Atolls (some bla bla to make 15 characters)

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:

For comparison Darwin’s map of atolls:

Darwin's Atoll 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).

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