Many structures exist that could significantly affect the safety of your UA flight when operating below 400 feet AGL and particularly below 200 feet AGL.
Most skeletal structures like radio and television antenna towers are supported by guy wires, which are very difficult to see, even in good weather. These wires can extend up to 1,500-2,000 feet horizontally from a structure…so really, unless you’re specifically inspecting or photographing the structure and have done a proper pre-flight survey of the area, you should avoid these kinds of structures. The FAA recommendation is to fly at least 2,000 feet horizontally from these structures to be clear of guy wires. It’s not an actual regulation, just a strong suggestion.
Scouting out your flight environment ahead of time is really important. It’s not just large antenna towers. It’s overhead transmission and utility lines, construction cranes, large tree branches, large gatherings of birds on a building’s roof — you name it.