Over the past few days I have been using Osmose (docs) to find errors in administrative polygons in Ireland. OpenStreetMap does not have a native area type, so we have to create type=boundary relations. It’s easy to break these areas, which means software which wants to extract boundary data from OSM is unable to “see” these areas.

As someone who consumes OSM data for administrative boundaries (Townlands.ie for townlands in Ireland), I have a certain affinity for admin boundaries in OSM in Ireland, and would like T.ie to be as good as possible, and that’s possible if the OSM data is the best possible.

I’ve been turning off all checks in Osmose and enabling just the open polygon error. Click on each blue marker to open the popup with details. Then click the “josm” link for the relation with the problem, which will load that relation in JOSM using JOSM’s remote control. The JOSM validator will then tell you where the open polygon is. You should then look at the problem and see what’s wrong and how to fix it. The most common error is a small gap, and there’s a missing way that you need to (re-)add to the relation. However I have seen more complicated errors sometimes.

Osmose will update regularly, but it can take a day before it’s updated, so the “polygon error” will still be on the website. You can click on the “corrected” link in the popup to tell Osmose that this problem has been fixed. The popup & marker will then disappear from the map (for everyone). You can then clear the errors and work on the next problem. Sometimes I’ve seen many copies of the same error on top of each other, so if you click “corrected” and it looks like the popup & marker don’t disappear, it probably did get recorded as corrected, it’s just that there’s an identical error in the same place which you’re seeing. Just click “corrected” on all of them until it goes away.