Finding level 3 islands in OSM data?

General talk
6

You can also look at objects tagged place=island and see if they are contained within another place=island. I did this to test out Geodesk and it does bring up some genuine level 3 islands (e.g. this one) but it’s not entirely reliable, it also brings up a whole bunch of QA issues / questionable tagging. For example

  • the same island being mapped twice, once as a node and once as an area
  • place=island on a construct that actually includes multiple separate islands (example) which themselves have place=island
  • part of an island being mapped as place=island (example) despite a land connection to the “outer” island
  • the admin boundary for an island being tagged place=island (example), in addition to the island itself

Sometimes they all combine and place=island is on the node, the way, and the admin boundary, resulting in a “fake” level 3 island.

Your example of a level 3 island doesn’t even have a place tag so it wouldn’t be found this way. The opposite is also not uncommon, where someone maps an island with place=island/islet, but forgets to make the surrounding water body a multipolygon and add the island as an inner. (When I get around to it I might make a Maproulette challenge to fix these.) So in some cases the approach via place can find islands you would not otherwise find, but overall it’s probably less reliable than the approach based on multipolygons and inners.

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