Remarkably difficult things to identify from vertical aerial photography. I have been trying to identify one next to the line from Godalming to Guildford. Not on OS▸ maps and virtually invisible on aerial photography (and StreetView isn't in focus enough to read the signs on the gate), obviously have to be investigated on the bike.
Also remarkably difficult to spot from a light aircraft. Fortunately, there are relatively few and they are marked on the charts, but they are tall and slender, with little to see in some lighting conditions. The diagonal cables are all but invisible, and there are frequent warnings throughout the textbooks. You can't even count on sympathy if you black out a million TV sets at a crucial moment in Eastenders.