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

Paul Allen pla16021 at gmail.com
Sun Oct 7 17:11:38 UTC 2018

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:

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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