Not the subject, but… That’s because the FAA has nothing to do with surveying. Elevation at take-off point for a zero reference, like used in drones to convey elevation, is meaningless when traveling very far. The FAA went with what it uses for manned aviation. With manned aviation, it’s more than just how high you are above the surface, which is important for sure. But also, how high you are absolutely due to it’s effect on aircraft performance. We don’t need centimeter level accuracy there, though!