will chip in the proper
tags.

ael


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Re: Double, double, toil and trouble (how to map Witch's Cauldron?)

Paul Allen


On Sun, Oct 7, 2018 at 8:10 PM ael <[hidden email]> wrote:
Is that the stream tagged with source = npe?

Yep.

 Just because you can't see it from your armchair, doesn't mean that it isn't there. Although 
I admit npe is often a bit approximate, and it may have dried up.

NPE is very approximate.  And prone to blackouts from heavy drinking.  And some hallucinations.

Maybe the NLS 2 1/2" historic map might shed more light.

OS OpenData StreetView matches what ESRI shows.  What is mapped is a stream with a
right-angle turn into the basin and no drain from the basin.  What is not mapped is the stream
continuing straight on to the sea as shown in OSODSV and is very visible in ESRI.  What is not
mapped is the underground channel from the basin to the sea, marked on OSODSV.  What is
mapped doesn't seem to match any of the write-ups I've seen for this feature whereas OSODSV
does.  And even if the mapping is right for the stream, there is no arch where it has been placed.

Just ATM, I can't recall the right terminology, but these partiallycollapsed sea caves are quite common, and I am sure I have seen them with a dedicated tag. No doubt someone else will chip in the proper
tags.

There are tags for caves in general and could be applied to a sea cave.  But the only way the
cave tags can be applied to a collapsed sea cave is to pretend the collapse is a sink hole, which
(according to the wiki and to my understanding of what a sink hole is), it isn't.

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Re: Double, double, toil and trouble (how to map Witch's Cauldron?)

ael-3
On Sun, Oct 07, 2018 at 08:32:56PM +0100, Paul Allen wrote:
> On Sun, Oct 7, 2018 at 8:10 PM ael <[hidden email]> wrote:
>
> There are tags for caves in general and could be applied to a sea cave.
> But the only way the
> cave tags can be applied to a collapsed sea cave is to pretend the collapse
> is a sink hole, which
> (according to the wiki and to my understanding of what a sink hole is), it
> isn't.

No. I am sure that I have seen a dedicated/specialist term for exactly
these features. Just can't recall the details ATM. Pretty sure I have
seen examples in Cornwall and else where. I think there was a news story
about a rescue from one such relatively recently. It may come to me
later...

ael


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Re: Double, double, toil and trouble (how to map Witch's Cauldron?)

Paul Allen
On Sun, Oct 7, 2018 at 9:52 PM ael <[hidden email]> wrote:

No. I am sure that I have seen a dedicated/specialist term for exactly
these features. Just can't recall the details ATM.

As I mentioned in another part of this thread, I subsequently found that the correct term for
one of these is a littoral cave (if you want to be fancy) or a sea cave (if you want to be plain).  This
particular one is a collapsed littoral cave.  Well, that's the best I could come up with after looking
at various wikipedia pages.  As for existing, documented tags, nothing seems to be an exact
match for this.
 
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Re: Double, double, toil and trouble (how to map Witch's Cauldron?)

Joseph Eisenberg
A collapsed cave on land is a sinkhole. Is that not appropriate? 

The natural=cliff idea is also sensible and correct for the edge of the hole

Natural=arch on a node is a great idea for the natural bridge between the collapsed section and the open sea. There are plenty of similar features on rocky coasts, Eg Oregon, Baja California, and in deserts due to wind erosion (eg Natural Arches monument in Utah)

Small collapsed sea caves can become a “blowhole” if wave action causes water to erupt up through the hole, but it doesn’t sound like this is the case in this particular place.

On Mon, Oct 8, 2018 at 7:17 AM Paul Allen <[hidden email]> wrote:
On Sun, Oct 7, 2018 at 9:52 PM ael <[hidden email]> wrote:

No. I am sure that I have seen a dedicated/specialist term for exactly
these features. Just can't recall the details ATM.

As I mentioned in another part of this thread, I subsequently found that the correct term for
one of these is a littoral cave (if you want to be fancy) or a sea cave (if you want to be plain).  This
particular one is a collapsed littoral cave.  Well, that's the best I could come up with after looking
at various wikipedia pages.  As for existing, documented tags, nothing seems to be an exact
match for this.

 
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Paul

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Re: Double, double, toil and trouble (how to map Witch's Cauldron?)

Paul Allen
On Sun, Oct 7, 2018 at 11:56 PM Joseph Eisenberg <[hidden email]> wrote:
A collapsed cave on land is a sinkhole. Is that not appropriate? 

The wiki for natural=sinkhole seems to imply it is for any type of sinkhole, however formed.  But
the wiki for sinkhole=* doesn't have anything that precisely matches this, although sinkhole=pit
might do.

The natural=cliff idea is also sensible and correct for the edge of the hole

In this case, I think so.   I watched a youtube video of somebody walking along the footpath nearby
and it gave me vertigo. :)

Natural=arch on a node is a great idea for the natural bridge between the collapsed section and the open sea. There are plenty of similar features on rocky coasts, Eg Oregon, Baja California, and in deserts due to wind erosion (eg Natural Arches monument in Utah)

It's not widely used and it's not documented, but it's definitely appropriate.   One of the examples
I found of it on an area also had layer=1 but I have my doubts about that.  The sea is at sea level
and the ground is at ground level, so is it layer 1 or 0?  Probably best not to bother with layer.

Small collapsed sea caves can become a “blowhole” if wave action causes water to erupt up through the hole, but it doesn’t sound like this is the case in this particular place.

You can fit quite a few kayaks in there and there's a small beach that could hold a dozen people.  You
might get a water eruption if there were a tsunami.

That still leaves me wondering what to do with the water.  It's been mapped as natural=water
without specifying what type of water because there's nothing that matches.  The closest is
water=lagoon, but there's no barrier.  I'm half-inclined to map it as the sea by tweaking the
coastline: if the natural arch collapsed then it would be mapped as sea.  If the natural arch collapsed
and were replaced by a man-made bridge it would still be sea.

It's complicated. :)

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