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<div>I would tag it as a waterway in tunnel (though I have no idea about a suitable value) or</div><div>as water area with covered=yes and natural=bare_rock area mapped, both with a proper tags.</div><div><br /></div><div>It also sounds like it is a tourism=attraction .<br /></div><div><br /></div><div>Sounds a bit similar to <a href="https://en.wikipedia.org/wiki/Blue_Hole_(Red_Sea)" target="_blank" rel="noopener noreferrer">https://en.wikipedia.org/wiki/Blue_Hole_(Red_Sea)</a> - though</div><div>this one is entirely underwater and of a different origin.<br /></div><br />7. Oct 2018 19:11 by <a href="mailto:pla16021@gmail.com" target="_blank" rel="noopener noreferrer">pla16021@gmail.com</a>:<br /><br /><blockquote class="tutanota_quote" style="border-left: 1px solid #93A3B8; padding-left: 10px; margin-left: 5px;"><div><div><div>I've encountered a feature called, in English, "Witch's Cauldron" (also "Witches Cauldron" and "Witch's Pit") and</div><div>called, in Welsh, "Pwll y Wrach." It was mapped by somebody else around 4 years ago and the mapping has</div><div>one definite error and a couple of things that may be wrong. The problem is I'm not sure what the correct</div><div>mapping would be.</div><div><br /></div><div>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</a></div><div><br /></div><div>It's a complicated geological feature. I've yet to come across any description of this particular feature that</div><div>gives a name for that type of feature, and perhaps there isn't one. The only way I can describe it is by the</div><div>processes that led to it. It once consisted of a stratum of soft rock at sea level, overlain by harder rock.</div><div>Tidal erosion formed a cave by removing the soft rock, leaving a layer of hard rock forming the roof. Eventually,</div><div>part of the roof collapsed. The result is a hole in the ground with sea water at the bottom, with a tunnel from <br /></div><div>the hole to the sea. Depending upon the state of the tide it's possible to traverse the tunnel from the sea to</div><div>the hole (but only in something like a kayak, nothing larger).</div><div><br /></div><div>It's been mapped as an area of natural=water (no other tags). In the centre of the water is a node tagged</div><div>natural=arch, which is not an arch at all. The arch is about 25m NW of that node. OS OpenData StreetView</div><div>(available as background imagery in iD and possibly in other editors) shows a thick, grey dashed line</div><div>connecting the water in the hole to the coast's high water mark and nearby are the words "Natural Arch."</div><div><br /></div><div>It currently shows a tributary of nearby river connecting the hole in the ground. Such an interpretation is</div><div>not backed up by the OS or ESRI backgrounds (Bing is too unclear to cast any light on the issue). Nor</div><div>is this backed up by any description of the feature I've found.<br /></div><div><br /></div><div>So natural=arch is in the wrong place. Arguably it should be a closed way covering the water passage <br /></div><div>underneath, possibly with layer=1. The tributary that isn't visible in OS should be removed. And some sort</div><div>of water should be mapped (whether it renders or not, just for routeing) under the arch connecting sea to</div><div> hole in the ground. But what sort of water? And what additional tag to use for the water in the hole? It's</div><div>not really a pond, it's part of the sea.<br /></div><div><br /></div><div>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</div><div>arch collapsed. In that case the HWM would extend inland to encompass the hole in the ground, which might</div><div>perhaps be tagged as a cove, because that's what it would be. So why not do that with a natural=arch over it?</div><div><br /></div><div>Any better ideas? If nobody can come up with anything convincing, I'll leave it alone and pretend I never saw</div><div>it. :)</div><div><br /></div><div>-- <br /></div><div>Paul</div><div><br /></div></div></div></blockquote> </body>
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