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: https://www.openstreetmap.org/node/3200691239#map=18/52.07127/-4.77079 <https://www.openstreetmap.org/node/3200691239#map=18/52.07127/-4.77079https://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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