A remote pilot mapping a 4-mile pipeline corridor in Class G airspace discovers the final mile passes under a Class D shelf with a 1,200-foot AGL floor. The planned survey altitude is 300 feet AGL throughout the corridor. What ATC authorization is required?

Why →A Class D shelf with a 1,200-foot AGL floor means Class D airspace does not exist below 1,200 feet AGL in that segment. At 300 feet AGL the drone remains in Class G. No ATC authorization is required under § 107.41. Airspace is three-dimensional; the student must read both lateral boundary and altitude stratum.

The trap →Many students see the Class D boundary on the chart and assume authorization is required anywhere inside the lateral line. The number painted inside the boundary is the floor. Below it sits whatever class is underneath, usually Class G for shelves.

Field note →Shelved Class D airspace with 1,200-foot or 1,500-foot floors is common near regional airports. Your 400-foot ceiling under Part 107 sits well below those shelves without any authorization at all.