Getting Started Fixing Polygons in OSM
There are many ways of helping with this effort depending on your OSM skill level and time.
The priority for the following jobs is specified according to the visibility of the problems on the map: The priority "High" is for larger (multi)polygons such as forests that can possibly affect a large map area. "Low" is used for problems that are invisible or nearly invisible on usual maps. "Medium" is for everything in between, such as buildings, etc.
We are creating several Maproulette challenges for different skill levels, regions, and problems. They will be rolled out piece by piece. We are starting with simpler ones and build up towards the more complex problems.
Same tags on member way in multipolygon relation
Difficulty level: Medium
Challenge started: 2017-08-08
Description: Quite often some of the outer or inner ways in a multipolygon relation have the same tags as the relation itself. This is usually wrong and leads to wrong renderings. Note that some tags can be the same, say you have a coniferous forest with a deciduous forest in the middle. But all tags being the same usually doesn't make sense. (Certain tags such as "source" are ignored in this comparison.) A member way should only have the tags describing that specific way only, not anything belonging to the whole multipolygon.
To fix this: Displayed are ways that are a member of a multipolygon relation and have the same tags as the relation. Often you can just remove the tags from the way, but look at the tags of the relation and all member ways and figure out what's right. Look at satellite images to help guide you.
The challenge has not been split up by region:
Using the OSM Inspector
The Areas View in the OSM Inspector show a lot of different information about broken areas. It is a great tool for the advanced mapper, but can be daunting for the beginner. Old-style multipolygons do not show up in this view, because it would be too much data.
Fixing duplicate nodes
Difficulty level: Easy
Challenge started: 2017-02-23
Description: One very simple task is to fix duplicate nodes using OSMI and JOSM. Duplicate nodes are two nodes at the same position or one node that is in a way twice.
To work on this: Go to the OSM Inspector and only activate the duplicate nodes layers. Find an area you want to fix with lots of orange dots and load it into JOSM (using the edit button with the little "J" in it). Activate the validator in JOSM and run it. Click on the "Mixed type duplicated nodes" warning and then the "Fix" button to fix them.
Go to OSMI (todo)
Fixing multipolygons in South Korea
This is mostly done, South Korea doesn't look worse than other places on the planet now.
Difficulty level: Medium
Challenge started: 2017-03-29
Challenge finished: 2017-05-27
Description: South Korea has a lot of broken multipolygons. The problems include self-intersections, overlapping areas, and other problems, mostly on forest and other landuse polygons. The problems are too dense to fix using Maproulette.
To work on this: Go to the OSM Inspector and look at South Korea. Choose a random starting point, load the area into JOSM and start fixing problems. Upload often to minimize duplicate work.
You can load the OSM Inspector Area view into JOSM as a background layer to simplify this. In JOSM, open the menu "Imagary -> Imagery preferences", then find "OSM Inspector: Area" in the "Menu Name" column in the table. Click on it and then the "Activate" button below the table and close the dialog with "OK". You can now choose "OSM Inspector: Area" from the "Imagery" menu whenever you want to add the layer. The layer is transparent, so you can use it together with a satellite image layer.
Go to OSMI (todo)
Fixing old-style multipolygons with JOSM
Cleanup of all the old old-style multipolygons is done. New ones will occasionally be created by editors/mappers who don't know yet how to correctly map multipolygons. So some cleanup might still be necessary now and then.
Fixing old-style multipolygons isn't very difficult with JOSM. Look at this step-by-step description which also has a lot of handy "productivity" tips.