For day16 of the #30dayMapChallenge I pulled down objects with natural=atoll from OSM. As I suspected that not all would be present I also queried Wikidata for atolls with co-ordinates. This is resulting map:
In doing this I noticed a lot of discrepancies (and there are more which I haven’t investigated yet:
natural=atoll on administrative boundaries (sometimes at more than one level), see this relation as an example (Overpass query). The worse case is probably the Line Islands.
natural=atoll being mapped as the complete area including islands, lagoon and off-shore reefs. This is the common approach, and conforms to the wiki description. (I quite like this as it enables various features to be separated & avoids difficulties in counting atolls).
Some odd, and very recent, relations like this one.
Well-known atolls, such as Midway, being nicely mapped in detail, but without an atoll tag. Also mapped solely as a place=island with a node located in the lagoon.
Only 69 of the natural=atoll elements with a wikidata tag matched items from wikidata marked as atolls. I haven’t investigated this at all thoroughly, but at least some wikidata objects have erroneous data.(this one seems odd, Pitcairn is unusual for a Pacific Island in not having much in the way of surrounding reefs).
According to wikipedia there are about 440 atolls across the warmer oceans of the globe. We probably have them all mapped, it would be nice to tidy it up a bit (and possibly Wikidata too).
Anyway, you should write a guide to “what do things look like and how do i tag them”. You seem like the kind of person who could do something like that.
I think what the wiki advocates, and which I’m inclined to agree, the atoll way should encompass the islands, and reef, probably to where the sea stops being shallow. Otherwise with large atolls you end up over counting atolls, if each significant island is tagged atoll. Also the lagoon within the atoll was always regarded as a significant feature.
+1 Définition | Atoll describes it well. First a volcano, later eroded. One or more lsland can subsist with lagoon in the middle and the coral reef around.
I’d keep reef for things which are close to the surface: in colder waters reef generally refers to an area of rock which may have waves-breaking on it at low tide (i.e., it can be below low tide, but not much). Whereas the atoll perhaps belongs to something like the 10 fathom mark.
