This check identifies roundabouts that have an unusual valence. The valence can be for the entire roundabout or any of its intersections. By default, unusual overall valence is defined as less than 2. However, this value is configurable, and can be adjusted by setting connections.minimum in the configuration file. Each intersection in a roundabout should only ever have one or zero connections to outside roads.
- This roundabout id:5794760 has been incorrectly tagged as a roundabout. Because the valence of this roundabout is 1, it should be either labelled as a turning circle or turning loop depending on the traversability of the center.
- This roundabout id:182781989 has multiple connections at single nodes.
In Atlas, OSM elements are represented as Edges, Points, Lines,
Nodes & Relations; in our case, we’re working with Edges.
In OpenStreetMap, roundabouts are Ways classified with
the junction=roundabout tag. We’ll use this information to filter our potential flag candidates.
Our first goal is to validate the incoming Atlas Object. We know two things about roundabouts:
- Must be a valid main Edge
- Must have not already been flagged
- Must have
junction=roundabouttag - Must be car navigable
After the preliminary filtering of features, we need to get all the roundabout's edges. Sometimes roundabouts are drawn as multiple edges so it's important to get all the edges before getting its valence. This will also prevent us from creating a MapRoulette Challenge for each individual Edge.
We use a SimpleEdgeWalker to gather all roundabout Edges that are connected to the original Edge.
Once we have retrieved and stored all edges in the roundabout, we iterate over the connected Nodes to get the count of each Node's connected non-roundabout edges. These connected non-roundabout Edges must also be car navigable to avoid counting pedestrian walkways as part of the valence. If any Node has more than one connection the roundabout is flagged, as this can affect routing (see the second to last bullet under 'How to Map': OSM-wiki:junction=Rrundabout). Otherwise the valence of each Node is summed to get the total. If the total is less than the configured minimum (default 2), it is flagged.
To learn more about the code, please look at the comments in the source code for the check. RoundaboutValenceCheck.java