Remove unconnected walking areas from graph

[vesameskanen]

Summary

Walkable area builder is able to detect unconnected areas, but left them in the graph. Such areas often lack so called visibilityVertices, which are required when linking stops to areas. The result was that some isolated graph edges got linked with the stop, and island pruning did not discover to relink such stops after removal of the area.

It seems best to remove bad areas as soon as they get detected, instead of adding more complex code to deal with various linking problems. This PR does the required change.

Attached is a picture of a ferry platform, which got detached from the graph due to this bug.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (11d0094) 63.90% compared to head (b1ede04) 63.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #4981   +/-   ##
==========================================
  Coverage      63.90%   63.90%           
- Complexity     13501    13502    +1     
==========================================
  Files           1674     1674           
  Lines          66152    66156    +4     
  Branches        7155     7156    +1     
==========================================
+ Hits           42274    42277    +3     
- Misses         21492    21495    +3     
+ Partials        2386     2384    -2     

Impacted Files	Coverage Δ
.../graph_builder/module/osm/WalkableAreaBuilder.java	81.19% <100.00%> (+0.21%)	⬆️

... and 3 files with indirect coverage changes

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[leonardehrenfried]

Under what circumstances does this happen? Is it an error in OSM or a bug of the walkable area code?

[vesameskanen]

This usually happens in OsmBoardingLocation linking with a public transit platform. If the platform has simple shape like 4 point square (i.e. no convex points) and no entrance to platform area (unconnected area), the area gets no visibility vertices at all. In my opinion, that is not a bug but a correct result.

When trying to link a transit stop to this kind of area, absence of visibility points means that no connecting edges will be created. However, OsmBoardingLocation module adds some BoardingLocationToStopLink edges to the transit stop. Because of these links, island pruning thinks that stop is linked and does not relink it.

Instead of fixing such link errors, it seemed best to remove faulty areas.

[leonardehrenfried]

And the best fix is to add an entry in OSM, isn't it?

[vesameskanen]

Yes indeed. There will be an UnconnectedArea issue in the report, but this is the automated fix before data gets fixed.