There are a lot of (multi)polygons in OSM that are broken in one way or another. And we have to fix them. While some of the broken ones appear on the map just fine, some don’t appear and some mess up the map. And some of those that appear fine on the main OSM map will not show up on other maps where different software is used.
A while ago I set up a web page and a Github repository devoted to that issue that I never announced properly. Go there and read up in more detail where the problems are and how we are going to fix them.
Today I am launching several Maproulette challenges that allow you to easily help with the “cleaning up” effort. Read this webpage for more details on those Maproulette challenges. This is a huge effort that will take a long time and we really need any help we get. The challenges posted today are only the beginning. They only show the about 6,500 ways worldwide tagged as buildings (with less than 100 nodes) where the way intersects itself. I have decided to start with a simple problem like this, to learn how the Maproulette stuff works and how well, you, the community, responds. Especially for beginners fixing those building is much easier than starting with 10,000 node multipolygons with possibly multiple errors in them. (If you want to, you can still do that. All multipolygon errors show up in the OSM Inspector areas view.)