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.
Duplicate segments in closed ways
Difficulty level: Normal
Challenge started: 2017-03-18
Description: There are many cases of duplicate segments where a closed way doubles back on itself. Usually they are left over from way back before we had multipolygons or they are from mappers who don't know about multipolygon relations and are creating several outer or inner rings by "connecting" those rings with parts of a way. There are two common cases:
- Two or more outer rings connected
- Outer and inner rings connected
Other cases than this are possible, too.
Here is an illustration of the first case, where outer rings connect to other outer rings. This is either fixed by having separate closed ways (if the ways can stand by themselves, for instance if they are just forest polygons), or by creating a proper multipolygon relation (if the ways can't stand by themselves, for instance when they form two parts of an area with a common name). You have to look at the tags to decide which case you have.
And here is an illustration of the second case, where outer rings connect to inner rings. This is usually fixed by creating a proper multipolygon relation.
To fix this: The segment(s) connecting two different rings of a multipolygon are shown. Remove the connecting segment(s) and create a proper multipolygon or separate (closed way) polygons. In JOSM "Tools/Join overlapping Areas" and "Tools/Create multipolygon" can often be used to do this quickly.
The challenge has been split up very roughly 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 fix 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 old-style multipolygons with JOSM
We are currently focusing more on fixing geometrically invalid (multi)polygons, but if you want to get started on fixing the old-style multipolygons, that's great, too.
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.