↖ Fixing Polygons in OSM

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.

Using Maproulette

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.

Open Rings

Difficulty level: Normal

Priority: Medium

Challenge started: 2017-03-29

Description: Multipolygon relations containing an unclosed ring. So the boundary of the (multi)polygon is not closed. These can not be rendered and must be fixed. Careful, many of these are small, but some of them are huge!

To work on this: Lines show the ring with the markers at the open ends. Look at the aerial images first and determine whether the multipolygon makes sense. Often it can be removed, because there is another (complete) polygon or multipolygon that already does its job. If it can be removed, make sure to remove the multipolygon relation as well as all the member ways that are not needed for something else. If it needs to be kept, close the ring by drawing a new way over the gap or extend an existing way. Some of these need local knowledge to fix. Do not spend too much time on these tasks, if it looks to hard, just mark it as "Too hard to fix".

"Spikes" in Buildings

Difficulty level: Normal

Priority: Medium

Challenge started: 2017-04-27

Description: Sometimes ways contain "spikes" where the way doubles back on itself. So you have a way going from node A to B to C, back to B and then on to D. Or something like it. Some of these spikes are easy to fix (just delete node C), but sometimes they are more complicated (A, B, C, B, C, D). This challenge contains about 3000 such cases around the world.

To work on this: The marker shows the tip of a "spike", the lines the extent of the spike. Remove the spike from the way either by removing the nodes or disconnecting them from the way (Alt-J in JOSM). Fix any other multipolygon or tagging problems on the way.

You can take a look at all completed challenges.

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 multipolygons in South Korea

Difficulty level: Medium

Priority: High

Challenge started: 2017-03-29

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 duplicate nodes

Difficulty level: Easy

Priority: Low

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 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.