I say that you drive up to the bridge, and that makes you the same height as the bridge because the AGL is from the point you are on. But let's flip the scenario. You are on the bridge and fly down to the river. Can you fly back up? Technically, you are -XXXX feet below where you began. We all can read into this or that. And what if you fly over an Eagle's nest, DNR will scream, and PETA will say you made it abort a new egg. Dept of Foresty will have the butt because the Redwood tree felt the vibration from the props and stunted its growth. While shaking, it registered on the Richter Scale and Japan is freaking out. DJI will shut down, and nobody will have their crappy customer service to complain about since they are closed waiting for the aftershock. All this; do this or don't do that is, "Killing me Smalls." Fly wrong pay the fine, fly straight out the gate.