Some bridges have no gap under them, and now the game says which
Since yesterday the game has been pointing pilots at bridges to fly under. A day of flying showed a problem we had partly measured but underestimated. In Google's photoreal mesh a lot of bridges are not bridges at all. The deck is a skin draped from the road down to the water, solid all the way, with nothing underneath. You can see the arches in the texture, but there is no way through. Flying at it ends in the mesh, and the hint that told you to "fly under" it was promising something that does not exist.
It is mostly the small and middle-sized bridges. Big, high ones usually keep their gap: Köhlbrandbrücke in Hamburg, with its deck around fifty metres over the water, is a good example. Suspension and cable-stayed bridges tend to survive. Low stone bridges in a city centre tend not to. Which ones are fused is a property of how Google built the mesh in that place, so there is no list we could copy. The only way to know is to look.
The game looks for you. When you fly within about 300 metres of a bridge in the photoreal world, the game sends a few rays down from just under the deck, at several points along its middle. If one of them finds open air a few metres below, the bridge has a gap. If every point answers solid, the bridge is fused. A bridge only counts as fused after three such answers in a row, because the detailed tiles sometimes arrive a second or two after you do. A missing tile never counts as an answer.
What changes on screen. A fused bridge stops getting the "bridge ahead" hint with the arrow. If you fly right up to one anyway, the pill says the bridge has no gap in Google 3D instead of telling you to fly under it. On both maps a fused bridge is a smaller grey marker and is placed last when markers compete for space, so the bridges you can actually fly under win. The bridge card says which case it is: no gap, or a gap that pilots have found.
Everyone's flights add up. Each check is sent to the server, which keeps it only if your own flight path really passed near that bridge. The answers are pooled, so a bridge one pilot found fused is grey for the next pilot before they get there, and a bridge someone has flown under is always marked open. Your own check in this flight wins over the pooled answer, since you are looking at the same mesh right now.
Nothing about counting changes. A pass still has to go under a real deck with air below, and each bridge still counts once. If you find a bridge the game calls fused that you can fly under, or the other way round, tell us in Discord with the bridge's name. The check is new, and real bridges will show us where it is wrong.