Inspired by Jochen Topf's old style multipolygon writing, I have just finished manually cleaning up (most? all of?) the remaining old style MPs in Luxembourg.
The process is currently quite tedious. If we want to put old-style multipolygons behind us, we need to make it easier to find them, convert them, maybe even get editors to automatically convert them on the fly when they're touched. QA tools like Osmose could also possibly make it a one-click JOSM remote fix.
The method I used was downloading a Luxembourg extract, then searching for
type:relation type=multipolygon tags:1 in JOSM. I also had
-((type:relation tags:1) | (child (type:relation tags:1))) set up as a filter, but didn't end up using it, because the MPs would disappear as soon as I'd tag them.
For each MP I found, I would zoom to it (
3), click on the outer way, alt-click on that way to select the relation, copy tags from previous selection (utilsplugin2,
shift-R) from the way to the polygon, click the way again, and delete the redundant tags on the way.
Sometimes, for joined geometries, I had to select the relation by right-clicking on the parent relation in the tags list.
Since most of these old-style relations haven't been touched for a long time, I also did some drive-by cleanups. In Luxembourg, the old-style MPs were mostly forests.
Did I get all old-style MPs, or only most of them? If there are multipolygons out there on which both the outer way and the relation are tagged, my find pattern didn't find them. These are possibly the most ambiguous ones, e.g. conflicting tags on outer way and relation, and should be fixed. Has anyone got a good easy way to find them? I tried
hasRole:outer -untagged but that found a lot of false positives.