I calibrated the compass far away from the bridge and off the concrete path that leads to it. As far as the solar k level, from what I saw after looking it up, it was at normal levels. As far are electrical interference goes, I can't rule that out entirely either because the bridge does have lights on it but I'd think that flying in a city would cause more issues than out on a remote bridge. This bridge is miles out of any town and I had to hike 1/2 just to get to it. I didn't see any nearby transformers, cell towers or high tension lines that would give me reason to pause.